Compression, any rule of thumb?

[woojr]

I recently checked compression of a couple old, "good running" saws. It lead me wonder what others here think about when its time to address compression drop off. I can assume answers that can range from when it no longer runs to after the fifth tank of fuel. But I was curious about the saw that seems to run "good", has a nice looking piston and cylinder, but comes in under a hundred.
I've had my eye on these mid sized Huskys but not sure I want to invest in a full rebuild on a saw that isn't worth that much more than parts. Can replacing just the ring be a worthwhile venture? Is there a rule of thumb for rebuilding vs passing for monetary reasons?

 

[fishhuntcutwood]

If all else is good, but it's got low compression. Change the rings. That's what they're for. Anything under 100 is bad. Change the rings, and if that ain't enough or it still doesn't run, then it's up to you as far as what the rebuild is worth to you.

Jeff

 

[hillbillycycles]

Hello, I'm a new guy here and new to working on saws. I can, however, speak from the side of rebuilding two stroke motorcycle engines. If compression is low, you might be able to simply get away with a new set of rings and honing the cylinder. However, you need to check the cylinder for roundness. On motorcycles the cylinder can be in good shape, but be worn out of round where overboring is required to true up the bore. Not sure if it applies to saws as I don't know if they make overbore piston/rings or if you have to purchase a new cylinder, piston, and rings.

 

[TimberPig]

The first thing I'd do is check your tester, not start tearing down the saw. If it runs good, starts easy and then test as having less than 100lbs of compression, it makes your gauge seem suspicious. A saw with compression that low will not normally run well, and start easily. You need a gauge that has a shrader valve at the cylinder end of the hose, and pull til it stops rising with the throttle open.

 

[woojr]

I thought like you did about the validity of the tester so I also checked a newer saw that pulls like bear.. it came up to 145. tried them both again and got the same numbers.
Well anyway I tore the top end off and it really looks good. not a mark on the cylinder walls and lower end is tight. The piston isn't perfect. I don't know how to discern if the ring is worn. Its springy with sharp edges and moves freely in the grove.

I think I'm going with a new piston and ring.

Do you guys use threadlocker on cylinder and muffler screws? What type?

 

[PWB]

Unless the piston is really chewed up, why not try just rings to start with? Not a lot of dollars.............

 

[davefr]

On the last jug I worked on Stihl recommended Loctite #243 (ie medium strength) on muffler to cylinder and muffler to crankcase.

They did not recommend it on cylinder to crankcase or carb to cylinder.

 

[Lakeside53]

Loctite (type varies, not all 243)- only on mufflers of a few saws... On most saws Stihl does not recommend it. If you look in the service manual, just after "specifications" there is usually a table of torque values and whether any locitites or other adhesives are required. Definitely NOT on the cylinder screws. It's important to get the correct temerature/type of locite if it's going to be used.

It's more important to use torque wrench and get the correct torque value than to just apply loctite...

 

[Lakeside53]

If all else is good, but it's got low compression. Change the rings. That's what they're for. Anything under 100 is bad. Change the rings, and if that ain't enough or it still doesn't run, then it's up to you as far as what the rebuild is worth to you.

Jeff

I doubt most saws would start at 100... I find 135 is getting pretty bad, and at 115-125 they are dogs and usually start poorly.

 

[v8titan]

Can anyone recommend a decent compression tester and where to buy it? I would like the ability to check several small engines, my saws included.

Tom

 

[musher]

Compression Tester

Bailey's has one here (catalog item #G 320HD) for $24.95.

 

[v8titan]

Bailey's has one here (catalog item #G 320HD) for $24.95.

Thanks musher. Looks like that one should be adequate for my needs.

Tom

 

[bvaught]

If the saw runs acceptably to you, dont worry about the compression reading. If you have a problem with the saw and thats how you came about finding the low compression reading, then by all means replace the piston and rings. A cylinder swap should be unnecessary, most saws have chrome plated bores that do not wear out or out of round in all but the most severe of cases. Run a hone through the cylinder anyway (it aint gonna do much good) to try and rough up the surface and aid in break-in.

 

[ozflea]

Hello, I'm a new guy here and new to working on saws. I can, however, speak from the side of rebuilding two stroke motorcycle engines. If compression is low, you might be able to simply get away with a new set of rings and honing the cylinder. However, you need to check the cylinder for roundness. On motorcycles the cylinder can be in good shape, but be worn out of round where overboring is required to true up the bore. Not sure if it applies to saws as I don't know if they make overbore piston/rings or if you have to purchase a new cylinder, piston, and rings.

Good point my friend but with chrome plated or nickasil bores it becomes a bit of a problem as these cylinders ain't really suitable to rebore.

I don't think in all the time i have been reading the threads on this forum i have see or read comments that mention checking the cylinder to piston clearance on saws with a feeler gauge something no doubt overlooked when chasing poor performance the engine is a pump is it not ?

Mc Bob.

 

[TimberPig]

If the saw runs acceptably to you, dont worry about the compression reading. If you have a problem with the saw and thats how you came about finding the low compression reading, then by all means replace the piston and rings. A cylinder swap should be unnecessary, most saws have chrome plated bores that do not wear out or out of round in all but the most severe of cases. Run a hone through the cylinder anyway (it aint gonna do much good) to try and rough up the surface and aid in break-in.

Most saws use nikasil, or similar plating. Chrome was used, but has fallen out of use as the other platings types are superior.

The idea of the honing is mostly to break the oil glaze. It doesn't cut very deep into the plating, it merely refreshes the crosshatching so the rings will seat properly.

 

[woojr]

Well all this information is fantastic.. however for my job at hand (272xp) I have new news. After reading the porting and polishing thread I decided to take a better look at the cylinder. After a little carb solvent and some compressed air I found a small chip out of the wall. It wasn't noticeable prior to the cleaning. So how did it run so well? Last week I ran it hard through some large oak (ripping and cutting up blocks 20" bar all the way in for most of 5 tanks of fuel). Only symptom of a problem was eratic idle after a long cut.

So now I need a cylinder. Any suggestions out there? Woojr

 

[bvaught]

Most saws use nikasil, or similar plating. Chrome was used, but has fallen out of use as the other platings types are superior.

The idea of the honing is mostly to break the oil glaze. It doesn't cut very deep into the plating, it merely refreshes the crosshatching so the rings will seat properly.

Yes, when I said "chrome" I meant "Chrome, nikasil, or similar plating".