What causes low compression

[Matt_M]

Hi fellas

Replaced the scored piston and cylinder in one of my project saws, a husky 351, and the compression is still low. The saw runs, but was just expecting a bit more.
When I get home I will compression test it and post the results.

So, what things would cause low compression after replacing piston, rings and cylinder.

 

[moody]

Lack of lubrication on the piston during installation

 

[pioneerguy600]

The amount of squish limits compression the most, new rings need to break in to reach top compression, this may take up to 10 tanks of fuel run through the saw to get them seated completely.

 

[moody]

The amount of squish limits compression the most, new rings need to break in to reach top compression, this may take up to 10 tanks of fuel run through the saw to get them seated completely.

Little off subject but these newer strato saws seem to take a little longer to break in. Do dual rings take any longer time to break in ? You've got more experience than I do so I thought I'd ask while in the appropriate thread.

 

[cutforfun]

why do you think compression is low?

 

[sarge3604]

what is the compression it is hard to tell unless we have what the compression is

 

[pioneerguy600]

Little off subject but these newer strato saws seem to take a little longer to break in. Do dual rings take any longer time to break in ? You've got more experience than I do so I thought I'd ask while in the appropriate thread.

The strato`s are taking longer to break in, there are more than one line of thought on this subject. The use of synthetic oils is causing some of it as it takes much longer using synthetic oil over the older petroleum based oils. Some think the extra gulp of air is causing the piston to run cooler, thus the ring takes longer to heat and rub in . Another possibility is that there are different alloys used in the rings or the cylinder coatings, all hypothetical at this point however. The same goes for the tolerances these new saws are machined to, the engineers are always trying new materials and machining techniques.
A two ring piston creates more friction and thus heat so I would expect them to break in sooner than a single ring. The verdict is still out as far as I am concerned on what lasts longer, a single ring or two ring piston and ring combo.

 

[zogger]

Could be, if the original problem that caused the p/c roaching hasnt been addressed, the brand new p/c are now getting roached. It might still run, but may have serious damage already.

Just a thought, pull the muffler, take a look.

 

[Matt_M]

I guessed they had run straight fuel through it which scorched it. More then likely wrong though. I'm only getting into my chainsaw repair addiction. Could also be tuned incorrectly?

I will give you the reading when I get home. The original cylinder had zero compression.

 

[gmax]

A vac test is mandatory if I replace a P/C, guessing what caused the failure is not a good option.

 

[Andyshine77]

I agree, pressure and vac test is a must. Not every saw is designed to have lots of compression, the 351's normally have about 140 psi of compression.

 

[Chris-PA]

The strato`s are taking longer to break in, there are more than one line of thought on this subject. The use of synthetic oils is causing some of it as it takes much longer using synthetic oil over the older petroleum based oils. Some think the extra gulp of air is causing the piston to run cooler, thus the ring takes longer to heat and rub in . Another possibility is that there are different alloys used in the rings or the cylinder coatings, all hypothetical at this point however. The same goes for the tolerances these new saws are machined to, the engineers are always trying new materials and machining techniques.
A two ring piston creates more friction and thus heat so I would expect them to break in sooner than a single ring. The verdict is still out as far as I am concerned on what lasts longer, a single ring or two ring piston and ring combo.

Has this been true of the Husky stratos too, or just the Stihls?

 

[sunfish]

351 is a very good saw! Hope you get it running right.

 

[7sleeper]

The strato`s are taking longer to break in, there are more than one line of thought on this subject. The use of synthetic oils is causing some of it as it takes much longer using synthetic oil over the older petroleum based oils. Some think the extra gulp of air is causing the piston to run cooler, thus the ring takes longer to heat and rub in . Another possibility is that there are different alloys used in the rings or the cylinder coatings, all hypothetical at this point however. The same goes for the tolerances these new saws are machined to, the engineers are always trying new materials and machining techniques.
A two ring piston creates more friction and thus heat so I would expect them to break in sooner than a single ring. The verdict is still out as far as I am concerned on what lasts longer, a single ring or two ring piston and ring combo.

As far as I have understood 2 rings are cooler because they facilitate heat transfer to the jug.

7

 

[pioneerguy600]

As far as I have understood 2 rings are cooler because they facilitate heat transfer to the jug.

7

Two rings may transfer more heat to the cylinder wall but they also create more friction than one ring. This is very evident when we run one ring on a piston designed for two rings a saw, it will pick up 500+RPM if everything else is left set just the way it was before removing one ring. If there is less friction then one would surmise that there would be less heat being created.

 

[Chris-PA]

Two rings may transfer more heat to the cylinder wall but they also create more friction than one ring. This is very evident when we run one ring on a piston designed for two rings a saw, it will pick up 500+RPM if everything else is left set just the way it was before removing one ring. If there is less friction then one would surmise that there would be less heat being created.

But there might be more friction <i>on that ring</i>. If there is, say, 30% less friction with one ring, there are 50% fewer rings. So from a wear point of view you would expect the ring to wear faster and the cylinder wall to wear slower.

It depends on what the reduction really is.

 

[zogger]

But there might be more friction <i>on that ring</i>. If there is, say, 30% less friction with one ring, there are 50% fewer rings. So from a wear point of view you would expect the ring to wear faster and the cylinder wall to wear slower.

It depends on what the reduction really is.

I kinda always thought that two rings was a tradeoff. They would not run as high RPM, and more friction, but would help to keep the piston centered.

 

[Matt_M]

Ok, I did a compression test and got a reading of 120psi or 800kpa. This is the low end of the dial. What sort of reading should I expect from a cold saw with a new P/C?

I will do a leak down test, but need to wait till pay day to buy one. Also will invest in a RPM tacho for learning how to tune!


Sent from my GT-P5110 using Tapatalk 2

 

[Zippy]

Don't know about the 351 but I just put an a/market piston in an MS200T cylinder that I cleaned up and got 145psi straight away.

I did wipe a film of oil over the piston/rings prior to installation.

 

[SkippyKtm]

Ok, I did a compression test and got a reading of 120psi or 800kpa. This is the low end of the dial. What sort of reading should I expect from a cold saw with a new P/C?

I will do a leak down test, but need to wait till pay day to buy one. Also will invest in a RPM tacho for learning how to tune!


Sent from my GT-P5110 using Tapatalk 2

Be sure your compression gauge is accurate, whenever I suspect my gauge is out of whack, I have a "control saw" (or calibration saw if you will) that I check it with. Its also important your gauge has a valve in its tip, or it will read too low.
Is this an aftermarket kit or OEM? Some aftermarket pistons have an incorrect deck height, so its always a good idea to check the squish before reassembly.