Husqvarna 372xp normal compression and squish?

[Derrick Sawyer]

HI Ya'll,
Could use a little help here, rebuilding a 372xp old school version, i have a used piston that looks fine, piston rings have same width as new ones. Cylinder is smooth as well. With gasket, the squish is more than my soft solder of 0.050, so when i take out the gasket and use motoseal, the squish is still only 0.038. Don't know why its still so high, i tried a different used cylinder i have too and squish was similar.

Compression (without running to break in saw) was only 125psi-130psi without base gasket. I think thats too low and while it might go up after breaking in, by how much? Would it ever reach 145-150psi?

I'm thinking it needs either base cut, but more likely also needs piston, the skirt width can also affect the compression quite a lot, not just based on rings width, right?

Vac/pressure is fine, don't want to put a lot of money into it since i just plan to sell and already have $300 into it.

Thanks.

 

[RIDE-RED 350r]

I have noticed that some saws have inherently tighter squish than others. I have deleted base gaskets on two different 394's recently and after the delete, squish was .032" +/- on both. When I did the same to my 357xp squish came in at .023". My brother deleted the gasket on his NE346 and it came in at .019" if memory serves. I say this just to show that different models have different squish clearance, so I wouldn't worry that something is wrong with yours coming in where it did. It is most likely typical for that model of saw.

Now on your compression numbers, that does seem quite low, especially with the base gasket delete in play. I have a 372XPW that I deleted the base gasket from. And before I raised the exhaust port it would pull 180-190psi if memory serves. I have since ported that cylinder, raising the exhaust and it has been awhile since I did the basic gasket delete in that stage. So I could be off a few PSI on the numbers going from memory. I have been into that saw about 6 more times since as I made changes to the porting. She's pulling 175psi now.

So at this point, given the piston you are using looks fine, I would have to wonder if the cylinder you are using is worn causing the loss of compression. The cylinder may well be "undamaged", but if it has alot of run time on it, the bore could be worn out of spec for taper, out of round, and overall bore. These cannot be checked without knowing the service limits and the use of snap gauges and a micrometer. A cylinder bore that is worn will do a couple of things that will negatively effect compression: 1) ring gaps can be too wide, which gives compression wider gaps to escape and also lets the rings relax a tad more reducing the snug fit they should have to the cylinder wall. 2) A well worn yet undamaged cylinder wall will get "glazed" thus reducing the ability of the rings to seal properly.

Also it should go without mentioning, but I will say it anyway, make sure you don't have a leaking decomp. As a matter of fact, I would double check that first before suspecting a problem with the cylinder, even if that means spending a few dollars on a plug and replacing the decomp with it just for testing purposes.

 

[sarge14]

Last year I decided to check the compression of my 372 XP (2000 model) and I about fell over when it read 135psi! It was slow to reach the ground on a drop test and ran great. I decided to pull the jug and check it out. To my surprise everything looked great. I took it to a friends shop (who is a Husqvarna dealer and tech) and he called Husqvarna tech support and was told that 130 - 135psi was the normal compression for that saw. He said that brand new out of the box, they might have 140psi.

Just thought I'd throw my two cents in.

 

[SawTroll]

Last year I decided to check the compression of my 372 XP (2000 model) and I about fell over when it read 135psi! It was slow to reach the ground on a drop test and ran great. I decided to pull the jug and check it out. To my surprise everything looked great. I took it to a friends shop (who is a Husqvarna dealer and tech) and he called Husqvarna tech support and was told that 130 - 135psi was the normal compression for that saw. He said that brand new out of the box, they might have 140psi.

Just thought I'd throw my two cents in.

As I recall it (from reading over the years), a bit less than 150 the norm on a new (as in not run in yet) 372xp (original edition), but less than 140 sounds low anyway? Not all saw engines need a lot of compression to make good power, and at some point (no doubt variable) it becomes counter-productive (then we are talking about much higher numbers though).

 

[Derrick Sawyer]

Thanks guys, compression running around 140 and cuts fine now.

 

[spudulike]

The factory squish is 1mm, leaving the base gasket out it becomes 0.5mm. Standard compression is circa 150psi which rises to 170 - 180 when the base gasket is dropped.

I have ported a fair few of these and that is what I have found. Make sure the carbon is cleared from the squish band and piston crown when comparing squish.