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Terry, so you're thinking that the 4 wear marks aren't from the motor not being 100% warmed up first like CamP did?
Why would there only be detonation on those 4 areas, and not all over?
As for where the opening was- you can actually see it in the 3rd picture. I snapped that shot before removing the circlip. The spot that it broke at was right about at 9:00, and the actual opening was at about 7:00.
Old tiddler, that's how I install them, but without a screwdriver. I just use my thumb to lever it in there. It's funny, because I've always been paranoid about installing circlips, thinking of the carnage that could be caused by one coming loose. In this case, I don't see how where I did anything wrong, but am thinking that there was a metallurgy issue. I'm not opposed to finding out that I did something incorrectly, as I don't want to have issues in the future. I'm not thinking that this is a common Namura issue, heck I also have a Namura in my CR250R that was installed at the same time the RM85 piston was. It seems to be fine, though I will be pulling the pipe to look for blowby even though it only has about 5 tanks of fuel run though it.
I have now been in contact with Namura. I'll be filling out their return form and sending them the piston. The gentleman I spoke to was helpful and professional, hopefully that trend continues and they take care of the cylinder and head in addition to a new piston. I will keep everyone informed.
Thanks again to everyone for the comments and suggestions (err, at least most of them, lol).
And hey, don't insult my heritage yo! I'm 3/4 Scottish and thats good enough! And who told you I didn't know who Robbie Bruce is yer wanker?
Awa an bile yer heid yer walloper!
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Id still bet that was the cause, or it was damaged a bit when it was installed. Not being a smart ass.... just saying from experience. Mod two stroke engine building was my business for a few years. Be careful when installing them.
I'm not sure what I could have done that caused it to fail like that. No that it's impossible, but I don't see what I could have done to cause the circlip to break, especially where it broke at.
@6000 rpm, your RM-85's piston is traveling approx. 30 feet per second. It's going from full speed to a dead stop (TDC/BDC), to full reverse speed 200 times in that same second. The gravitation (G) force on the circlip are huge. Try to picture what happens to the circlip at BDC. You've got G force pushing down on the top of the clip. The bottom of the clip is against the piston and it's now being pushed up by the crank. The circlip is being squeezed by two opposing forces (upward mechanical and downward gravitational) It's like a small vice. The same thing happens in reverse at TDC.
Now take an old circlip and squeeze it with a pair of pliers. Try it with the circlip opening to the side (3 or 9 o'clock) in relation to the jaws. It compresses at the gap fairly easily. Now repeat this with the opening on a jaw (6 or 12 o'clock). You'll feel how much harder it is compress. The load is now being applied against both sides of the circlip. That same "squeezing" is happening in your engine 100's of times per second.
Unfortunately, I believe the pictures show that circlip position may have been a contributing factor. The top of the circlip was compressed and became unseated at BDC. It protruded out into the transfer port. As the piston traveled up, the end of the circlip was literally sheered (cut) off by the piston and the top of the transfer port. Based on the location of the groove, (say 1 o'clock), the end of the circlip was 1/8" past that. That puts the end of the clip (start of the gap) close to 2-3 o'clock.
The circlip may have been weak, but it also may not have failed if the gap was at the top or bottom. That's between you and Namura.
Thankfully you were able to figure out my problem nearly 3 years later!
Yep, Namura pistons do suck, as does their customer noservice department.
In the end I replated the cylinder and moved on. Replaced the ring every 5 hours, new OEM piston every 10 hours. It was great. Matter of fact, all the RM85 stuff is being sold, he's on 125's now.
Ignore me razzing you for bringing this thread back to life, now maybe someone considering buying a namura piston won't make the same mistake I did.
He's on a late model YZ125. I'm going to re-do my KX125 and YZ250 this winter. Bit of a 2 stroke garage here.
Pit Row
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