What has happened to this piston?

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dirtyhandslopez
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J1MS wrote:Pencil marks in line with ring pegs helps align the piston rings on assembly.
If piston rings are accidentally pushed against the pegs on assembly the pegs can be loosened even if the piston dosnt fully slide into the barrel.
They are an interferance fit so any undue pressure can efffect their later performance.
Don't understand this at all. Rings are free floating in the grooves. What stops them spinning is because they press on the peg. Sooner or later, they are going to want to put pressure on the peg.
Sounds like the problem is ring pegs aren't being set in the pistons hard enough. Put a small curl in the tail of the ring peg that bites into the ali. on the inside. Or something. Can't be that hard to work out a solution surely.
Jagged teeth on the ring peg pehaps?

Form another thread posted by another member:
Now finally, the most perplexing cause of failure. Ring pegs. On the face of it, if a ring peg moves either in or out, it would be natural to assume that is has not been a tight enough fit. Whilst we have seen batches of pistons where the peg could be pushed in with a finger nail, we have also seen peg failures where the peg has simply been worn away by the piston ring or “worried” until it has loosened. Under these circumstances, we need to look elsewhere for the cause of the ring peg failure. Obviously the peg is being loosened by the sideways forces exerted by the piston ring. These are brought about primarily by the ring moving in and out of the exhaust port (and to a lesser extent the transfer ports) causing it to change shape. If the ports have been enlarged, this will put extra load on the ring peg and may cause failure. If the peg is in the centre of the piston, the loads on the peg will be balanced and less likely to dislodge it.

I see said the blind man.
So, it would appear it's the fault of the exhaust port. Hmm, what to do.
That's not going anywhere...
J1MS
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So you are saying the reason for failure is loose pins and you dissagree with both post quoted.
I agree with your reasoning that loose ring pegs fail, I also know that wide exhaust ports put larger side thrust forces on pegs which can greatly reduce reliability. And depending on the type/position of ring peg in the casting, as some have blind drillings some are drilled through.
Ive seen on assembly a piston peg pushed back out of the groove. Where tge ring was missaligned and the taper on the base of the barrel about 30 degrees helped push the pin out so it sat flush with groove. I knew it wasn't going right by the for e used to push the barrel down over the piston.

But you might be right the pegs might just be a poor fit.

But surely a good way to align is to mark the position even If it's just to make assembly easier.
dirtyhandslopez
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Joined: Sun Mar 22, 2009 5:15 am
Location: Richmond Virginia, USA
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What I didn't understand was why you though that you could exert enough pressure, on assembly, on the ring pegs to cause problems. Once one end is touching, the ring should slide in the groove alowing fitment of the cylinder.
I fully understand the tapping aspect loosenng them up.
trouble is, you can't just start putting oversize pegs in cause that will affect end gap, and if not done correctly, jepordise the integrity of the piston itself.
Dot puch 'em inplace from the inside of the piston?
A quick dot punch in the ring land to secure them?
Use Wiseco pistons :) ?
Good call on the pencil mark for the pegs. The little buggers like to play hide and seek.
Thinking about it, pencils are lead centered, the mark might even add a little extra start up lube, or conversely make also add contamination to the cylinder, putting a little score mark where the pencil mark was made. Ever seen anything bad where you put a mark?
That's not going anywhere...
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