Wrist Pin Clip Failure

[Brandon Atwell]

Hey guys!
Having an engine that keeps wanting to throw clips and not sure why.
Backstory, CMB 45 RS EVO I’m running as an outboard. The original setup lasted for maybe 4 weekends of racing plus a few test days.

This most recent piston/ liner/ clip/ wrist pin combo (all new with new bearings) lasted for maybe 3/4 heats worth of racing.

No excessive leaning, this last combo didn’t make it out of break in. Motor ran flawless with good speed on the first set.

Not sure why I would be eating through them now. Engine was built by two different people (liner set 1 vs. 2) so I’m strongly doubting improper installation based on their track records.

 

[user 27523]

Is the piston crown or combustion chamber peppered? Detonation can do that!

 

[timothy johnson]

Are the clips from CMB . I have bought some clips that weren't from CMB and the diameter and spring tension was less than CMB. Brandon I would suggest you learn how to install these yourself since your having issues. If the clip is not properly in the groove of the piston there will be problems. With a magnifying glass you should be able to see if the clip is in the groove.
Talk to the Florida Scale Boater, Rick, he likes the RS style motors and can help.

 

[Rod Fonda]

Also,the clip ends/opening should face the top of piston or the skirt,and not to the sides

 

[Brian Sorgente]

Hi Brandon, here's a few things I'd check.

Make sure the clip isn't twisted, if it is, straighten it out so the ends are in line with each other.

Check the fit of the clip in the groove. Use a Jewlers loupe so you can get a good look at it and make sure it's fully seated.

I deburr/ soften the ends of clips before installing to ensure it doesn't catch or scratch the groove. This also helps so I can push them around after install and ensure they're sitting nicely. I've never worried about clocking a clip to a particular position, though I do typically try to clock them to a noticeable position to see if they ever move (which I admittedly never remember to check lol).

Make sure the wrist pin isn't preventing the clip from seating fully. The pin should be able to be slid in and out of the piston smoothly and have a little click of end play after the clip is installed.

Hope this helps, let us know what you find.
Brian

 

[K F Wheeler]

This very subject came up about 20 years ago... Can't remember who suggested it... Their idea was to cut off most of the straight section of the end of the wire clip. The idea was to eliminate the extra weight that may tend to weaken the clip.

Ken

 

[John Beardslee]

Brandon, check your collet to see what the run out is and also check the prop to see if it’s hit something and slightly changed the balance.
John

 

[steve wood]

I have run into cmb wrist pins that were too long from factory. It won’t let the clip seat in the groove. Install out of the engine,and tap on both ends of the pin,if the clips come out,it’s probably too long

 

[Bill Eilers]

NEVER reuse clips, there SO inexpensive. but if they fail it will cost u hundreds in ruined parts. Orientation of the open section is VERY important. It should be on top or bottom, not fwd or back. If it's top or bottom acceleration will expand clip