Stihl 046 at 190psi compression - Too high?

[TonyRumore]

I have rebuilt quite a few Stihl saws and nothing has blown over about 175psi. I just finished up an 046, squish is about .023" and it blew 190 pounds. I am not sure why this one is so high. It's a Mahle cylinder, not sure about the piston, but it's not a pop-up or anything.

Thoughts on this? Safe to run?

Tony

 

[PogoInTheWoods]

Safe to run?

Tony

No!!!:hmm3grin2orange:

BTW, does it?

opcorn:

 

[Mastermind]

I have rebuilt quite a few Stihl saws and nothing has blown over about 175psi. I just finished up an 046, squish is about .023" and it blew 190 pounds. I am not sure why this one is so high. It's a Mahle cylinder, not sure about the piston, but it's not a pop-up or anything.

Thoughts on this? Safe to run?

Tony

Run it....

I build them regularly with well over 200psi. The 046 has a smaller D-shaped combustion chamber that can cause higher than "normal" compression readings. One other thing.......has the saw been started yet? The oil that is used for assembly lube can give a higher reading as well.

 

[TonyRumore]

I haven't run it yet. There is a thin layer of two stoke oil on the cylinder walls. I'll run it a bit and see where it's at.

Tony

 

[o8f150]

the 2 stroke oil you use to build it will cause the comp to be higher,, run it for a few minutes in wood and that will burn all the oil out,, let it cool and see where its at,, i would be willing to bet the comp will go down,,, i have a little 37cc chinese saw that i ported and it blew 200 after i got it done and run it,, and it feels like 200 too

 

[Trx250r180]

dont believe a thing that mastermind says ,i sent a couple saws to him ,now the pullstart cord about rips out of your hand on startup :msp_w00t:

 

[Jimmy Hedge]

That much compression is actually good. I wager it'll be back to your 175 PSI after break-in. If not, still good.

 

[BloodOnTheIce]

It's not broken in run and then check after 5, and 10 tanks of fuel.

If it's still reading that high toss another or thicker base gasket in it.

A fella who has forgotten more than I'll ever know once told me
"Even ported work saws I don't like running over 180#'s compression, that much compression
is a good way to destroy crank bearings"

 

[Roll Tide]

dont believe a thing that mastermind says ,i sent a couple saws to him ,now the pullstart cord about rips out of your hand on startup :msp_w00t:

I know what your saying. He's built a couple of saws for me . My 261 blew 223 psi after 6 tanks.I told him I wanted a screamer

 

[Anthony_Va.]

That much compression is actually good. I wager it'll be back to your 175 PSI after break-in. If not, still good.

Most of them actually build a little more compression after break-in. It may drop after burning the oil out of the cylinder but after that it'll only gain if it does anything. Maybe another 5-10psi, maybe nothing, but it should'nt loose anything.

 

[albert]

Checking comp. before runnning with even a lite oiling on the p/c will give a false high reading. There is no point doing it.

 

[blsnelling]

I know what your saying. He's built a couple of saws for me . My 261 blew 223 psi after 6 tanks.I told him I wanted a screamer

Too much compression can fight you too. IMHO, you don't need that kind of compression, and stress, to make a super strong saw. A 261 will make close to 190 PSI compression with no machine work.

 

[FATGUY]

Too much compression can fight you too. IMHO, you don't need that kind of compression, and stress, to make a super strong saw. A 261 will make close to 190 PSI compression with no machine work.

Jury's still out on that... I do agree that the more comp, the harder on the bearings it is, but if the load on the bearings is still within the bearing's capacity, then longevity is no issue.

 

[blsnelling]

A 261 can run nearly like a 70cc saw with no machine work. At some point, more compression will make less power.

 

[Mastermind]

A 261 can run nearly like a 70cc saw with no machine work. At some point, more compression will make less power.

A some point I'm sure you are right.........:msp_rolleyes:

 

[lambs]

When we say "no machine work", what does that encompass? I'm thinking muf mod, thinner gasket, retune. Is there more than that?

Thanks.

 

[Mastermind]

When we say "no machine work", what does that encompass? I'm thinking muf mod, thinner gasket, retune. Is there more than that?

Thanks.

On the 261 the stratos are done away with, and the timing numbers are optimized....

 

[Roll Tide]

A some point I'm sure you are right.........:msp_rolleyes:

Not at the point mine is at.:msp_rolleyes:

 

[Mastermind]

Not at the point mine is at.:msp_rolleyes:

At 225psi or so you saw is under the point that pumping losses cause any issues......at least that's been my experience. Heat is the thing that you have to watch on an engine with that much compression. Keep the fan clear and the fins clean.....watch out for it on real hot days and you shouldn't have any issues.

I've got a bunch of saws out there making a living that are blowing north of 200psi......I wouldn't recommend these saws to folks that don't know Jack about saws though......

 

[Mastermind]

Oh.......one other thing.....on a small bore engine I'll go higher than a larger engine......