identifying 2-stroke [oil] ring & cylinder wear, teardown pics

[r7000]

on a km90r trimmer, run with stihl gray bottle 2-stroke oil for its life of 5-10 years. Basic home lawn use. At worst maybe the oil mix was done incorrectly, unknowingly, over the years. Trimmer acted up this year, would not get full throttle and when feathering throttle could get it to near full rpm but would not sustain that rpm and have power. Did everything on it including crank seals and resealing the case. Would start right up and only idle nice.

Decided to keep it and got a new cylinder with piston/ring. Shop even took a look at it, after everything had already been replaced, said cylinder/ring was shot. So, ok. My question is, since I have everything apart and have a brand new unused cylinder + piston & ring in hand how can one determine wear? In the cylinder given its small and deep you're looking at it from a 10-20° angle, the new one inside looks nearly the same I don't see any crosshatch like on a 350 sbc. I do see some wear lines in the old cylinder but nothing that stands out. I guess the new piston ring looks better than the old one. The old piston was clean and shiny just like the new piston.

I'll post pics tonight.

 

[Mastermind]

The piston wear is determined by whether or not the machining marks are still visible on the skirts, or by measuring the size compared to a new piston. The cylinder is plated with a very hard coating that rarely shows signs of wear. A new ring on the piston, a light scuffing of the bore with 180 grit sandpaper by hand, a good rinse.....and back together. Your problem was likely never compression related. A new carb....or proper carb rebuild would probably have been all it needed.

 

[Wolverine]

Take the old ring off the old piston, put the ring in the cylinder and push it down in the bore beyond the exhaust port with the piston (which makes it straight). Measure the gap like this:


Obviously this isn't a pic of a saw but it shows what to do.
If that gap is too large, indicates your ring is worn. If those machine marks Randy mentioned are still on the piston, maybe just a new ring and back in business.

Also agree with him on the possible carb issue.

 

[SOHC]

 

[Mastermind]

May of 1992.

 

[Al Smith]

On a high hours engine often times the ring or rings will show a slight ovality caused by a slight tilt of the piston ..As such the cylinder is no longer perfectly round .However a new set of rings will "wear in " after some run time .A plated cylinder cannot be honed true you just have to deal with it .Although highly debated the oil mix ratio in spite of what many think has a lot to do with ring/cylinder wear .
. .

 

[slackinoff]

I've come across a lot of saws with very worn out rings and piston. Never really come across a truly "worn out" cylinder, instead they are damaged.

 

[Junk Meister]

on a km90r trimmer, run with stihl gray bottle 2-stroke oil for its life of 5-10 years. Basic home lawn use. At worst maybe the oil mix was done incorrectly, unknowingly, over the years. Trimmer acted up this year, would not get full throttle and when feathering throttle could get it to near full rpm but would not sustain that rpm and have power. Did everything on it including crank seals and resealing the case. Would start right up and only idle nice.

Decided to keep it and got a new cylinder with piston/ring. Shop even took a look at it, after everything had already been replaced, said cylinder/ring was shot. So, ok. My question is, since I have everything apart and have a brand new unused cylinder + piston & ring in hand how can one determine wear? In the cylinder given its small and deep you're looking at it from a 10-20° angle, the new one inside looks nearly the same I don't see any crosshatch like on a 350 sbc. I do see some wear lines in the old cylinder but nothing that stands out. I guess the new piston ring looks better than the old one. The old piston was clean and shiny just like the new piston.

I'll post pics tonight.

Is a KM r 90 a 4mix that needs valves adjusted?