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Acellerated piston ring wear

16K views 33 replies 9 participants last post by  Jaguar57 
#1 ·
Hello everyone

I'm looking for opinions on my ring wear, I seem to be going through rings too quickly and can't nail down the root cause.
I'll start with the bike...
2007 Honda CR250 that I ride mostly in tight and steep single track / x-country riding.
Engine is leak-free and gets pressure tested every time I assemble it. Jetting is safe-rich, mixed @ 32/1, air filter always clean.

Piston #1 I started the season with a brand new oem cylinder and top end, the bike ran awesome to 60 hours when I changed the top end as a precaution (admittedly a little past my usual 40 hours) At this point the compression was still good, but the ring showed wear on the exhaust side... not shocking for 60 hours. Everything else internal looked brand new.

Piston #2 lost compression at around 10 hours. One day I noticed a lack of compression at the kick starter, and looking back... I seem to recall it had been running a little rich leading up to this.
-ring worn very thin on exhaust side, and has collapsed into the ring groove
-spark plug reads very rich
-looks to me like good piston wash, a little on the rich side
-no scrub marks on the piston sides
-cylinder plating and cross hatch in good shape
With no found cause, I second guessed my ring end gap, and replaced the top end being more careful with my measuring and filing.




Piston #3 just lost compression again at around 15 hours. Same as last time... engine has seemed slightly richer the last couple rides, and I noticed low compression at the kicker yesterday. I will tear it apart this afternoon, expecting to find the same situation. I had done a compression test a week or so ago, and it was still good.


I am left wondering what is causing this. Tight ring end gap seems like a suspected cause, but I was very precise setting it last round. The only other two things I can think of are:

1- Overheating. I do boil my bike over pretty regularly trying to climb nasty hills in the trees, possibly the exhaust side of the cyl is swelling when hot?
I would expect to see piston scrub marks if this were the case, but there are none.

2- High speed transfer sections. We are forced to cover some ground on old roads at times, and I am geared very low so the bike sees a little time cruising in top gear at moderate RPM (5000 - 6000).
Although I monitor my RPM and try to keep it low, I always hate sustained RPM riding like this. Even at rich jetting, is this a possible culprit?

Any insight would be greatly appreciated.
 
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#28 ·
I agree that it is a possibility that the bore is not perfectly round or the exhaust port might be too large due to porting.
Also, If it was bored...were the port edges chamfered afterwards so they aren't like a "razor blade finish" basically trimming the piston ring?
 
#30 · (Edited)
Maybe it might be a bad crank or rod bearing causing the piston to "cock meaning angle against the bore".
What brand of oil are you using?
What oil/gas ratio are you using?
Just wondering, If it's lean, it may wear the ring sooner.
If it's lean, just thinking...how about a bad crank seal...Just thinking here.
Of course, a single ring piston is gonna wear much quicker than a 2 ring piston.

Also, I've seen KLOTZ Benol mixed with race gas...Separate meaning the fuel and oil don't stay mixed.
 
#31 · (Edited)
The cylinder may still be crooked from the factory. Back a page or so a dial bore gauge was suggested & the comment was that they are too expensive. Not So; An acceptable model is available on eBay for about $60.00. I purchase around 1 per year since they are easily damaged.



The digital units are too sensitive & very tricky to work with. Sometimes drilling larger or another oil hole in the piston at the exhaust bridge area is very helpful in lubricating.
JT
:Cheers:
 
#32 ·
True point John.

The Cylinder might have Not been decked properly on the base side...Even tho it is a factory cyl. They can send you one that is not trued properly.

It could have been completed on a Friday at 4:45 p.m. When the person wanted to leave for the weekend or on a Monday when the guy doing the cylinder was hung-over after a hard partying weekend!

I guess the best bet would be to chuck it up on a lathe with a dial indicator and check the run-out.
 
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