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Ovaled piston skirt
Posted: Mon Mar 14, 2011 7:18 pm
by mattcfish
I have a friend who just got some used R90 pistons that should fit his new bores (R75 jugs bored up to standard R90 specs). One piston slides right into either bore perfectly. The other one slides in halfway crown first and catches on the skirts. Both pistons are in nice shape other than this (obviously large) problem. Is this correctable? What would cause this to happen?
Re: Ovaled piston skirt
Posted: Tue Mar 15, 2011 3:56 pm
by mattcfish
Hmm....I hear nothing but crickets.
My friend, put the gudgeon pin in and then took a large C clamp to the skirts. After some measurements it appears to have worked. No noticeable damage to the piston.
Re: Ovaled piston skirt
Posted: Tue Mar 15, 2011 4:19 pm
by Garnet
I too was hopeing someone smarter than me would answer.
Pistons ARE supposed to be oval at the skirt and round only at the ring land area. But what the ratio is, I havn't a clue.
Re: Ovaled piston skirt
Posted: Tue Mar 15, 2011 4:33 pm
by vanzen
mattcfish wrote:Hmm....I hear nothing but crickets.
My friend, put the gudgeon pin in and then took a large C clamp to the skirts. After some measurements it appears to have worked. No noticeable damage to the piston.
Re: Ovaled piston skirt
Posted: Tue Mar 15, 2011 10:12 pm
by mattcfish
Garnet wrote:I too was hopeing someone smarter than me would answer.
Pistons ARE supposed to be oval at the skirt and round only at the ring land area. But what the ratio is, I havn't a clue.
Yes, this is true. One was way more oval than the other, to the point that it would not fit in the cylinder. Our guess was that the the PO/seller had used a large hammer to remove the pin...sans heat. I told him the clamp was a barbaric solution, but he got it to match the other one.
Re: Ovaled piston skirt
Posted: Wed Mar 16, 2011 12:07 am
by John Falconer
It might be prudent to heat cycle that piston a few times in the oven before deciding the c-clamp "fixed" it.
Re: Ovaled piston skirt
Posted: Wed Mar 16, 2011 12:22 am
by robtg
John Falconer wrote:It might be prudent to heat cycle that piston a few times in the oven before deciding the c-clamp "fixed" it.
I agree with John and look at it carefully with a magnifying glass for cracks. I admire you daring to try something
not normally done.
Re: Ovaled piston skirt
Posted: Wed Mar 16, 2011 12:46 pm
by vanzen
Unless I had no other possible alternative,
I'd make a decorative pencil holder out of that piston.
It has now been seriously distorted – TWICE !
If it chooses to go to pieces in the cylinder at any RPM ...
the cost of repairs will very likely far exceed that of a piston set.
Re: Ovaled piston skirt
Posted: Wed Mar 16, 2011 7:39 pm
by Duane Ausherman
Basically, you want to unknurl a piston. The stress must be relieved, as in the case of knurling. John is correct............ again.
Re: Ovaled piston skirt
Posted: Wed Mar 16, 2011 7:56 pm
by Major Softie
vanzen@rockerboxer.com wrote:Unless I had no other possible alternative,
I'd make a decorative pencil holder out of that piston.
It has now been seriously distorted – TWICE !
If it chooses to go to pieces in the cylinder at any RPM ...
the cost of repairs will very likely far exceed that of a piston set.
I agree. The piston has been made to "fit." The chances that the distortion has been perfectly (as in "proportionally") repaired, is nil.