i have had some trouble with the wristpin snapring failing on a rear exhaust tuned pipe engine with only the rear snapring failing. it of course ruined the piston and sleeve. its an ABC engine. its happened twice now with one end of the ring breaking off and doing the damage. should the ring be placed in a certain position in the groove? maybe i should try and find a snap ring with no holes for removal. the picture is of the ruined one and one that is new. any suggestions? thanks

I have many Fox engines with that setup and never had a failure, this must be happening at some surreal rpm.

I would think it's more heat-related since it's only happening to the rear snap ring. Does the engine run pretty hot? Lots of scratches on the piston. What's the other side look like? How's your oil content?

CR

Man, is the end of that wrist pin exposed to an open exhaust port? I've never seen anything like that on any two cycle engine. I was going to recommend that you obtain or have machined either teflon wrist pin retainers or brass wrist pin retainers (little buttons that fit on the end of the wrist pin) and lose the circlips. But, if the end of that wrist pin passes by the open port it would not be possible.

Bill

I remember reading that when assembling a rear exh. engine, when possable, the blind end of the pin should be facing the E. opening and the open end toward the F. intake port(s ) so theoretically the pin can be cooled by the incomming mixture.


One thing I noticed is the orientation of the clip...
If you look at the clip in the new one, it is orientated properly...so vibration will have less effect. Look at the position of the failed one for comparison.
I'm not sure how to word this...but if the clip (e clip, snap ring, whatever ) is positioned so that the tabs or tang is horizontal, the up and down (rapid and abrupt ) changes in direction can cause the tang/tabs to wiggle/twang, resonate until metal fatigue causes failure. That and the heat perhaps???

Unfortunately, no way to know how the failed clip was oriented initially.

CR

You can see one of the tabs remains...and actually, it looks like it was positiioned OK...unless it rotated? Could be a simple case of "got a bad one"? (C clip I mean ) Or it might have been damaged during installation?

This reminds me of an incident I had with one of my Webra .50's..
I bought a couple of new wrist pins, and one of them was a bit on the long side...about .020" and that prevented the retainer clip from getting down fully in the groove in the piston. (I didn't use that wrist pin...sent it back )

All the rear exhaust engines I worked on with the port actually at the rear, where the pin was, had a blind hole "step" in the wrist pin bore of the piston. The blind side of the piston went to the rear. The only retainer was on the front side?

The circlip / snap ring open end positioning has always been a critical problem

When I raced boats we clipped the tail off of the circlips to prevent them from jumping out of the groove. You might want to try it on the exhaust side. There were also motors that had weak clips and we would replace them from other engines that had better clips of the same diameter.

Did I see it right that the ring was still in, but one side had worked its way out? If this is the case, it is quite likely that the ring wasn't seated properly. Just letting the ring snap in place isn't enough if the grive is a tight fit. it is good practice to tap the ring home after getting it in the grove. A snap ring should be slightly loose in the grove. Just a thousands or so clearance on the OD of the ring. It should be a tight fit on the sides, not a press fit, but very close to it. The clearance on the OD is to allow for expansion.

I've never seen an application where the location of the lugs was fixed. As long as the ring is fully in the grove, it doesn't matter where the gap is.

Prior to assembling the engine, test fit the rings into the piston. Make sure there is no flashing or crud in the grove. Also, check the wrist pin for fit and for burs. The wrist pin should float between the rings, not be tight. You may need to stone the ends of the wrist pin if there is a rough finish on them. Just make sure it is well cleaned after so none of the stone is trapped in the center hole.

Don

D

well, after installing a nw engine overnight and this morning, all went well on the few flights i got with it. i double checked the snap ring to be sure it was seated and in the right position. ran it plenty rich, it flew fast and no problems. maybe i just had a couple bad ones and some bad luck, time will tell.