parts saw Dolmar PS-7900 re-build

[buzz sawyer]

Broke at the throw.

 

[meglodon]

While I'm waiting for the short block to show up I decided to remove the baffle from muffler and add a deflector I got from Dyno Joe's site (https://worksaws.com/collections/all)

I think it'll work pretty decent. I have some nut serts that

I used a mapp gas torch and levered up the crimp, removed the baffle and started to mock things up.

I also decided to fix up my old HF 3.5 gal parts washer (the pump went in it many years ago and it's just been on a shelf) I have a new pump on order and plan to do some upgrades to help filter better and make it easier to drain/store the solvent.

 

[old guy]

Had the piston ready to go and was excited to start assembly, and then a stag as I was doing some more cleaning to put things back together.

I found that the crankshaft has a crack in it.

View attachment 1051792

Which sucks... but I guess that it’s better to find it now and not after the damage would be done.

Since I'm this deep in may as well move forward. I found a short block on ebay with the following description:

OEM Crankcase assembly with piston. Bearings are good, comes with oiler pump and gear, Piston is good.

Once that arrives this project can move forward.

I was about to jump on that shortblock, I snoozed, I loozed

 

[meglodon]

I was about to jump on that shortblock, I snoozed, I loozed

Ah. gotta move quick (although, there are a couple more the bay, same seller)

 

[meglodon]

Case arrived (packaged well, no damage)
- has a slab sided 50mm piston
- was missing case bolts & gasket was damaged
So the investigation began. I was hoping I could pull things apart, replace the gasket and be good to go, but upon removing the one side I the bearings cages were gone. But, the crank and oil pump appears to be in good shape.




So, I pulled the other case apart to assess how it looks

And, as you can see, the bearings are in much better shape (although the clutch side came out on the crank but the case it's self seems undamaged)

I was intially debating if I should try and reuse the bearings. But since I've taken things down this far I'm going all in I guess.

I have some loctite 680 on order and new bearings & seals ready to go in.

 

[old guy]

Well crap, I hope the next one in line is better, cuz it's on it's way here.

 

[meglodon]

Well crap, I hope the next one in line is better, cuz it's on it's way here.

Fingers crossed for you. In my case I was planning to split the case so it wasn't a huge deal. After it arrived I was looking at the description of the two auctions and I suspect (I could be wrong) that they mixed them up. But, the parts I was looking for appear to be in ok shape so charging ahead.

 

[old guy]

All I need is a good crank, but was hopeing to just transfer my stuff to the new lower.

 

[meglodon]

All I need is a good crank, but was hopeing to just transfer my stuff to the new lower.

that was my hope as well... ah well, this will probably end up better.

I didn't take any pictures, but I got my parts washer together tonight and tried it out on a couple things. I think it's going to make cleaning things much quicker, easier and less messy.

 

[meglodon]

Playing around with the parts washer and cleaned up the two case halves tonight. The first one I tried was the PTO side, which turned out ok. But I’m going to do a second round and I need to pop out the seal to prep for the new parts bearing and seals. But I added a bit more cleaning solution, heated up the flywheel side bearing and popped it out. Then ran it through the parts washer.

Should have fixed this parts washer a while ago, but now that I have it working again and I’m happy about that. I'll be doing some additional upgrades to help filter the cleaning solution and make it work a bit better. (Filter system, some method of being able to heat the solution up, etc)

But with the case halves cleaned I’m closer to putting things back together!

I’ve also been reading through a few threads on these saws (covering things like porting, stroker cranks, & carb mods)
But the realistic side of me knows I should start with just getting the saw running. But I'm tempted to just do some gasket matching and lower transfer work since I'm already here. The optimistic side of me wants to go all in and start doing all the work... I guess we'll see how ambitious I get trying to balance the two approaches.


Mostly Clean

 

[old guy]

Got my case today, all looks well, no movement in the bearings. Has a clutch and all anti-vibe stuff, even has wires for hot handle, not that I can use that.
Will move stuff over, my cyl is making 168 comp on the old saw, we'll see what a new crank does for it, if possible I will even dump the base gasket.

 

[ballisticdoughnut]

I realize this is a bad time to mention this but, those black oil hoses were prone to leaking. The updated ones are made of a different material and are green. Also I’d suggest leaving the cylinder gasket in place, it’ll be over 170 psi compression after it’s broken in.

 

[old guy]

This piston & cyl. is well broken in, it's just going on a different bottom end.
If There is room the base gasket goes away. Thanks for the tip on the oil line.

(edit) Just checked, got the green line!

 

[meglodon]

Got my case today, all looks well, no movement in the bearings. Has a clutch and all anti-vibe stuff, even has wires for hot handle, not that I can use that.
Will move stuff over, my cyl is making 168 comp on the old saw, we'll see what a new crank does for it, if possible I will even dump the base gasket.

Thats great! (I should have snagged that one ... ah well. Still congrats!)

I realize this is a bad time to mention this but, those black oil hoses were prone to leaking. The updated ones are made of a different material and are green. Also I’d suggest leaving the cylinder gasket in place, it’ll be over 170 psi compression after it’s broken in.

Thanks for the tip. I had read that the black ones are prone to leaking. Luckily the second case I purchased came with green lines, I'll be running those. I ended up using the base gasket I have, but since it's used I added with some 1184 as the squish was quite tight. (.018ish) I just got things together last night and haven't measured what it is now.

 

[meglodon]

The journey of learning that things are probably just cheaper to buy them new continues. But for me, it's always been about the journey, seeing how things work and satisfaction of getting them running… not necessarily the destination. I put the part washer to work and it's doing an excellent job compared to how I was doing things before.

Got the cases together, I ended up going with the clutch side from the second set of cases as the original pto side had more movement than I was comfortable with. In terms of how things went together… Well, I did it. But the process involved me having to take it apart at least once because a bearing didn’t seat correctly when I heated cases up. I’m still learning and let’s call this portion of the journey ‘wisdom gained by doing it twice (or in some cases more.)’



I’m still not 100% on it, but seems ok now, but I suppose we'll see how this goes I might be tearing things back apart.

As mentioned above, I checked squish and decided to use the base gasket in the build. (haven’t checked squish now but without the gasket the squish would have been tight.



The air filter I bought is obviously not OEM and has a gap... 1184'd that and will look around for a new one. This will work to see if the saw starts.

 

[meglodon]

Got the muffler back together (just opened up the existing hole a bit to begin with) I elected to get the saw running then when I go back in to port things I’ll take on the second muffler opening.


And… the chain brake is back in (which … was a learning experience but I might be able to do better in the future.

And the oil driver… welp as you can see, it’s in the same state as the other, one I have. The cheap 372 one would work across the clutch drum, but the teeth are too small to mesh with the oil pump gear. (And the depth is not the same)


I'll hunt for a replacement and ponder that problem. Business opportuninty if tons of people are trying to find them. If I can't find one, I think that the 372 one almost would work, i may be able to modify it. (press the bearing in, see if I could switch the oil pump drive gear, etc)

I suppose I could to 3d print one but I highly doubt my 3d printed version would stand up to the abuse, OEM are probably glass filled nylon or some other high impact resistant plastic.

But, i've pulled it over and it's got spark! (and compression) but I need to source some bolts to get the carb attached. Most likely will need to order some, as I'm skeptical I'll be able to find any M4x55mm locally.

I have a few other parts inbound and then the saw will have earned a placed on the "I have put to much into it to sell at the price it would require to make my money back" shelf...

 

[ballisticdoughnut]

Starting to look like a chainsaw again. I really like the 7900 (I have three) they run great with minimal work. Just a muffler mod wakes them up pretty decent.

 

[Lightning Performance]

Got the muffler back together (just opened up the existing hole a bit to begin with) I elected to get the saw running then when I go back in to port things I’ll take on the second muffler opening.
View attachment 1055062View attachment 1055063

And… the chain brake is back in (which … was a learning experience but I might be able to do better in the future.
View attachment 1055064
And the oil driver… welp as you can see, it’s in the same state as the other, one I have. The cheap 372 one would work across the clutch drum, but the teeth are too small to mesh with the oil pump gear. (And the depth is not the same)

View attachment 1055065View attachment 1055066View attachment 1055067
I'll hunt for a replacement and ponder that problem. Business opportuninty if tons of people are trying to find them. If I can't find one, I think that the 372 one almost would work, i may be able to modify it. (press the bearing in, see if I could switch the oil pump drive gear, etc)

I suppose I could to 3d print one but I highly doubt my 3d printed version would stand up to the abuse, OEM are probably glass filled nylon or some other high impact resistant plastic.

But, i've pulled it over and it's got spark! (and compression) but I need to source some bolts to get the carb attached. Most likely will need to order some, as I'm skeptical I'll be able to find any M4x55mm locally.

I have a few other parts inbound and then the saw will have earned a placed on the "I have put to much into it to sell at the price it would require to make my money back" shelf...

Stop pissing around and use the old center to bush the new part with a bigger hole.
Does the pump bolt in from a Husky?
K.i.s.s.

 

[meglodon]

Stop pissing around and use the old center to bush the new part with a bigger hole.
Does the pump bolt in from a Husky?
K.i.s.s.

More to it unfortunately. (As there's a height and thread difference) but I've been thinking along similar lines and if it was just height, I'd be doing exactly that!




Unfortunately, I don't have a 372 pump to test so I can't say, but based on the pictures on ebay their a bit different on where the inlet side. sits and are press vrs barbed fitting probably easy enough to add a fitting to it though. https://www.ebay.com/itm/163615950694

Maybe because they both originate from Germany a Stihl drive gear might be a closer fit? Looking at this https://www.ebay.com/itm/294014626917 From the measurements I'm doing the dimensions on the pump gear compared to the Dolmar are a pretty close match. Note: The rest of that pump wouldn’t work based on where the inlet & bolts would line up compared to oiler adjustment.

But the gear may, so I might pick this https://www.ebay.com/itm/374036669364 up to see if the bushing fits and teeth are correct. If they do, it could be a good platform to start from.

 

[Lightning Performance]

More to it unfortunately. (As there's a height and thread difference) but I've been thinking along similar lines and if it was just height, I'd be doing exactly that!
View attachment 1055070

View attachment 1055073

Unfortunately, I don't have a 372 pump to test so I can't say, but based on the pictures on ebay their a bit different on where the inlet side. sits and are press vrs barbed fitting probably easy enough to add a fitting to it though. https://www.ebay.com/itm/163615950694

Maybe because they both originate from Germany a Stihl drive gear might be a closer fit? Looking at this https://www.ebay.com/itm/294014626917 From the measurements I'm doing the dimensions on the pump gear compared to the Dolmar are a pretty close match. Note: The rest of that pump wouldn’t work based on where the inlet & bolts would line up compared to oiler adjustment.

But the gear may, so I might pick this https://www.ebay.com/itm/374036669364 up to see if the bushing fits and teeth are correct. If they do, it could be a good platform to start from.

I can check Stihl parts on my Makita saw. Not sure if Dolmar and Makita interchange. I do have a 2186 here now. If they can be swapped using gears and pumps as a set seems logical.

The parts you referenced are all junk so don't waste your time on the AM worm gears trust me I've been there already.