I had a problem i guess

And sometimes the universe gives you warnings before really bad things happen. I only took this apart to fix a exhaust port

concours said:

The failure mode on Mike's piston is improper installation of the circlip. Squeezed too hard with the tool, the clip was bent at the thinnest area just behind the loop. This leaves the loop end unsupported by the circlip groove, where it flexes with each start/stop at the end of each piston stroke. Eventually it fatigues & breaks. Then, it does it's visible damage as shown by the witness marks.

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I think that's a likely possibility in this case because the rest of the circlip is still in the groove but there's another explanation besides improper installation...

MY CIRCLIP STORY:

When I had one of those wire styled circlips come out of it's groove on my commando at 65mph it hit the cylinder wall. The bike let out a "POP" and a blue puff of smoke which I saw in my mirror. I immediately pulled the clutch, shut the bike off and coasted to a stop. From what I saw and heard, my bike was going to be taken apart for an inspection. I had my friend pick me up with my van and haul me back to my shop. I tore the engine down to see a flattened spot on the loose circlip and a few scratches on the piston wall.

In my case one of those horrible wire circlips that came with the new pistons when I did my last rebuild a few weeks before had popped out. I didn't like the looks of them and I shouldn't have used them, but figured they must be ok if this dealer sold them with the pistons, but I know better now. After doing some research I concluded that they were the reason that the circlip popped out so I mentioned it here in the forum on one of the other "Circlip threads". A few people immediately said they were fine, and it was the fault of the person who installed them,....

At least that's what they said until Jim Comstock entered the discussion and said that he wouldn't use those particular circlips in anything except maybe a lawnmower, and he had seen that type of circlip pop out before. He was sure that they can pop out even with proper installation. Suddenly there were no more people accusing me of installer error and people accepted that an actual expert says that they pop out on occasion... As I recall he said that circlips popping out on commando engines is not limited to just those crappy wire version that I had, and that other kinds also occasionally pop out... It's a "Thing"...

My theory,... yes, I have a theory,... is that the violent reciprocation of the commando pistons can create enough inertia in the circlip for it to oscillate in the grove IF the wrist pin has worked itself against the opposite circlip so there's no friction on the oscillating one. In this case the force was focused on the thinnest part of the circlip. Notice how the broken circlip is positioned and the break is near to the 6 o'clock position. IF,... what I say is true then the entire mass of the oscillating circlip was focusing it's force on a single arm of the circlip at it's thinnest part and eventually it fatigued and broke off.... thousands of miles after it was installed.......

I don't see how it could be improper installation if the bike went thousands of miles without incident. My piston circlip failure happened 2 weeks after a rebuild and I'm certain that it was caused by the long tangs on the wire circlips acting like "bob weights" due to reciprocation and compressing the circlip enough for it to pop out of it's groove.... in my case I was doing 65mph, so the reciprocating force was great at that speed. This fits with my analysis and has been another factor that added to my theory...

So,.... from that theory,

1) I would only position cut steel circlips like a horseshoe facing up, NOT on their side like the failed one in this thread is showing.

2) I would only use circlips without long tangs.

3) If I used wire ones, I would only use the Jim Schmidt type without any tangs and are used with a bevel on the end of the wrist pin so they can't push the circlip out of the groove.

MikeG said:

Cant speak for the installer, especially if its the one i was told it was. He's been gone since 2010 and back in the day he was a dealer, racer, and very helpful and knowledgeable guy. If it was or wasn't it's hard to place blame after thousands of miles, some not so gentle.

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So what's the plan moving forward?

The circlips that Jim recommended are available at Mcmaster-Carr.
The circlips that came with my RGM 920 kit were loose when in place- a very common problem with other supplied pistons as well.
Fortunately I had read about piston circlip problems and solutions here on this site.
Having to stop working on assembly while waiting for the circlips to arrive was annoying, but we have it on good authority that the loose goosey ones eventually show up as in the op photo.

I wonder how many Commandos and other old Brit bikes are rebuilt with faulty piston clips in place?
It could give trouble right away or take quite a few thousand miles to show up.

Glen

worntorn said:

The circlips that Jim recommended are available at Mcmaster-Carr.
The circlips that came with my RGM 920 kit were loose when in place- a very common problem with other supplied pistons as well.
Fortunately I had read about piston circlip problems and solutions here on this site.
Having to stop working on assembly while waiting for the circlips to arrive was annoying, but we have it on good authority that the loose goosey ones eventually show up as in the op photo.

I wonder how many Commandos and other old Brit bikes are rebuilt with faulty piston clips in place?
It could give trouble right away or take quite a few thousand miles to show up.

Glen

Click to expand...

Never had a circlip issue in 4 rebuilds over the years . My new build ( which has not been fired up yet , winter ) is J.S. Motorsports circlips , no end ring holes to bust off . I trust they are better .

Schwany said:

So what's the plan moving forward?

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Buy new parts and rebuild. Strangely enough with all the wear on the rod bearings the crank journals are still standard size and ok. Also no issues with the cam and followers. Motor will go back together stock, it's fun enough as is.