I buddy of mine called me last night and said his son's KX85 cylinder had been badly scratched below the transfers after what appears to be a circlip failure allowing the wristpin to slide over. This is the 2nd time he's had this same problem with the bike in the last year-14 months. He used Wiseco pistons and circlips. I've had failures in the past of the type circlips with the "ears" but never with the Wiseco style. He asked me why I thought this was happening to him and I couldn't really think of any reason but I did tell him I knew where a bunch of mechanical genuises hung out on the net and would pose the question there and try and find out for him. So how about it guys? What do y'all think would be the most likely cause of circlip failure. I'm not discounting the fact that he may have just overstressed the clip on assembly but I struggle with what could be causing the side load on the wristpin if that's what is causing the problem.
I've never had problems with them either, and the cause of this puzzles me. I've had people tell me never to put the opening of the clip at the 9 o'clock or 3 o'clock position, because the clip's own inertia as it changes direction can help to compress it and let it pop out. I suppose you could weigh a section of clip and try to get a close calculation of the force involved at rpm X. I just decided to take this advice, since it harms nothing and there might be something to it.
Once in a great while, I've seen pistons that came with burs in the groove where the clip rides, and I removed them before using the piston. What are the odds that he could've had something like that happen twice???
I've had a type of circlip failure,a few times, with forged pistons in my various CB350 engines. What happens is the wristpin gets shoved out the side of the piston,deforming the piston material,much like a piece of clay. The circlip does't come out of it's groove, but gets imbedded into the piston deeper into the groove by the end bevel of the wristpin. In a compete failure, the wristpin moves sideways enough to provide insuficiant support of the piston and the rod yanks the pin and boss out the bottom of the piston. In a partial failure, the circlip forms a nice burr on the outside of the circlip groove, necessitating a knife to deburr the groove to get the pin out. My best guess as to the cause is excessive end-play in the connecting rod,the motion grabbing the pin and pushing it sideways out of the piston, the cure is to use teflon wristpin buttons. This seemes to be a problem with the old street cranks I use, Todd Henning never had a failure of this kind, but he always bought new cranks, big bucks. I don't know if this will help in your situation, but is something to think about. What does the piston look like? Any burrs? Any remains of the circlip? Probably out the exhaust. Forged pistons, of any make, are very soft by their nature. To make a forged piston, a lump of aluminum gets sqeezed into a mold like a lump of clay. I'd use a cast piston, of any make, before a forged in any two-stroke. The hype of forged being better over cast just dosen't work in all motors.
Thanks of the replies guys. Carl I wonder about the odds of it happening twice like this too if properly installed . I haven't seen the piston with my own eyes yet Stan but he said there does appear to be some clip material left in the groove like you mention but he said the pin slid right out easily when he disassembled the top end. Maybe this is a case of excessive end play since the crank wasn't changed between the failures. BTW it happened on the same side of the piston both times. Is there a source for small teflon buttons or do they have to be custom made? I would love to put them in both of my twins just for the safety factor. It seems to me the twins might be even more apt to have this type of problem. If they have to be custom made does anyone have a material supplier source?
I've bought a Teflon rod from an industrial suplier, McMaster-Carr, I think, and turn up my own buttons on my lathe. I haven't made any for two-strokes, no need so far. You have to have the cylinder wall support the button throughout the whole stroke, not possible on all strokers(what are all those holes for?) I would find the problem (worn crank, etc.) first, use a Pro-x,ART(stock)piston in the rebuild. There is no inherant problem in these motors as far as I know. I can make buttons for any aplication if you want, the piston must also have support for the button top and bottom, slipper skirts won't work.
I agree with stan,, either there is some problem with the rod/crank or I would be suspect of some installation error, one must check to see that the clip is carefully installed and fully seated in an undamaged groove. care must be taken not to over-flex the clip when it is installed. I have always carefully rotated the clip after install to align the opening at the bottom, this also lets me see that it is fully seated in the groove. Once I get the clip started into the pin hole I use an old pin to carefully push it into place. This places even pressure on it and prevents damage to the groove.
I never use a metal tool to install the circlips. I {painfully} push them in with my thumbs and then use the handle of a small screwdriver to push the clip into the groove. The little screw drivers that Snap on gives away at christmas are perfect. I also try to put the upper section of the clip directly in it's groove on installation instead of having the clip fully compressed in the pin bore and then sliding it into the groove. I am trying to compress the clip as little as possible. I then use an appropriately sized screw driver to forcefully push the pin against each clip to make sure they are fully seated. Kawasaki got a bad batch of clips for the G series in 1973 and all the mechs got circlip religion Rau
So now the argument of whether it's best to avoid compressing the clip vs avoid twisting it will arise. Personally I prefer teflon buttons (thanks for the tip on where to get some Stan, my stock is nearly depleted) but then I never had any problems with circlips. I used Yamaha Bob's method. Metal = Bad is an oversimplification of a rule of thumb intended to keep ham fisted "mekaniks" from doing damage to soft aluminum engine parts when using a file end, screwdriver or drift as their tool of choice. Kind of like giving an infant a plastic tool kit rather than a set of Snap-Ons. An old clean wrist pin certainly couldn't do any damage. It has a mirror smooth finish and was designed to fit the pin bore and clip. [This message has been edited by Diesel (edited 04-12-2002).]
Agreed,, I try and avoid asymmetrically flexing the clip causing a stress at the peak and possibly egg shaping it. the wrist pin cannot damage the piston , pin hole or circlip. There are times I have to use a small screwdriver also, the key here is to be GENTILE with a clip. The other technique I use is to have a wrist pin in the piston when I put the first (inner clip) in the piston before I put the piston on the rod, this prevents getting it to deep in the hole or wedged sideways, then on a twin I can put the outers in, the pin is in place to keep the clip aligned and prevent the clip from going in to deep.
I also do not twist the clip. I put the open end up angled in toward the groove and force the bottom of the clip into the pin bore with the tips of both thumbs. Painful but worth it for piece of mind. Ray
That's the way I do it, too. Don't you just hate it when it bites a piece of skin off and traps it under the clip? I've thought about just leaving it, as I'm sure it would rapidly deteriorate to nothing in a running engine. But I always end up taking it out and trying again.
I just leave it in there. It will be long gone before the engine ever gets revved up. I have had some clips that were too long and the ends would butt up before you could get the clip in the groove. Hodaka's and others come with a "G" clip. We used to snip off most of the leg leaving just enough to get some needle nose pliers on. Ray
One thing that wasn't mentioned, was never reuse a clip. If I hve to remove one for any reason, A new one goes in. Also I have run forged pistons for 30 years now. Never lost a clip. Soft (forged) takes a lot more abuse than cast. I have bent a few pistons where a cast piston would of "shrapped" and destroyed the motor.
