Hi Robert, that is interesting thanks.
E: Engine Nip up Comet
- Thread starter Tony Wilkinson
- Start date
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.
Thanks I will check.
Basically ring gaps should be set by the manufacturer, but then, I once had a brand new Mahle auto piston taken from the original packing. When trying to slip the cylinder over it It stopped at the oil control ring and even a lot of push would not get the piston inside. So in the end I stopped trying and inspected the ring, had a nasty scratch in the cylinder bore from trying. Obviously that ring was for the next oversize when I took measures !!
In our case would a too small ring gap produce nip ups on the piston skirt like shown ?? I would not think so.
How is the fit of the cylinder liner in the crank case ? An easy slip fit with not the least resistance when shoving down ?? Otherwise I´d suspect a deformed liner from an imperfect case, less so from head studs.
Vic
In our case would a too small ring gap produce nip ups on the piston skirt like shown ?? I would not think so.
How is the fit of the cylinder liner in the crank case ? An easy slip fit with not the least resistance when shoving down ?? Otherwise I´d suspect a deformed liner from an imperfect case, less so from head studs.
Vic
Lack of piston ring gap in cast iron causes the ring to break into many small pieces, not cause piston pick up.
From the images I would first check the piston for adequate cam and taper, and if enough, put this down to the head being torqued down too tight if the liner is a snug fit into the crankcase.
Either way I would replace the piston while the engine is apart.
From the images I would first check the piston for adequate cam and taper, and if enough, put this down to the head being torqued down too tight if the liner is a snug fit into the crankcase.
Either way I would replace the piston while the engine is apart.
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!
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!
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I agree with passenger0-0 100 % throw the piston away and start again. The hot spots on the piston are from thermal differences around the barrel due to the large diameter holes for the hold down studs, and as he said, the head nuts have probably been pulled down too much......max 32 Ft Lb's and no more. You need to check the depth of the liner spigot in the head verses the height of the hat section of the liner. They will most likely be either the same or the liner will show a lesser figure, this means the liner cannot seal correctly at the head joint. The head recess should be 0.125" and the liner ideally slightly more. This will give you a slight gap across the broad alloy faces of the head and barrel with head sat atop the barrel.......... You need to check this before you go any further, as any further action could become a wasted effort.
Actually I just looked over your pictures and one shows the top of the liner which has been leaking at the head joint anyway.........to be honest, I would start again with a new liner/muff assembly, new omega or CP piston, bored and honed to 0.005" clearance, lap the barrel/head joint to give the correct 2 to 3 thou air gap at the alloy broad faces, and carry on from there...........sounds expensive, not really if you want to sort it out properly and keep it reliable on the road. I don't mess around with old barrels/pistons as the grief in having to redo it over again is simply not worth it.
Its not the omega piston that most of us are running and which the club supplies. I know that at least.
