# Building a 372XP Big Bore With 272XP Piston



## Mastermind (Jun 23, 2013)

Here's a few pics......I'll be adding to this one. 

Opened up the transfer inlets a bit since the 272XP piston is a windowed design.









Here we are test fitting to the case. The match is worse than you can see here.....





Here it is after being case matched.......





Like I said......I'll post more as I set the squish and port it.


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## ELECT6845 (Jun 23, 2013)

That 372 looks clean. Did some idiot power wash it for you?


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## barneyrb (Jun 23, 2013)

Randy, who built the big bore kit?????

Thanks.....


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## Mastermind (Jun 23, 2013)

ELECT6845 said:


> That 372 looks clean. *Did some idiot power wash it for you?*



Sure enough. :msp_sneaky: 



barneyrb said:


> Randy, who built the big bore kit?????
> 
> Thanks.....



It's a no name kit from Weedeaterman. Very nice ones these are.


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## Jacob J. (Jun 23, 2013)

Interesting how Husqvarna chose to shroud the lower transfers on the 372 jug, but chose not to on the 385/390...


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## Mastermind (Jun 23, 2013)

Jacob J. said:


> Interesting how Husqvarna chose to shroud the lower transfers on the 372 jug, but chose not to on the 385/390...



I think it's because of the relationship with the bottom of the piston at BDC......


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## Rudolf73 (Jun 23, 2013)

Woohoo a 372BB hotrod build! :cool2:


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## workshop (Jun 23, 2013)

Mastermind said:


> Here's a few pics......I'll be adding to this one.
> 
> Opened up the transfer inlets a bit since the 272XP piston is a windowed design.
> 
> ...



It's time for a dumb question now. What is squish?:msp_confused: Besides what a bug makes when you step on it.


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## Mastermind (Jun 23, 2013)

Rudolf73 said:


> Woohoo a 372BB hotrod build! :cool2:



It's gonna get a big assed carb and stuff too.


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## Mastermind (Jun 23, 2013)

workshop said:


> It's time for a dumb question now. What is squish?:msp_confused: Besides what a bug makes when you step on it.



Squish is the clearance between the top of the piston and the head.....


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## Jacob J. (Jun 23, 2013)

Mastermind said:


> I think it's because of the relationship with the bottom of the piston at BDC......



That and desired base volume when the piston starts back up I always figured...


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## workshop (Jun 23, 2013)

Ah! I understand. Thanks for answering Mastermind. I see others talking about proper squish clearance. Is there a rule of thumb standard for it? Also have seen others talk about removing the gasket at the bottom of the cylinder. Does this put squish in danger of being too close?


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## Mastermind (Jun 23, 2013)

workshop said:


> Ah! I understand. Thanks for answering Mastermind. I see others talking about proper squish clearance. Is there a rule of thumb standard for it? Also have seen others talk about removing the gasket at the bottom of the cylinder. Does this put squish in danger of being too close?



I try to shoot for .022 - .024 

On some saws removing the base gasket will make it way too tight. Always check it to be sure......


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## mt.stalker (Jun 24, 2013)

Mastermind said:


> It's gonna get a big assed carb and stuff too.



That thing's gonna scream :msp_thumbup:


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## Rudolf73 (Jun 24, 2013)

mt.stalker said:


> That thing's gonna scream :msp_thumbup:



Literally and metaphorically :eek2:


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## bryanr2 (Jun 24, 2013)

been real boring on here lately. Glad to see something worth reading.


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## greg storms (Jun 24, 2013)

*squish*

so, would not using a base gasket yet maintaining the deck clearance you mention be the same as raising the compression ratio? I'm trying to comprehend....


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## Mastermind (Jun 24, 2013)

greg storms said:


> so, would not using a base gasket yet maintaining the deck clearance you mention be the same as raising the compression ratio? I'm trying to comprehend....



You'll see us talk about cutting the squish band in these engines. That decreases the volume of the combustion chamber......then we turn the base of the jug to set the squish clearance which raises compression. At times lowering the jug also allows us more control over the port heights.


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## watsonr (Jun 24, 2013)

Mastermind said:


> You'll see us talk about cutting the squish band in these engines. That decreases the volume of the combustion chamber......then we turn the base of the jug to set the squish clearance which raises compression. At times lowering the jug also allows us more control over the port heights.



Come back from your retreat and be all technical and stuff.... welcome home:msp_wink:


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## Mastermind (Jun 24, 2013)

watsonr said:


> Come back from your retreat and be all technical and stuff.... welcome home:msp_wink:



Bonnaroo was a blast..........Sundance in two weeks. That's my last getaway till next spring.


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## nmurph (Jun 24, 2013)

Jacob J. said:


> Interesting how Husqvarna chose to shroud the lower transfers on the 372 jug, but chose not to on the 385/390...





Mastermind said:


> I think it's because of the relationship with the bottom of the piston at BDC......





Jacob J. said:


> That and desired base volume when the piston starts back up I always figured...



Any thoughts on why the OE is shrouded and the NE cyinder isn't?

I have seen the OE shroud removed, left in place, and I have seen a really strong OE that left it at about 1/2 it's original height.


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## thomas1 (Jun 24, 2013)

Mastermind said:


> Bonnaroo was a blast..........Sundance in two weeks. That's my last getaway till the next Central Virginia GTG.



fixed.


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## Mastermind (Jun 24, 2013)

thomas1 said:


> fixed.



Oh yeah........no way in hell I'm missing a GTG at Ernie's.


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## Rudolf73 (Jun 24, 2013)

Mastermind said:


> Oh yeah........no way in hell I'm missing a GTG at Ernie's.



You Mericans with all your fun GTG's... wish we had something similar over here :msp_unsure:


I might have to have my own gtg and just dress up different for each saw so that it looks a bit more populated. Could be fun


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## MCW (Jun 25, 2013)

Mastermind said:


> It's a no name kit from Weedeaterman. Very nice ones these are.



Were they the same as that kit I sent you originally Randy? If so they are the best 372 BB kits I've seen and the best performing as well.



Rudolf73 said:


> I might have to have my own gtg and just dress up different for each saw so that it looks a bit more populated. Could be fun



I'll come Rudy and also bring a few changes of clothes and some wigs.


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## Jimmy in NC (Jun 25, 2013)

Mastermind said:


> Bonnaroo was a blast..........Sundance in two weeks. That's my last getaway till next spring.



So that means you'll need visitors to break the monotony and bring pie. Have to see what I can do....

OK..back to the Swede.




Jimmy


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## thomas1 (Jun 25, 2013)

MCW said:


> I'll come Rudy and also bring a few changes of clothes and some wigs.



So, it'll be just like the GTG's here, then?


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## MCW (Jun 25, 2013)

thomas1 said:


> So, it'll be just like the GTG's here, then?



Nearly the same but there won't be any hamsters.


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## thomas1 (Jun 25, 2013)

MCW said:


> Nearly the same but there won't be any hamsters.



Wallabies.


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## MCW (Jun 25, 2013)

thomas1 said:


> Wallabies.



They're not as cute as you think in real life. They are smelly, have sharp claws, and they have these stupid little front legs that serve no real purpose apart from picking their nose.


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## wyk (Jun 25, 2013)

thomas1 said:


> So, it'll be just like the GTG's here, then?



I tried to organize a GTG in Ireland

We never left the pub.

Well, we did eventually. They booted us when we tried to start the saws out back in the smoking area.


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## thomas1 (Jun 25, 2013)

MCW said:


> They're not as cute as you think in real life. They are smelly, have sharp claws, and they have these stupid little front legs that serve no real purpose apart from picking their nose.



Sounds like Blazin.


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## Mastermind (Jun 25, 2013)

MCW said:


> Were they the same as that kit I sent you originally Randy? If so they are the best 372 BB kits I've seen and the best performing as well.



Yep.....same ones. 



MCW said:


> They're not as cute as you think in real life. They are smelly, have sharp claws, and they have these stupid little front legs that serve no real purpose apart from picking their nose.



Little Jons huh?


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## wyk (Jun 25, 2013)

Mastermind said:


> Yep.....same ones.
> 
> 
> 
> Little Jons huh?



How much changes to the timing didja make, or did i miss it?


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## Mastermind (Jun 25, 2013)

It's all together with an XT carb........


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## cowroy (Jun 25, 2013)

opcorn:


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## Rudolf73 (Jun 25, 2013)

Mastermind said:


> It's all together with an XT carb........



An dennn...


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## Mastermind (Jun 25, 2013)

Rudolf73 said:


> An dennn...



We get to run it tomorrow?


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## Rudolf73 (Jun 25, 2013)

Mastermind said:


> We get to run it tomorrow?


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## Mastermind (Jun 25, 2013)

Rudolf73 said:


>




Intake timing ending up a little longer than I would have liked it.........but we'll see if it's a runner. I've got a few more pics.....but my camera is in the shop and I ain't unlocking the door. :msp_wink:


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## Rudolf73 (Jun 25, 2013)

Mastermind said:


> Intake timing ending up a little longer than I would have liked it.........but we'll see if it's a runner. I've got a few more pics.....but my camera is in the shop and I ain't unlocking the door. :msp_wink:



Did you have to do much to the exhaust roof? Some of the 372 BB kits have a funny shaped one.


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## Mastermind (Jun 25, 2013)

Rudolf73 said:


> Did you have to do much to the exhaust roof? Some of the 372 BB kits have a funny shaped one.



I raised it about .100 anyway. :msp_sneaky:


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## 4492011 (Jun 26, 2013)

*Its tomorrow where is the vid*



Mastermind said:


> We get to run it tomorrow?



ok its tomorrow where is the vid?


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## Mastermind (Jun 26, 2013)

4492011 said:


> ok its tomorrow where is the vid?



We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs. 

If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:


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## blsnelling (Jun 26, 2013)

Mastermind said:


> We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs.
> 
> If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:



Ahh man! I hate it when that happens:mad2:


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## Rudolf73 (Jun 26, 2013)

Mastermind said:


> We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs.
> 
> If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:



Hmmm that's no fun, but a little bit of extra tightness can do that to ya. 


But at least you didn't have zero squish issues right...


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## tbone75 (Jun 26, 2013)

opcorn:


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## Mastermind (Jun 26, 2013)

blsnelling said:


> Ahh man! I hate it when that happens:mad2:



Jon pulled it apart.......we cleaned up the piston and the jug......it was pretty minor scuffing. A new ring in it and we'll try again tomorrow. I really want to see how this does with the 272 piston. It's a little looser now. 



Rudolf73 said:


> Hmmm that's no fun, but a little bit of extra tightness can do that to ya.
> 
> 
> But at least you didn't have zero squish issues right...



It's all good...these kits make it cheap enough the play with some stuff I probably wouldn't be able to otherwise.......it's fun at least. 



tbone75 said:


> opcorn:



I hope you brought enough for us all. :cool2:


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## davebull (Jun 27, 2013)

Mastermind said:


> Here's a few pics......I'll be adding to this one.
> 
> Opened up the transfer inlets a bit since the 272XP piston is a windowed design.
> 
> ...




Hey Randy,

Got any info you want to hand over on the popup 272 piston? How much did you take off the top and how far in from the skirt did you come?
Any info would be great, I'm doing they same build with an xt carb and BB kit
Cheers from down under


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## Mastermind (Jun 27, 2013)

davebull said:


> Hey Randy,
> 
> Got any info you want to hand over on the popup 272 piston? How much did you take off the top and how far in from the skirt did you come?
> Any info would be great, I'm doing they same build with an xt carb and BB kit
> Cheers from down under



No popup here.......


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## thomas1 (Jun 27, 2013)

Mastermind said:


> No popup here.......



Have you tried the blue pills?


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## Mastermind (Jun 27, 2013)

thomas1 said:


> Have you tried the blue pills?



Nope......that's "fuel doping".

:bang:


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## davebull (Jun 29, 2013)

Mastermind said:


> No popup here.......



Mmmm.....


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## Mastermind (Jun 29, 2013)

davebull said:


> Mmmm.....



I cut the squish band instead.


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## greg storms (Jun 30, 2013)

*squish band & relationship to compression ratio*



Mastermind said:


> You'll see us talk about cutting the squish band in these engines. That decreases the volume of the combustion chamber......then we turn the base of the jug to set the squish clearance which raises compression. At times lowering the jug also allows us more control over the port heights.



thanks for the explanation. I have a few saws to rebuild & would like to do it right!
Once, a 1978 evinrude 100hp x 4 cyl haqd a piston melt & when I rebuilt it, the replacement piston came with the top ring about 4-6 mm lower than the original....as it turned out, the mfg designed the original with the top ring too high, therefore too close to the combustion chamber/intake& exhaust ports. Learning is fun!


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## Mastermind (Jun 30, 2013)

greg storms said:


> thanks for the explanation. I have a few saws to rebuild & would like to do it right!



You are welcome. I don't mind sharing at all. 

As a matter of fact I ran this saw some yesterday at the East TN GTG.....and will share my thoughts. 

I don't like the way it runs at all. I'll not be using windowed pistons in these engines anymore. I just needed to try it to see how well it works.......not as well as a non-windowed piston as far as I can see. It has too little crankcase pressure in my opinion......Now I need a different top end.


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## greg storms (Jun 30, 2013)

*windowed piston vs non type*



Mastermind said:


> You are welcome. I don't mind sharing at all.
> 
> As a matter of fact I ran this saw some yesterday at the East TN GTG.....and will share my thoughts.
> 
> I don't like the way it runs at all. I'll not be using windowed pistons in these engines anymore. I just needed to try it to see how well it works.......not as well as a non-windowed piston as far as I can see. It has too little crankcase pressure in my opinion......Now I need a different top end.



exactly what's the difference? I went to eKartingNews.com & they discuss strengthening the piston by glass beading the high stress area of piston lightly to increase resistance to melting. Also, they seem to advocate making windows in thinner areas on non windowed pistons. If we can get the saws to last like 2 stroke bike engines, they may be better, eh?


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## Mastermind (Jun 30, 2013)

greg storms said:


> exactly what's the difference? I went to eKartingNews.com & they discuss strengthening the piston by glass beading the high stress area of piston lightly to increase resistance to melting. Also, they seem to advocate making windows in thinner areas on non windowed pistons. If we can get the saws to last like 2 stroke bike engines, they may be better, eh?



Getting them to run like a two-stroke bike is what we long for......they already seem to last just fine. :msp_wink:

In the 372XP the designers used a non-windowed, full skirted piston. I think that since the transfers are fed from the bottom of the jug they used this type piston the help more fully evacuate the crankcase on every stroke. Using a open type piston seems hinder the performance on the engine.....at least it does with it ported the way I port one. There is probably something to be gained using this piston that I'm missing......but I like the way they run with a full skirt better at this time. 

Yesterday there was a Dolmar 7900 with a new slab sided type piston at the GTG and a Solo 681 with a 272 windowed piston. I built both saws for two different guys. The 7900 was holding over 1000 rpm more in then cut. That helped me to decide on what piston design was working better in that saw as well.....


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## davebull (Jul 3, 2013)

Mastermind said:


> I cut the squish band instead.



Ah yes I was being a little narrow minded, cheers 
I noticed on my project the 272 piston doesn't really have enough meat between the ring land and the top to made a pop worth while...
I think I'll do the same


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## MCW (Jul 3, 2013)

davebull said:


> Ah yes I was being a little narrow minded, cheers
> I noticed on my project the 272 piston doesn't really have enough meat between the ring land and the top to made a pop worth while...
> I think I'll do the same



How do you intend to cut the squish band and how much do you need to take out?

Pretty funny after mentioning you'd ordered 5 x 272 pistons in the other thread for pop ups  You'll have a few spares now


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## davebull (Jul 3, 2013)

MCW said:


> How do you intend to cut the squish band and how much do you need to take out?
> 
> Pretty funny after mentioning you'd ordered 5 x 272 pistons in the other thread for pop ups  You'll have a few spares now



I went and seen a guy in Sydney, he's going to cut the squish for me for $100 using cnc, he can also do a popup for me for $30 but I don't think that will work.
I was planning on taking out a little over .045
I'll give him the current squish reading with 372 piston and then just give him the lot and he can work it out,
He pretty good with 2 stokes, he's built race bikes race kart motors and bike saws.


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## davebull (Jul 3, 2013)

MCW said:


> How do you intend to cut the squish band and how much do you need to take out?
> 
> Pretty funny after mentioning you'd ordered 5 x 272 pistons in the other thread for pop ups  You'll have a few spares now



I was on YouTube the other night checking out your vids, you've got a nice 7900!! I think I won't one now after seeing it pull a 42" bar!!!
I'm only guessing it is you on there though?


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## MCW (Jul 3, 2013)

davebull said:


> I was on YouTube the other night checking out your vids, you've got a nice 7900!! I think I won't one now after seeing it pull a 42" bar!!!
> I'm only guessing it is you on there though?



Yeah I'm the main culprit there.
Funnily enough the 7900 pulling that 42" bar buried in Redgum is actually way less powerful than the pop upped one that Brad Snelling has built me since. That one has over 600 hours on it now and is still pulling 210psi compression


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## Mastermind (Jul 3, 2013)

I forgot the pics of the grinding......













I still don't like the way it runs.


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## Jacob J. (Jul 3, 2013)

Mastermind said:


> I still don't like the way it runs.



You should be able to fix that...


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## morgaj1 (Jul 3, 2013)

Mastermind said:


> Very nice ones these are.



What, are you Yoda now?:tongue2:


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## Mastermind (Jul 3, 2013)

Jacob J. said:


> You should be able to fix that...



Yeah........I figure you're probably right. :msp_wink:



morgaj1 said:


> What, are you Yoda now?:tongue2:



No. I'm Randy. R A N D Y opcorn:


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## Jacob J. (Jul 3, 2013)

Mastermind said:


> Yeah........I figure you're probably right. :msp_wink:



I usually am...


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## Mastermind (Jul 3, 2013)

Jacob J. said:


> I usually am...



That's weird for a hillbilly too huh?!?!? :msp_unsure:


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## davebull (Jul 3, 2013)

I still don't like the way it runs. [/QUOTE]


If you would like to part with that cylinder, I've got a nice little home for it on my 372...
I've got $$$ :msp_biggrin:


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## Mastermind (Jul 3, 2013)

davebull said:


> If you would like to part with that cylinder, I've got a nice little home for it on my 372...
> I've got $$$ :msp_biggrin:



200.00 and shipping


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## davebull (Jul 4, 2013)

Mastermind said:


> 200.00 and shipping



Sold randy, I just sent you an email
Cheers Dave


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## davebull (Jul 4, 2013)

MCW said:


> Yeah I'm the main culprit there.
> Funnily enough the 7900 pulling that 42" bar buried in Redgum is actually way less powerful than the pop upped one that Brad Snelling has built me since. That one has over 600 hours on it now and is still pulling 210psi compression



If I ever get a 7900 that's where I'll be sending it, 

Or mastermind..


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## MCW (Jul 4, 2013)

davebull said:


> If I ever get a 7900 that's where I'll be sending it,
> 
> Or mastermind..



Can't go wrong either way


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## wyk (Jul 5, 2013)

Mastermind said:


> No. I'm Randy. R A N D Y opcorn:



MY GF tells me the same thing.

Nice work on those ports, btw. 

Oh, hey, my GF also tells me that as well.


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## ckelp (Jul 5, 2013)

Randy, my 1000's of hours of inexperience made me think of could it be the ignition timing be off for your half baked saw


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## wyk (Aug 26, 2013)

Randy,

How much blow down are you running on that BB?


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## Mastermind (Aug 26, 2013)

reindeer said:


> Randy,
> 
> How much blow down are you running on that BB?



About 20°

I just put a different piston in it.....


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## wyk (Aug 26, 2013)

Mastermind said:


> About 20°
> 
> I just put a different piston in it.....



Thanks for a quick answer! I got one in front of me right now and was running the numbers.


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## Mastermind (Jul 31, 2014)

This is weird........I got a notification......but no one has replied. 

Good thread to bump though. 

This 272 piston in a 372 BB was a waste of time. I didn't have anywhere near the grunt that a full circle piston had. Case compression suffers with the 272 piston, it takes up less volume in the case...

Another case of the original designers knowing more than a redneck with a grinder.


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## nmurph (Jul 31, 2014)

So, what's your take on any of the AM BB 372 kits? Can they be made to run as well or better than OEM?


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## Mastermind (Jul 31, 2014)

Oh yeah.......the one's I've seen have been very nice. But the damn pistons........I worry about them not holding up.


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## Rudolf73 (Jul 31, 2014)

This one time I made a piston out of a bit of hardwood, it worked, sort of...


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## jd548esco (May 23, 2015)

i wonder if anybody has tried a slab piston in a saw that had a windowed piston??

i wonder how would that do??? -- LOL


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## KG441c (May 24, 2015)

Mastermind said:


> Here's a few pics......I'll be adding to this one.
> 
> Opened up the transfer inlets a bit since the 272XP piston is a windowed design.
> 
> ...


Randy will the amount u took from the cylinder and crankcase matching affect crankcase compression much? Increse rpm ?


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## KG441c (May 24, 2015)

KG441c said:


> Randy will the amount u took from the cylinder and crankcase matching affect crankcase compression much? Increse rpm ?


Sorry I saw your comment on the case compression after reading the whole thread


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## Huskybill (Jun 29, 2018)

I seen a 2100 cylinder that was ported by a husky salesman I think he got his info from husky. He raised the exhaust port 1/8” in the center and arch it to the sides by 1/16”. He lowered the intake port by 1/8”. He opened up the transfer ports. He cut 1/8” off the bottom of the piston and I advanced the timing. The 2100 was a beast. We plugged the govenor.

I followed his lead on a 81/250cr husky motocross bike, I did more porting. And advanced the timing. This bike was unbeaten down the straights. She was an animal. With the cylinders that had ribs in the transfer ports I thinned them, lowered them and put a c shape in the middle to keep the flow centralized.


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## MG porting (Jun 30, 2018)

Mastermind said:


> Here's a few pics......I'll be adding to this one.
> 
> Opened up the transfer inlets a bit since the 272XP piston is a windowed design.
> 
> ...


That 272 piston look right at home in that cylinder. Good work Mastermind.


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