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"Y Alloy" Pistons

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi all, can anyone tell me the expansion rate for Y alloy pistons (thou per inch of diameter), I cannot find my info on it anywhere.
I am sure that one of you guys will have this little gem of info hiding in the deep dark recesses of the mind.

Regards
Kevin
 

Big Sid

Guest
The motorcycle industry in England all employed for their vertical twins con rods made of RR 56 while their single cylinder motors retained the use of steel rods I seem to recall . Sid .


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redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
.000022" per degree Centigrade. "Tuning for Speed" page 240 fifth edition 1969.
Hi Chankly, I don't have that edition (I have 3rd) and mine doesn't mention same info, could you please supply more detail - because if Y alloy expands at .000022" per degree C, 3.25"dia piston, typical piston temp of 300 deg C, then I need minimum of 0.02435" clearance...Houston we have a problem :)

Kevin
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
Kevin, you must consider the different clearances allowed down the piston and also the cam grind, usually a "C" in the old Repco catalogue. Look at "Piston Clearances" in the same worthy tome, page 55 in my edition. The coefficients of expansion are given in the Appendix of my edition. I get nearer .020" because the piston ain't starting frozen and everything else is expanding too, including the Universe! Also, hopefully, the piston is not uniformly hot if you have oil doing its job and good thermal paths in the motor, which as I've said before, Phil. Irving designed that way. Cheers.
 

Big Sid

Guest
On skirt clearance . With the lack of uniform heat rejection through to the muff fins it's better to go a bit more than 2.5 thou . The cooling efficiency is very spotty with old muff sleeve assemblies ,the inner contact between muff and iron liner poor .
Better to go with a snug 4 thou . To lessen the chance of the piston being scored . Sid .


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Big Sid

Guest
A lovely piece on modern pistons but it appeared in issue 554 , we are now 214 issues later , much water under the dam since .
All well and good , except if you motor still uses its original cylinder assemblies . We're those liners changed properly during your last motor rebuild ? Do you know ? Or we're the pretty new pistons fitted into the old liners ? That's the bone in your throat . Sid .


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redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Kevin, you must consider the different clearances allowed down the piston and also the cam grind, usually a "C" in the old Repco catalogue. Look at "Piston Clearances" in the same worthy tome, page 55 in my edition. The coefficients of expansion are given in the Appendix of my edition. I get nearer .020" because the piston ain't starting frozen and everything else is expanding too, including the Universe! Also, hopefully, the piston is not uniformly hot if you have oil doing its job and good thermal paths in the motor, which as I've said before, Phil. Irving designed that way. Cheers.
Thanks Chankly, If you read the pdf http://www.voc.uk.com/net/docs/3.1/3.1-554-29.pdf supplied by David Dunfey it says a skirt clearance of .004" - .006" Now I am getting very confused?

Kevin
 
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