removing barrells

[nkt267]

Here is another. I am pretty sure that I did not ream the small end big enough, and there seems to be excessive side movement of the conrod on the bigend. Unfortunately the photos of the inside of the barrell did not show the timing side wear so i did not post them, but the wear pattern is the same shape as the piston below the oil ring. There was certainly more lateral movement of the conrod than there is with the rebuilt flywheels and Carillo rod.. John

 

[Howard]

I seem to remember a batch of GPM pistons being made "round" in the 70s which gave simillar failures - we didn't have internet back then you know, so it's all hearsay.

How about this for a suggestion? Tight gudgeon pin overheats due to friction at the little end bush and expands. Much malignied circlips stay put. Expanding pin presses on circlip and distorts the piston. As van Drenth said it's down to the tight little end bush.

H

 

[stumpy lord]

You are quite correct about G.M. pistons I have two brand new ones on the shelve never used, sold to me by Chas Guy.
stumpy lord.

 

[wmg73141]

The one thing that hasn’t been mentioned as a possible root cause of this problem is the alignment of the little-end bush relative to the big-end.

In my opinion it is essential that the little-end bushes be bored to size using the big-end of the connecting rod as an alignment datum. This of course means that such work has to be done before the crankshaft is assembled and also requires access to a milling machine and some careful setting up.

I did notice a reference to reaming the little-end which did make me cringe a bit! It is worth noting that as small an alignment error as a ¼ thou. that is 0.00025” (0,0064mm) in the bush, which is less than 1/10th the thickness of a human hair by the way, will tip a Vincent piston by 0.0008” (0,02mm) over its diameter.

If that misalignment happens to be on the axis of the gudgeon pin and is also quite possibly more than the above figures then it is easy to see that the compromised skirt clearances will result in extra friction leading ultimately to this sort of disaster. Such misalignments will do no favours to the big-end bearings either!

 

[redbloke1956]

The one thing that hasn’t been mentioned as a possible root cause of this problem is the alignment of the little-end bush relative to the big-end.

In my opinion it is essential that the little-end bushes be bored to size using the big-end of the connecting rod as an alignment datum. This of course means that such work has to be done before the crankshaft is assembled and also requires access to a milling machine and some careful setting up.

I did notice a reference to reaming the little-end which did make me cringe a bit! It is worth noting that as small an alignment error as a ¼ thou. that is 0.00025” (0,0064mm) in the bush, which is less than 1/10th the thickness of a human hair by the way, will tip a Vincent piston by 0.0008” (0,02mm) over its diameter.

If that misalignment happens to be on the axis of the gudgeon pin and is also quite possibly more than the above figures then it is easy to see that the compromised skirt clearances will result in extra friction leading ultimately to this sort of disaster. Such misalignments will do no favours to the big-end bearings either!

I stand ready to be corrected, but i would be very surprised if the tolerances you mention in alignment between little end bush and the big end would have been achievable in the 1950s?

 

[tatty500]

Dear all,
I've finally found the information you need. See page 55 of this: http://www.boosttown.com/engine/piston_damage.pdf

 

[jellywrestler]

The plot thickens
there's two different size circlips in the bike. Brand new set of VOC Omega 7.3:1 standard pistons and one set of circlips are 54thou while the other are 64thou.
64 thou ones came from the piston that lost one of it's clips also very little tension on that one to hold it in it's groove...
Did they get supplied with different circlips???
who knows I'm picking

 

[jellywrestler]

oops forgot the piccie