E: Engine Nip up Comet

[Bill Cannon]

Anyway, continuing the thread. I have now stripped the top end. The bore looks absolutely fine and the piston will be OK with a bit of a clean up. As I thought the nip up area on the worst part just seemed to be catching the oil ring. I measured the oil hole in the liner and the top of the hole is a good 3-4mm BELOW the bottom edge of the oil ring so I am assuming this is also fine. I have yet to strip and check the cylinder head but will do that next.
Can anyone identify the piston please as I might want to change the rings?
View attachment 29723View attachment 29716View attachment 29717View attachment 29718View attachment 29719View attachment 29720View attachment 29721View attachment 29722

I had GPM in my Egli when it was !000. There was an article by Neville Higgins in MPH many years ago explaining that they are good pistons but as Bill says they are round. You need to relieve the skirts at the four points close to the cylinder studs with a fine file and they will be fine. Mine did many thousand miles without a problem.
Cheers Bill

 

[Martyn Goodwin]

Thanks I will check.

ring gaps top 0.020" second 0,025" oil 0.018"

 

[CoreyL]

I have just looked again at the piston, it does indeed show signs of nipping up in four places, in line with the head studs.

I think the line up with the studs is coincidental. The scoring on the piston also coincides with where the piston has the most metal (around the wrist pin) and that's where the most expansion occurs.

 

[Cyborg]

I think the line up with the studs in coincidental. The scoring on the piston also coincides with where the piston has the most metal (around the wrist pin) and that's where the most expansion occurs.

I would agree with that... "sort of" looks like a cold seizure.

Tony... I PM'd you a bunch of info on diagnosing piston seizures. Hopefully you can open the file. There is one in there that is similar.

 

[Tony Wilkinson]

OK so here is a picture of my GPM which looks just like yours. And here is the reason you should probably chuck it in the bin.

This standard bore pair measure 1 1/2 thou bigger across the gudgeon pin faces than they do across the thrust faces, and the other pair which are 9:1 and plus .020 measure about 1 1/4 thou more again across the gudgeon pin faces that across the thrust faces.

Like I said before, I was told they nipped up at the 4 corners and that common practice was to nip them up, take the whole plot apart and file the high spots, rinse and repeat. Just like they had to do in the 20's until they figured out not to make round pistons!

Thank you for the information, I will take on board all that you have said. I will measure the piston accurately and see if mine is the same.

 

[Tony Wilkinson]

I would agree with that... "sort of" looks like a cold seizure.

Tony... I PM'd you a bunch of info on diagnosing piston seizures. Hopefully you can open the file. There is one in there that is similar.

Great thanks.

 

[Cyborg]

Although, after thinking about it during my morning coffee... how snug was the cylinder in the cases? Oexing had me thinking about that. If the cylinder is a really snug fit and the cylinder studs were over torqued the case could distort enough to in turn distort the cylinder and possibly give you those results.

 

[Tony Wilkinson]

Although, after thinking about it during my morning coffee... how snug was the cylinder in the cases? Oexing had me thinking about that. If the cylinder is a really snug fit and the cylinder studs were over torqued the case could distort enough to in turn distort the cylinder and possibly give you those results.

Yes I really think this was the problem. Cylinder is quite a snug fit in the cases. The head had been pulled down VERY tight. I have since checked and the torque setting is only 32ft Ibs. I am going to clean everything up, check out the head, valve guides etc. If all is OK I will replace what is needed and put back together.

 

[Tony Wilkinson]

I had GPM in my Egli when it was !000. There was an article by Neville Higgins in MPH many years ago explaining that they are good pistons but as Bill says they are round. You need to relieve the skirts at the four points close to the cylinder studs with a fine file and they will be fine. Mine did many thousand miles without a problem.
Cheers Bill

Thanks for that Bill. I intend to do just that. Head was pulled down much too tight. Someone else said just replace absolutely everything with new parts. I am not going to do that. I think cleaning everything up and re-assembling properly will sort it all out.

 

[Cyborg]

If any aluminum has transferred onto the bore, you can clean it off with muriatic acid. Better to do it before to use a hone or glaze breaker.

Ps... you could explore the option of putting valve seals in there while you have it apart... assuming it doesn’t already have them. Available through VOC Spares I believe. Only thing is, you’ll need the valve guide retainer removal/installer tool and a suitable reamer for the new retainer. If it’s the same setup I used on my Comet, the new retainer’s ID is smaller than the stem OD, so needs to be rearmed,