high starting compression sheared flywheel key MS250

[r7000]

so I believe this to be an unmodified saw, it is an Stihl MS250 and the cylinder and crank pan have Stihl markings on them. There was a red colored sealant between cylinder and crank pan which i assume is the OEM Dirho HT stuff.

In fixing a disintegrated clutch drum bearing, I noticed the motor has very high compression pulling the starter cord and also had a sheared flywheel key. Not wanting to just put another flywheel on so it can shear the aluminum key again...

I have the whole thing apart and piston and crank assembly is removed from cylinder, inside of cylinder looks pristine no scoring. Only thing I can guess at is what carbon there was on top of piston and in the top end of cylinder maybe... maybe... caused the high starting compression? Otherwise is it possible that there was a run of MS250's that were factory built with higher than normal compression, is a high starting compression not uncommon with MS250's ? I don't believe there was excessive carbon.

Looking for recommendations, was contemplating a new cylinder and piston kit but after having the thing apart to see first hand I think I want to reuse existing cylinder and maybe just put new rings for $10 on the piston. I have cleaned the piston top and sanded it down with some 600 and 1000 grit so it is shiny now, need to find a scrubber/sander type brush to reach top end of cylinder? Also contemplating trying to sand out a tiny bit of cylinder material where spark plug comes through to increase volume to alleviate the high compression problem, I know it's a 42.5mm bore, piston diam above top ring comes out at 42.35mm, and stroke I think is 32mm, so I'm not sure how critical any minuscule change in volume might affect compression if I should just clean the piston top and call it good or go further cleaning/sanding cylinder head.

 

[T.Roller]

I'm guessing it's not high compression, it's the sheared flywheel key has the timing all messed up. I've never seen a 250 with anything close to high compression. Put another flywheel on it and pull it over.

 

[T.Roller]

Throw the cylinder back on it without the flywheel, I bet it pulls over like it should.

 

[r7000]

is a sheared flywheel key, the kind that the key is part of the flywheel which is aluminum, is that common?

 

[Deets066]

Did the flywheel come off easy? Or did you still need a puller.

My guess is flywheel wasn’t seated properly.
New flywheel and you should be fine

 

[T.Roller]

Did the flywheel come off easy? Or did you still need a puller.

My guess is flywheel wasn’t seated properly.
New flywheel and you should be fine

+1

 

[T.Roller]

is a sheared flywheel key, the kind that the key is part of the flywheel which is aluminum, is that common?

I wouldn't call it common but it happens.

 

[NCFarmboy]

I've had 3 recently 2 on same 371 and 1 346. Seals were replaced on both. I blame the mechanic "ME" not torqueing flywheel nuts.
No big deal as I want to advance timing on both.
Shep

 

[r7000]

I only had to tap on crank with a hammer not much and flywheel unseated, then noticed sheared key on it when it would not seat and lock back on the crank.

 

[Deets066]

Doubt compression is your problem. Stock MS 250 is probly half this.

 

[T.Roller]

No compression is definitely not the issue here. At best it's blowing 150

 

[r7000]

i got the MS250 together last night, only new parts were new crank seals and new caber piston rings, and new flywheel. There was nothing wrong with the old piston rings.
And I have cleaned and polished the oem piston top.
with ignition off the effort to pull the starter cord feels same as before, takes a bit of effort really have to start hard pulling and be committed to pulling.
I got up to 170 psi on my compression gauge, this is before even starting engine (still no fuel in tank)

i am thinking the starting problem of being strong and committed to pulling the cord lies in the design of the starter pull cord mechanism and diameter of that part of the flywheel.

after having pulled the ms250 completely apart, my opinion it's a junk model and should be sold in depot and walmart alongside other saws of that size, the fit of parts on the MS250 is terrible.

 

[T.Roller]

i got the MS250 together last night, only new parts were new crank seals and new caber piston rings, and new flywheel. There was nothing wrong with the old piston rings.
And I have cleaned and polished the oem piston top.
with ignition off the effort to pull the starter cord feels same as before, takes a bit of effort really have to start hard pulling and be committed to pulling.
I got up to 170 psi on my compression gauge, this is before even starting engine (still no fuel in tank)

i am thinking the starting problem of being strong and committed to pulling the cord lies in the design of the starter pull cord mechanism and diameter of that part of the flywheel.

after having pulled the ms250 completely apart, my opinion it's a junk model and should be sold in depot and walmart alongside other saws of that size, the fit of parts on the MS250 is terrible.

I've ran and worked on many. Never had an issue starting one. 170 compression is higher than I would expect but should still be easy to pull over.

 

[Stump Shot]

I'll agree and say compression isn't your problem. Some models are just prone to this failure being bone stock. Seeing as Stihl has a memo out on "degreasing" the flywheel and crank taper before installing, I d hazard to say that they know of the problem.

 

[r7000]

can you elaborate on what is meant by "degreasing" the flywheel and crank taper?
and point me to that stihl memo?
thanks.

 

[T.Roller]

can you elaborate on what is meant by "degreasing" the flywheel and crank taper?
and point me to that stihl memo?
thanks.

Degrease the crank and flywheel ....

 

[Deets066]

i got the MS250 together last night, only new parts were new crank seals and new caber piston rings, and new flywheel. There was nothing wrong with the old piston rings.
And I have cleaned and polished the oem piston top.
with ignition off the effort to pull the starter cord feels same as before, takes a bit of effort really have to start hard pulling and be committed to pulling.
I got up to 170 psi on my compression gauge, this is before even starting engine (still no fuel in tank)

i am thinking the starting problem of being strong and committed to pulling the cord lies in the design of the starter pull cord mechanism and diameter of that part of the flywheel.

after having pulled the ms250 completely apart, my opinion it's a junk model and should be sold in depot and walmart alongside other saws of that size, the fit of parts on the MS250 is terrible.

If you assembled the topend with a decent amount of oil it can be giving you a higher reading on the gauge than normal. Fire it up and get it warm, then take a reading. I’d say that the concensous is the recoil assembly

 

[Chainsaw Jim]

Take the recoil spring assembly completely apart and clean all the crud out of it.

 

[r7000]

the saw is completely clean. there is no grease on the flywheel or crank.
Used the saw for some light duty tree pruning, ran great.
After that did a compression test when cold, got just under 180 psi.
Then started saw and rev'd to warm up, did another compression test got around 165 psi.
this is with new rings and less than 1/4 tank of gas used so far, using hp ultra oil mixed at around 30:1.

 

[fossil]

the saw is completely clean. there is no grease on the flywheel or crank.
Used the saw for some light duty tree pruning, ran great.
After that did a compression test when cold, got just under 180 psi.
Then started saw and rev'd to warm up, did another compression test got around 165 psi.
this is with new rings and less than 1/4 tank of gas used so far, using hp ultra oil mixed at around 30:1.

Any grease or oil on the flywheel to crankshaft taper will cause the flywheel to slip and could shear the flywheel key.

It shouldn't slip at all even when the key is missing. I use brake cleaner on both surfaces and wipe them until the cloth comes out clean.

There may have been some type of anti corrosion lube on the flywheel surfaces that could cause that.