# Makita / Dolmar 7900 Piston and Cylinder Install w/ PICS!!



## stock50s (Jan 22, 2010)

I installed the 7900 p/c today on my Home Depot Makita! I thought it would be worthwhile to post up some pics and give a few tips that my help a first timer. First off I want to start by saying that I am NOT a chainsaw mechanic nor do I claim to be one. If anyone has anything to add or feels I did something incorrectly, please post up the correct way or any tips that would make it easier on the next guy. Thank you.

To start off, remove both top covers and the handle as it will just get in the way. Before going any farther, blow off any dirt and grime around the carburetor, cylinder and exhaust. 







The next step is to detach the exhaust and carb. The exhaust is self explanatory but the carb can be a little tricky. To remove the carb, this rubber plate has to be removed to gain access to the screws. 






Once the carb is unscrewed and the impulse line is disconnected the cylinder can come off. This next step is not 100% necessary unless you need to reposition or trim the limiting cap but it will make reassembly easier. The intake manifold is mounted to the saw with a plastic ball and rubber socket. Simply pull up on the intake manifold and the ball will pop out of the socket. Now the carb is free... well sort of, disconnect the throttle cable and choke linkage. 






Now on to the cylinder, break all 4 screws loose (I like to use an “X” pattern) then remove them. Now the cylinder can be carefully removed. If the cylinder does not move, GENTLY tap it a dead blow hammer or rubber mallet. 











Before even breathing again, stuff a clean rag in the black hole you just created. The case just loves to suck down any kind of bolts, clips, or dirt. Now the fun part, remove the wrist pin clip from one side of the piston and push out the wrist pin. I like to use a small pick and get behind the clip to pop it out. Be sure to wear safety glasses and cover the side of the piston with your hand or a rag before you pop it out.






If the wrist pin doesn't slide right out, use the back side of a pen to push it out. Remove the old cylinder gasket and clean the surface well. This is easier to do before you put on the new piston.






I then swapped the intake boot, impulse line, decompression valve and AV mount onto the new cylinder. The decompression valve and AV mount can be put on after the cylinder is installed but I would defiantly install the intake boot and impulse line before. The AV mount spring has to be removed before you can get to the screw. It is as simple as turning the spring to unscrew it.





to be continued…


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## stock50s (Jan 22, 2010)

... Here is a comparison between the 6401 and 7900 cylinders. The 7900 cylinder is OEM but I was a little disappointed with the casting quality. The bore looked great but the intake port had casting marks and around the spark plug hole the cooling fin is missing some material. It is not poor quality by any means but I was surprised that 6400 cylinder was a much cleaner casting. 





















Comparing the pistons....






Alright, back to the install. If there is not a wrist pin clip in either side of your new piston, install one side before connecting it to the crank. Now lube up the wrist pin with some 2 cycle oil and slide it through the piston and connecting rod. The arrow on the piston is to point to the exhaust port. Now it is time to install the other clip. The clips can be tricky and even frustrating to put in. I have found that the easiest way for me is to put one end of the clip in the piston groove and then to work my way around the clip with the back of a pen. This prevents any kind of galling and won't hurt your finger if you slip.






Before slipping the cylinder on, make sure both the piston and cylinder are clean and the new gasket is on. I like to wipe them with a rag that’s dipped in premix. This will leave a small coating of oil once the gas evaporates. I also wipe a SMALL amount of oil on the piston ring. (Some say not to oil the ring but I personally have never had a problem when installing one this way). With the piston ring clamped, in the correct position (ends of the ring on either side of the locator pin) gently slide the cylinder over the piston. If it doesn't want to go, don't force it as the ring is probably caught.










Once all of the cylinder head screws are in, tighten them in steps using an “X” pattern. I usually snug them all up then tighten in two or the passes to ensure even seating and clamping force.
to be continued…


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## stock50s (Jan 22, 2010)

... The hard part of this install is over. Now you just have to reassemble a few things and you are ready for break-in. During the conversion, I also installed the HD air filter kit. I coughed up an additional $18 for the black cylinder cover so everything would match.  











Here is the final product. 











I truly would appreciate any critique on the install or steps that would make the install better or easier. I realize that a lot of these steps are so easy they may be insulting some but that was not my intent. Thanks again and I hope this will help some of you during your install.


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## Warroad50 (Jan 22, 2010)

Have you fired it up yet? Inquiring minds want to know how it runs with the upgrade. Looks good nice post.


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## howellhandmade (Jan 22, 2010)

Excellent contribution, way to go strong off the line. Great pics. Interesting that some surfaces, like the intake boot flange, that appear to be machined on the 64cc jug look as cast on the 79cc. Let us know how it runs.

Jack


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## grandpatractor (Jan 22, 2010)

Hey stock50s,nice job with the pics. I'm sure it will help some guys out.


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## stock50s (Jan 22, 2010)

Yep she runs!! I haven't even put half a tank through it yet but I can tell it will be a strong runner. 

The whole cylinder looks like it has a coating of oxidation on it. I was a little uneasy but then I noticed where the AV mount goes was nice and shiny. I now think that the only part machined after casting (besides the bore) was the AV mounting surface. Hard to tell but here is a pic.






One thing I completely forgot to mention is retuning. Make sure to richen up both the High and Low settings before firing it up. Obviously fine tune once running and warmed up.


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## mini14 (Jan 23, 2010)

excellent job, great photos, ive done 4 installs so far and just slipped the boot off the cylinder 1 less step that doesn't need to be done imho.


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## rms61moparman (Jan 23, 2010)

Fantastic step by step!!!
Mega shot of MOPAREP!!!!! coming your way!


Mike


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## FXDL (Jan 23, 2010)

Nice step-by step. one day i might do the 79er conversion


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## WACutter (Jan 23, 2010)

*Great Post!*

This is the step by step I have been looking for. Now all I need is a good deal on a 6401....Time to start trolling the local HD's!

Thanks for such detail.


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## firebenrt (Jan 23, 2010)

I did the same conversion to my 6401. I actually thought the P&C looked really nice. I was impressed with the exhaust port- smooth as a baby's bum!

Good post and good pictures!

I need to get the HD air filter kit- but all my money is tied up in project saws!


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## WoodChuck'r (Jan 23, 2010)

Awesome thread. Great write up. Excellent pics. Damn good contribution. And one kick a$$ saw. 

I dig.


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## Edge & Engine (Jan 23, 2010)

Just an FYI, you do not need to remove the cylinder-mounted AV spring to remove it from the cylinder. The outer screw hole (that attaches it to the handle) is just large enough to fit a T-27 wrench through to access the screw that holds the mount to the cylinder.


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## stock50s (Jan 23, 2010)

Thanks for the positive comments and tips guys.  It's good to know that it is easy to slip the intake boot off the cylinder. That would definitely save some time if you don't have to trim the limiting cap or reposition it. As for the AV spring, my T-handle wouldn't fit through but I never checked with one of the L wrenches. Good tips guys, lets keep them coming.


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## outdoorlivin247 (Jan 23, 2010)

Great thread...Definatly rep worthy, but the rep train is running late today...


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## Evan (Jan 23, 2010)

great thread.

i noticed those pistons have beefy port windows. i bet they could go through a pretty good weight loss program


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## busanova (Jan 23, 2010)

This is a great thread for anybody thinking about upgrading their 6401 to a 7901.I've been checking my local Canadian "Home Depots",but unfortunately they don't handle the Makita DCS6401 as their rental saw.They use the Echo 520 in my area.I'll have to keep my eyes peeled on the classifieds or ebay I guess if I want a Makita 6401.Anyway great post and good luck on your new 7901 upgrade.


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## howellhandmade (Jan 23, 2010)

stock50s said:


> Yep she runs!! I haven't even put half a tank through it yet but I can tell it will be a strong runner.
> 
> The whole cylinder looks like it has a coating of oxidation on it. I was a little uneasy but then I noticed where the AV mount goes was nice and shiny. I now think that the only part machined after casting (besides the bore) was the AV mounting surface. SNIP



In your initial pics it looks like the mating surface for the muffler isn't machined on the 79cc cylinder. Seems odd if so, you want a good seal there. I figured my eyes deceived me.

Jack


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## KMB (Jan 23, 2010)

stock50s said:


> I coughed up an additional $18 for the black cylinder cover so everything would match.



That's what I intend to do when I get mine. That's a good lookin' saw right there! 

Kevin


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## tjbier (Jan 23, 2010)

stock50s said:


> ... The hard part of this install is over.



Yep definitely a keeper!! I am waiting for the HD kit yet  , but I painted the stock bonnet and filter cover from grey to black yesterday with a rattle can and baked it in the oven for 25 min @ 175deg. 

Well done and appreciate the pictures!


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## buzz sawyer (Jan 23, 2010)

Evan said:


> great thread.
> 
> i noticed those pistons have beefy port windows. i bet they could go through a pretty good weight loss program



My thoughts exactly. That intake port looks considerably larger on the 7900 jug too.

Great post- rep sent!

Next-----VIDEO!!!!


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## KMB (Jan 23, 2010)

tjbier said:


> Yep definitely a keeper!! I am waiting for the HD kit yet  , but I painted the stock bonnet and filter cover from grey to black yesterday with a rattle can and baked it in the oven for 25 min @ 175deg.
> 
> Well done and appreciate the pictures!



Got any pics of that paint job?

Kevin


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## 04ultra (Jan 23, 2010)

stock50s said:


> …



Why didn't you use the 965.531.160 Metal base gasket ????





.


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## tjbier (Jan 23, 2010)

You bet! Might not be as nice as OEM but hey it works for me.


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## stock50s (Jan 23, 2010)

howellhandmade said:


> In your initial pics it looks like the mating surface for the muffler isn't machined on the 79cc cylinder. Seems odd if so, you want a good seal there. I figured my eyes deceived me.
> 
> Jack



It looked like the cylinder was only machined on that one AV mounting hole. The the exhaust surface was flat and I could barely see machine marks but did not look machined like the AV mount. The spark plug hole and mating surface both, and the decomp hole all looked dull like the rest of the cylinder. Maybe they machine the critical spots and the dunk the cylinder in something to prohibit corrosion? I know it is not, but it almost looks galvanized. I realize that would not explain the AV mount unless a correction had to be made later on. Anyone have any ideas as to why the cylinder would be that way?


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## stock50s (Jan 23, 2010)

04ultra said:


> Why didn't you use the 965.531.160 Metal base gasket ????
> 
> 
> 
> ...



That is the one that was included with the p/c. I also was not aware that a metal one existed. Is there a difference in thinkness between the two?


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## 04ultra (Jan 23, 2010)

stock50s said:


> That is the one that was included with the p/c. I also was not aware that a metal one existed. Is there a difference in thinkness between the two?



Metal one is the updated one ............






.


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## grandpatractor (Jan 23, 2010)

stock50s said:


> That is the one that was included with the p/c. I also was not aware that a metal one existed. Is there a difference in thinkness between the two?



I am not sure if there is a thickness difference, but the metal one is the newer updated part and I believe it to be more robust!


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## stock50s (Jan 23, 2010)

04ultra said:


> Metal one is the updated one ............
> 
> 
> 
> ...



Bummer!! Was there a problem with the other ones blowing out? I would hate to have it let go and loose a p/c over a $2 gasket. I may just have to pull it back apart and replace the gasket now that you have me worried.


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## 04ultra (Jan 23, 2010)

stock50s said:


> Bummer!! Was there a problem with the other ones blowing out? I would hate to have it let go and loose a p/c over a $2 gasket. I may just have to pull it back apart and replace the gasket now that you have me worried.





From what Im told the metal holds up a lot better........... The metal gasket has been used for a couple years now......








.


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## grandpatractor (Jan 23, 2010)

stock50s said:


> Bummer!! Was there a problem with the other ones blowing out? I would hate to have it let go and loose a p/c over a $2 gasket. I may just have to pull it back apart and replace the gasket now that you have me worried.



I don't know if I would tear it back apart, but I would check the base screws occasionally and make sure they stay tight!


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## nmurph (Jan 23, 2010)

where did you get that cylinder? i have done two of these conversions and neither of mine looked like yours.


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## rms61moparman (Jan 23, 2010)

stock50s said:


> Bummer!! Was there a problem with the other ones blowing out? I would hate to have it let go and loose a p/c over a $2 gasket. I may just have to pull it back apart and replace the gasket now that you have me worried.



Don't worry about it.
My base gasket is a thin bead of permatex#2.


Mike


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## KMB (Jan 24, 2010)

tjbier said:


> You bet! Might not be as nice as OEM but hey it works for me.



Looks fine...and you saved yourself a few bucks.

Kevin


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## wendell (Jan 24, 2010)

Great job!! I tried to rep ya but it said i have to spread it around.


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## CHEVYTOWN13 (Jan 24, 2010)

04ultra said:


> Why didn't you use the 965.531.160 Metal base gasket ????
> 
> 
> 
> ...



I agree. The metal gasket is tops. It even snaps into place and is handy when installing the jug. The best part about it is it lowers your squish.


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## stock50s (Jan 24, 2010)

CHEVYTOWN13 said:


> I agree. The metal gasket is tops. It even snaps into place and is handy when installing the jug. The best part about it is it lowers your squish.



Well guys I think I am going get the metal gasket. I can't imagine that Dolmar would replace the gasket if the old one was not prone to problems. I purchased the p/c from a site sponsor. If the new metal gasket has been out for a few years, it is a little disappointing that the old style was included. So no one has received a p/c that looked like the one I received?


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## 04ultra (Jan 24, 2010)

stock50s said:


> Well guys I think I am going get the metal gasket. I can't imagine that Dolmar would replace the gasket if the old one was not prone to problems. I purchased the p/c from a site sponsor. If the new metal gasket has been out for a few years, it is a little disappointing that the old style was included. So no one has received a p/c that looked like the one I received?



The place that shipped you it is a Dolmar dealer???




.


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## stock50s (Jan 24, 2010)

04ultra said:


> The place that shipped you it is a Dolmar dealer???



Yes they are. I would think that if a new part comes out because of an issue Dolmar should take back the old inventory as to prevent more problems. I think I will give the sponsor a call on Monday and see what they have to say. I don't want to make a mountain over a mole hill but at the same time this may be beneficial to others that do this conversion. The metal gasket is only $2 and I gladly would have payed that had I known it was the updated one.


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## 04ultra (Jan 24, 2010)

stock50s said:


> Yes they are. I would think that if a new part comes out because of an issue Dolmar should take back the old inventory as to prevent more problems. I think I will give the sponsor a call on Monday and see what they have to say. I don't want to make a mountain over a mole hill but at the same time this may be beneficial to others that do this conversion. The metal gasket is only $2 and I gladly would have payed that had I known it was the updated one.



Wow .......When I bought one from the Cutting Edge it had the metal gasket.......................




.


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## rms61moparman (Jan 24, 2010)

stock50s said:


> Well guys I think I am going get the metal gasket. I can't imagine that Dolmar would replace the gasket if the old one was not prone to problems. I purchased the p/c from a site sponsor. If the new metal gasket has been out for a few years, it is a little disappointing that the old style was included. So no one has received a p/c that looked like the one I received?





YOU GUYS SHOULD BE ASHAMED OF YOURSELVES!!!!!

This guy goes to the time and trouble to record the step by step tutorial on swapping out a jug and slug (and does a damned fine job I might add) on a 6401, and instead of giving him the kudos he deserves and congratulating him on the superb job he did and the fantastic saw he built, you have centered on one thing that wasn't up to YOUR standards of perfection and now have him ready to take his saw back apart.

There is NOTHING wrong with the saw you built stock50s, it will bring you many hours of pleasure and do an unfathomable amount of cutting in the years to come.

I know of guys running rubber gaskets, cardboard gaskets and oiled paper grocery bag gaskets and doing very well.
As I said, I don't even run a base gasket if the squish allows it and I have never had any issue with my method whatsoever.

Between the "safety police" and the "Overkill Engineers" it's a wonder people ever want to post anything here.

stock50s, do what will make you feel at ease with your saw! If the only thing that will give you 100% confidence in your saw, is to take it back apart and swap in the metal base gasket then by all means do it, but I will state right here and now that there is NOTHING wrong with your saw! And if you want to run it just the way you built it and it ever fries its p&c because of a base gasket leak, I WILL BUY THE NEXT PISTON & CYLINDER FOR YOU!!!!!
I am that confident in your work.


Mike


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## 04ultra (Jan 24, 2010)

*He did a great job on the thread*..........




It just surprised me that it was not the metal gasket.......The metal one kinda looks like the 660 Stihl style gasket......


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## atlarge54 (Jan 24, 2010)

Excellent step by step and great photos, too bad it was a few days late to be any help to me.

Did you do a trial assembly w/o piston ring and no base gasket to check squish?

Anyone considering doing this conversion, I just used my fingers to compress the ring and thought it was quite easy. Maybe get the ring tools if you plan on doing it often.

I seriously doubt I'd tear it apart over the base gasket, were base gasket blowouts extremely common? 

What are your squish and compression figures?

Did the packing have any markings to indicate where it was mfg? Is it possible you got a Chi-com substitute?


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## rmh3481 (Jan 24, 2010)

Im with Mike, he did a good job. Dont worry about the gasket. Check the bolts after a couple heat cycles and run the saw.


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## Edge & Engine (Jan 24, 2010)

Not 100% sure until confirmed by the OP, but I believe that this piston & cylinder was purchased from us. If so, it is definitely OEM.
As far as the old style base gasket, that was our/Dolmar's fault. I personally was aware of the new metal gasket (from the bulletin 2 years ago). However, for reasons unknown, Dolmar did not supersede the part number and is continuing to supply the paper gasket. Because of that our system continued to order the paper gasket, which is our fault because we could have manually updated it to start ordering the metal gasket. (BTW, the 3/2008 bulletin regarding the gasket change does not state the reason for the change).

If this was indeed a P&C sold by us, we'll have a metal gasket in the mail for you FOC whether you plan to replace it now or later. Just shoot me a PM. 

- Kyle


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## billmartin (Jan 24, 2010)

Edge & Engine said:


> Not 100% sure until confirmed by the OP, but I believe that this piston & cylinder was purchased from us. If so, it is definitely OEM.
> As far as the old style base gasket, that was our/Dolmar's fault. I personally was aware of the new metal gasket (from the bulletin 2 years ago). However, for reasons unknown, Dolmar did not supersede the part number and is continuing to supply the paper gasket. Because of that our system continued to order the paper gasket, which is our fault because we could have manually updated it to start ordering the metal gasket. (BTW, the 3/2008 bulletin regarding the gasket change does not state the reason for the change).
> 
> If this was indeed a P&C sold by us, we'll have a metal gasket in the mail for you FOC whether you plan to replace it now or later. Just shoot me a PM.
> ...



Not to try and sound like a gangster. But, Props to you for looking out. 

You don't even know if thats your customer but your willing to step up and take care of it.

That's one of the many reasons I really like this site. Honest people

Bill


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## 166 (Jan 24, 2010)

The PS-6400, PS-7300, PS-7900 Chainsaws and the PC-6412, PC,6414, PC-7312, PC-7314, PC-8116, PC-6430, PC-6435, PC-6530, PC-6535, PC-7330, PC-7335, PC-7430, PC-7435, PC-8140, and the PC-8240 Power Cut-Off Saws all can use the same cylinder base gasket.

They implemented the Metal - Crimped gasket for the Chainsaws back in December 2007. The Paper Gasket at that time was still used on the Cut-Off saws. So the paper number Would Not supercede to the metal gasket part number.

We had issues with the paper base gasket on the 64,73,7900 Chainsaws. 95% of the time it was with saws running 24" or longer bars. Very seldom did we see failures with saws running 20" bars.

So the part number on the gaskets are:
Paper - 965.531.121
Metal - 965.531.160

Steve


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## stock50s (Jan 27, 2010)

Hey guys I thought I would give you a little update. The p/c were indeed purchased from Edge & Engine. I just want to start off by saying that the p/c are definitely OEM and the fact that the intake port was not as smooth as expected has nothing to do with the vendor. As far as the metal gasket, Kyle said he will have one on the way ASAP. The true test of a company is how they deal with customer support and I can honestly say they pass with flying colors. I just wanted to thank you again Kyle for making it right. 

To get back on track, no I did not measure the squish when I installed the p/c. If decided to pull it apart and replace the gasket I will throw some Plastigage onto the piston and get the squish measurement. I tried to get some compression readings but I think my gauge is acting up. I tested 3 different saws and they all read 100 psi so it is safe to say the gauge is not cooperating. The saw has less that a tank through it so it will still pick up some performance.


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## Andyshine77 (Jan 27, 2010)

> We had issues with the paper base gasket on the 64,73,7900 Chainsaws. 95% of the time it was with saws running 24" or longer bars. Very seldom did we see failures with saws running 20" bars.
> 
> So the part number on the gaskets are:
> Paper - 965.531.121
> ...



What was the issue with the paper gaskets?


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## grandpatractor (Jan 27, 2010)

Andyshine77 said:


> [QUOTE We had issues with the paper base gasket on the 64,73,7900 Chainsaws. 95% of the time it was with saws running 24" or longer bars. Very seldom did we see failures with saws running 20" bars.
> 
> So the part number on the gaskets are:
> Paper - 965.531.121
> ...



What was the issue with the paper gaskets?[/QUOTE]

I have seen one where the gasket was getting pulled inward and really close to leaking, if it wasn't already. It had been straight gassed and I was replacing the piston and cylinder.


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## rayvil01 (Jan 27, 2010)

*Torque?*

_Once all of the cylinder head screws are in, tighten them in steps using an “X” pattern. I usually snug them all up then tighten in two or the passes to ensure even seating and clamping force._

Is there a torque value on these bolts?

Thanks for the post. I bought my 7900 in December of '07. 99-plus-percent chance I've a paper gasket. I may get a metal gasket, just in case. So that part of the discussion was helpful as well.


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## 04ultra (Jan 27, 2010)

stock50s said:


> I just wanted to thank you again Kyle for making it right.
> 
> .



:agree2::agree2:



.


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## caricob (Jan 27, 2010)

*question*

What type gasket does the BB kit come with? If not metal will the OEM metal gasket work with the BB kit. I would like to go with OEM but I don't know if I can come up with the extra cash. Is there enough difference to justify the cost?


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## rms61moparman (Jan 27, 2010)

caricob said:


> What type gasket does the BB kit come with? If not metal will the OEM metal gasket work with the BB kit. I would like to go with OEM but I don't know if I can come up with the extra cash. Is there enough difference to justify the cost?




The first question seems rather silly but, what are you going to use the saw for???
Yes I know, cutting wood right?
If this is going to be a 5-7 cord per year firewood saw, I believe you will be as well served with the BB kit, and yes the oem gasket should work just fine.
If this is a saw that is going to see a lot of HEAVY use, I would recommend saving up the extra cash and going with the OEM.

JMHO,

Mike


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## Edge & Engine (Jan 28, 2010)

rayvil01 said:


> _Once all of the cylinder head screws are in, tighten them in steps using an “X” pattern. I usually snug them all up then tighten in two or the passes to ensure even seating and clamping force._
> 
> Is there a torque value on these bolts?
> 
> Thanks for the post. I bought my 7900 in December of '07. 99-plus-percent chance I've a paper gasket. I may get a metal gasket, just in case. So that part of the discussion was helpful as well.



14.0 Nm +/- 1.0 Nm


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## ancy (Feb 29, 2012)

*Great post*

It will help when the kit comes in nest week. Any new things come up? Does it matter which way that metal gasket goes in? Do I need a longer wrench than the one that comes with the saw? Thanks


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## timmcat (Mar 1, 2012)

What did the HD air filter set up cost you? It looks way better than stock and the filter sure looks to have alot more surface area.
Tim


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## RiverRat2 (Mar 1, 2012)

grandpatractor said:


> Hey stock50s,nice job with the pics. I'm sure it will help some guys out.



Yep JD I agree!!!!!! Repped him too!~!!!!



timmcat said:


> What did the HD air filter set up cost you? It looks way better than stock and the filter sure looks to have alot more surface area.
> Tim


 Hey call Kyle @ Edge & Engine,,, (post #56 of this thread right above you a couple) He will hook you up on the best price and he is a site sponsor!!!!


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## ancy (Mar 1, 2012)

You know this is a few years old but I bought one last winter for $69.99 + shipping and haven't had to clean it in a year. I wished they had one for the 5100.


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## ECHO-Echo-echo (Mar 14, 2012)

So stock50s, do you still have the saw and how is it running now? It should be broke in by now? 
I'm looking to do basicly the same thing down the road. I'm scouting around for a deal on an oem 7900 kp/c kit. and the xl air filter.


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## ncfarmboy (Mar 15, 2012)

He lucked out and got an old stock P&C. The piston has windows. The new P&C kits come with a "lighter" non windowed piston. Got two of em to do. One on a new 6401 Makita and one for a 665 Solo to make it a 681. Mine came from Bailey's did not have a gasket of any kind in the box. Be careful tuning the limited coil will fool you.
Shep


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## ancy (Mar 15, 2012)

ncfarmboy said:


> He lucked out and got an old stock P&C. The piston has windows. The new P&C kits come with a "lighter" non windowed piston. Got two of em to do. One on a new 6401 Makita and one for a 665 Solo to make it a 681. Mine came from Bailey's did not have a gasket of any kind in the box. Be careful tuning the limited coil will fool you.
> Shep



You think the old piston is better? Why? There is another thread about it and it is lighter. http://www.arboristsite.com/chainsaw/182257.htm


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## ncfarmboy (Mar 16, 2012)

ancy said:


> You think the old piston is better? Why? There is another thread about it and it is lighter. http://www.arboristsite.com/chainsaw/182257.htm



They are lighter I have 2 of them. Don't know personally if they are better just going by what I've read. EC, Ctown etc all run windowed in their WP and race saws must be a reason. TW, EC cut windows in their WP 372, 359 to name a few. Must be a reason. My thinking is that it directs the fuel air charge to the transfers sooner and cools the piston better. 
Shep


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## stock50s (Dec 7, 2012)

*Update*

I haven't been on the forum for a while so I apologize for taking months to respond. The saw is still running strong and well broken in by now. I see that the piston was redesigned without windows. Can anyone confirm the motive behind that? It is interesting they would "improve" the old design without good reason.


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## nstueve (Dec 7, 2012)

stock50s said:


> I haven't been on the forum for a while so I apologize for taking months to respond. The saw is still running strong and well broken in by now. I see that the piston was redesigned without windows. Can anyone confirm the motive behind that? It is interesting they would "improve" the old design without good reason.



I'm no engineer so this is all conjecture...

The new piston has no windows so the initial flow of gas mix will be slowed. IE less to burn = less emissions 
The new piston is also lighter thereby reducing "resistance" and increasing power. letting less fuel through also results in lower fuel consumption. 

Basically I would guess it was a move to make same power with less emissions just by changing the piston casting. If I were "building" a saw I would want the old style that can be ground on a little to make them lighter and flow better through the windows.


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## dntbdmb (Feb 16, 2013)

*thank you stock50s!*

I know this is an old post, but the write up and knowledge of all the other posters on this site has helped me tremendously. I just purchased a Makita 6421, with the mahle 79cc kit. Upon receiving everything, I started the saw, just to make sure it ran and then tore it down. The whole process took about 2 hours, and I have never done one before.

The 6421 is slightly different. The decompression valve was relocated to the top right side, and the h setting on the carb appears to have a double d set screw. The decompression valve has way too much thread to seat all the way in, about half an inch of thread hangs out. I just bottomed it out and snugged it up. I had to drill a new hole in the plastic to relocate it, I did this with a step bit, I was off a little. I am considering buying another valve, so it seats better, but am holding out to do a compression test to see if its even an issue worth worrying about.

I am currently waiting on Amazon to deliver the double d adjusting tool before I run the saw wide open and set it with a tach. I plan on gutting the muffler tomorrow, if time allows.

Again, thank you for a superb write up, made my life much easier!


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## ancy (Feb 17, 2013)

Sounds like your on the right track. I do have a almost new set of 6401 top plastics that would line up with the decomp on the 7900 top end, if your interested PM me.


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