Bent tang wire circlips GPM pistons from andover.

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Kevin68fastback

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Hi all,

I am planning on using a set of GPM pistons purchased through my local Andover supplier here in Australia. It a resto of a 1968 750 with minimal perfomance upgrades.

My question purely relates to the wire circlips supplied with the pistons. I would wish to use the modern plain wire C clips instead of the ones supplied with the protruding tails. I know they would be fine on motor that revved slower but i read how the long stroke commando has quite high piston speeds. I have read recommendations against them and heard of failures although one cannot rule out they were correctly seated.
The GPM wire clips with tangs are 0.060" in diameter and I have sourced Wisco and triumph C clips of the correct size and they measure 0.047" and 0.048" in wire diameter.

The piston groove is too deep so that a clip of 0.055" or less would not stop the wrist pin from floating out.

Q. Are there Clips from another application I could use with these pistons?
 
Although it is not the correct application -I have used the stock Hepolite C-clip in that piston successfully.
I would trust the Hepolite c-clip further than I would trust the GPM wire ring -even in a groove that was not designed for it.
 
Triton Thrasher, is there a way to post pics without first putting it on a third party site, i only see the option for a URL.
 
Comnoz, I regret buying them now I know the potential failings with the clips.

Best solution I assume is to machine a slot and use some 0.060" to 0.067" wire clips. I just need clips to suit.
 
Hard to tell if you are talking about the same wire circlips from GPM that I have used with no issues in a 500 Dommie race bike reved to over 8000 for several seasons.

All I did was dremel off the longer tags so there was maybe 1mm or less of the tag remaining.
 
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Bent tang wire circlips GPM pistons from andover.



Here is a dental impression of the groove in the piston with a 0.047" circlip for comparison.
 
Bent tang wire circlips GPM pistons from andover.



This is the wire clip I don't want to use.

I've considered grinding off the tangs completely but it wouldn't leave much engagement in the groove
 
here's a picture of the circlips that failed on me. The flattened spot on the top circlip is where it hit the cylinder wall and caused a "POP" at 65mph. I pulled the clutch, shut the bike off and coasted to a stop, then got picked up by a friend in my van. I tore it down.... and rebuilt it back up.

Bent tang wire circlips GPM pistons from andover.


Here's one of the links where I explain what I think the problem is...

https://www.accessnorton.com/Norton...eals-bigger-problems.27252/page-2#post-411088

For the most part, someone, or maybe more than one person, made the accusation that possibly I (and others who had this happen) didn't seat the circlip properly. I can only say, I think I did but installing the circlips isn't like torqueing headbolts, where it's some precise memorable proceedure. I stuck the wire circlip in the recess and slide it inward until it popped into the groove. Then I moved on to other parts of the rebuild. Not something difficult to do and not very memorable as a "difficult proceedure".

The funny thing is that accusation bothered me a bit because I am pretty OCD about mechanical objects. Then Jim Comstock chimed in that he knows these shitty circlips pop out even when they are properly installed... I didn't need to fire a shot back at the internet experts who quickly claimed "human error!", and the "faulty installation" theory was done from a single comment from Jim...

Like JohnM said, you could probably remove the tangs and they would be better than the way they are sent to you stock. I bought the cut steel kind and used them in the round groove. I've put 15,000 miles on them without another circlip incident.

I wouldn't use those wire circlips... they look just like the ones that failed on my engine.
 
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Yes those look like the ones I cut the tags off.

But if your worried your probably best to use something else for your peace of mind
 
johnm said:
Yes those look like the ones I cut the tags off.

But if your worried your probably best to use something else for your peace of mind
Click to expand...


I would be worried. The piston speed on a 750 is quite a bit higher than on a Dommie.

If you have the stuff to do it you might cut the tangs off completely and then put a 45 degree taper on the end of the pins. Then you will not have to worry about them.

You will also need to machine a small slot in the piston so you can get a pick in if you ever want to remove them. Jim
 
o0norton0o, I've seen the thread and yours is a failure I've heard of on this forum. Others like comnoz do acknowledge they have a spotty history.

this mentions how even the plain wire type can be subject to self compressing under the piston speed extreme see 2:20 in. The wire with the tangs being extra weight would be even more susceptible.
 
Thanks Comnoz. I can machine a groove in the piston and chamfer on the wrist pin easy enough.

That will be my plan. Cut the bent section and tang off the original wire type if i cant find a plain wire C clip of the correct thickness.
 
Kevin68fastback said:
Thanks Comnoz. I can machine a groove in the piston and chamfer on the wrist pin easy enough.

That will be my plan. Cut the bent section and tang off the original wire type if i cant find a plain wire C clip of the correct thickness.
Click to expand...

I don't know of any wire circlips with a large enough cross section to replace the GPM clip. They work fine if you just cut the tangs off.

Make sure the bevel you put on the pin is smooth. If you leave a rough ground surface it will act like a file and cut through the circlip.
 
Late last year I had a circlip failure in my Roadster.
On the left hand cylinder , the eyelet from the LHS of the piston broke off and migrated through the hollow gudgeon pin and wore a hole and groove on the RHS of the bore.
It also came up out through the top and grooved the rings and piston.
Lots of white smoke needless to say!!!!!
These were some 0.020" piston supplied through a local vendor from Hepolite and the supplied clips came with the pistons.
I made sure the flat sides/sharp edge of the circlips were facing out towards the pistons as recommended.
This motor rarely sees 6000 rpm although often sees 5000 to 5500 rpm
Anyone have a good condition used set of cast iron barrels that will take a 0.020" or 0.040" overbore?
Regards Mike
 
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