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| Dave |
 Posted: Thu Nov 29, 2007 6:47 pm |
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Rookie
Joined: 22 Nov 2007
Posts: 4
Location: Tisselt, Belgium
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Axl,
recently bought a 812 Yoshimura piston set including cylinder block.
Trying to work out wether it would be ok to just assemble the whole lot using new rings I started measuring the piston clearance - 0.08mm approx.
Reading the article on the subject on you're website (simply amazing by the way) got me a bit confused.
The text says that forged pistons, in general require more clearance while the list below this text says the opposite : 0.0010" - 0.0020" for forged pistons, 0.0030" - 0.0045" for cast items.
If the list is correct - I assume it is - then I think my 0.08mm is quite acceptable assuming that my Yoshi pistons are cast (I'm quite sure they are). If not I have a problem  .
It would be nice to hear you're opinion on the subject.
Many thanks. |
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| mkramer1121 |
| Posted: Sat Dec 01, 2007 3:45 pm |
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Junior member
Joined: 11 Aug 2007
Posts: 11
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When i was contemplating my Yosh kit before selling it on eBay, I contacted MReick on the SOHC/4 forums about what the piston clearance should be. He stated I should have the cylinders bored out to .002" over. I know my forged Wisecos were done at .002" over as well.
Verify the cylinders are straight as well w/ a bore gauge. Sometimes, with used pistons, its better just to take them and a different cylinder to a machine shop and have them bored out. |
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| Dave |
| Posted: Sat Dec 01, 2007 7:19 pm |
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Rookie
Joined: 22 Nov 2007
Posts: 4
Location: Tisselt, Belgium
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Thanks for the info.
I understand that the Wiseco's should have a clearance of no more than 0.002" - they are forged items but if the Yoshi items are cast (I'm assuming they are) then they should, according to the slightly confusing info on the site, have greater clearance (0.0030" - 0.0045").
Greetings. |
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| Satanic Mechanic |
| Posted: Sun Dec 02, 2007 4:05 pm |
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Moderator
Joined: 04 Sep 2005
Posts: 309
Location: Munich, Germany
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Dave,
.08 mm is a bit on the loose side, indeed. The info about the recommended clearances look a bit contradictory, but the point is that pistons shrink during the break in phase (unless they are heat shrunk like practically all stock pistons are). The clearances in the list are the manufacturer's recommendations from their installation spec sheets, and they take into account if and if yes, how much their pistons will shrink initially. Arias even states different clearances for their different alloys.
The running clearance should be roughly in the range of .04 to .07mm. Now, if your pistons are still ok, and depending on how worn your cylinders are, I would either follow mkramer1121's recommendation and have another block bored-
The other option is having the pistons coated (only the piston skirt will be coated) - this will add about .03 to .04mm diameter, and it's proved in racing and street engines. Cost are approx. EUR 160 for four pistons.
And I promise to clear up the info on the big bore page in the next update 
Cheers |
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| Tim2005 |
| Posted: Mon Dec 03, 2007 11:34 pm |
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Member
Joined: 06 Sep 2005
Posts: 43
Location: Norfolk UK
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mkramer1121 wrote: When i was contemplating my Yosh kit before selling it on eBay, I contacted MReick on the SOHC/4 forums about what the piston clearance should be. He stated I should have the cylinders bored out to .002" over. I know my forged Wisecos were done at .002" over as well.
Are you including honing in that .002", as even .001" removed from the bored surface during honing increases the clearance by .002"?
My engineer showed me a textbook he has, which features a remarkably large section on honing; complex subject indeed! |
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| mkramer1121 |
| Posted: Wed Dec 05, 2007 8:56 pm |
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Junior member
Joined: 11 Aug 2007
Posts: 11
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| Honing was assumed. When you get the cylinders back from the machine shop they should be .002" over your piston size. |
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