Stihl 090 compression, piston?

[Moondogg73]

Could the piston pictured be the culprit for only 150psi out of my 090. Rings looked good but ordered new ones anyway. Should I buy a new piston also? Barely feel anything when I run my nail over it.

BTW: Does anyone know the stock cyl psi for an 090, I know it's greater than 150? I can't find it anywhere online.

Thanks
Ryan

 

[Moondogg73]

Well, I went ahead and ordered a new oem piston. I'll report back if that gives me some more compression. Going to do a gasket delete while I'm in there cause, why not. anyone know how much timing advance is needed to compensate for the .020 additional squish?

 

[Vintage Engine Repairs]

150 psi is a good reading. Piston is galled.

 

[Moondogg73]

150 psi is a good reading. Piston is galled.

It's good I know. But I want better than new. lol. I only wish someone knew what new compression psi was. I'm guessing like 180-185

 

[Vintage Engine Repairs]

I wouldn’t be surprised if It wasn’t far off 150 even new. They are of an old design with a long stroke and large squish clearances.

Edit:

found this: his 090 has 2mm squish / 0.08”

Newer saws will have closer to 0.5mm / .02”

 

[Moondogg73]

I wouldn’t be surprised if It wasn’t far off 150 even new. They are of an old design with a long stroke and large squish clearances.

Edit:

found this: his 090 has 2mm squish / 0.08”

Newer saws will have closer to 0.5mm / .02”View attachment 952762

Thats right about where I am. I'm at .078 now, and gasket measures at .019. Hoping with the new pistons rings and cylinder gasket removal to be at 180. otherwise it going to the lathe for another .035.lol. Than I probably will have trouble pulling er over. ha

 

[Vintage Engine Repairs]

I highly recommend asking for advice from someone who has ported this model before machining. I think you may end up wishing you left it stock in the long run. Best of luck with whatever you choose.

 

[scallywag]

Well, I went ahead and ordered a new oem piston. I'll report back if that gives me some more compression. Going to do a gasket delete while I'm in there cause, why not. anyone know how much timing advance is needed to compensate for the .020 additional squish?

I'd be more inclined to find out what caused the scoring on the original piston before performing any mods.....What was the inside of the cylinder / bore like?

 

[Maintenance supervisor]

I dropped my cylinder down to .027 squish, its difficult to pull over as expected. I also ported it . At that compression the saw holds itself up by the rope which is no small feat for a 30lb hunk of metal.
There are several build/port threads in this forum and others which are spot on, I HIGHLY recommend reading them before making modifications.

 

[thenne1713]

Could the piston pictured be the culprit for only 150psi out of my 090. Rings looked good but ordered new ones anyway. Should I buy a new piston also? Barely feel anything when I run my nail over it.

BTW: Does anyone know the stock cyl psi for an 090, I know it's greater than 150? I can't find it anywhere online.

Thanks
Ryan
View attachment 952527

IMHO, POLISH that out of piston, and if on INTAKE side, replace/improve air filter, trash getting IN; re: PSI, consider 120= MIN per Husqvarna, but new ring should show improvement OVER the 150, if cyl good and rings seat properly; have read synthetic cannot properly seat rings, and need to run petro for first few tanks? (But then that violates their warranty purchase requirement?) but do you also brush sawdust away from fill port before fueling/ oiling?

 

[Moondogg73]

I'd be more inclined to find out what caused the scoring on the original piston before performing any mods.....What was the inside of the cylinder / bore like?

I think it was simply run lean. As when I received it the carb was all out of wack.

 

[Moondogg73]

IMHO, POLISH that out of piston, and if on INTAKE side, replace/improve air filter, trash getting IN; re: PSI, consider 120= MIN per Husqvarna, but new ring should show improvement OVER the 150, if cyl good and rings seat properly; have read synthetic cannot properly seat rings, and need to run petro for first few tanks? (But then that violates their warranty purchase requirement?) but do you also brush sawdust away from fill port before fueling/ oiling?

Ya in engine building you always use a Dyno oil for break in. Makes sense you'd need the same with a 2cycle. Thanks.

 

[Moondogg73]

New piston and rings are in. Need to do a preliminary comp test then let the rings seat and check er' comp again.

 

[Moondogg73]

Started with 150psi. Did new rings and piston first. Then cyl gasket removal (used motoseal) and the results are in. 178psi ish. guess those rings/piston were tired. Then did a base gasket removal, now 195psi. Before seating the rings. Hell ya! I also may have fiddled with the timing a smidge. I can't wait to break this beast in.

 

[Maintenance supervisor]

Sounds like you got her buttuned up right .
After giving mine a shot of compression I broke 1 handle and 3 pull ropes , so its wearing a D handle with #5 True blue pull rope now. Just a heads up if you cord is not in good shape.

 

[Moondogg73]

Sounds like you got her buttuned up right .
After giving mine a shot of compression I broke 1 handle and 3 pull ropes , so its wearing a D handle with #5 True blue pull rope now. Just a heads up if you cord is not in good shape.

Put a new 4.5mm Stihl Elastostart from the 881's hopefully it holds up.

 

[Maintenance supervisor]

Those do hold up quite well.

 

[Moondogg73]

Well she's pretty well broken in now. So I gave er a good test run. She rips. Video of first cut below