How can I know the size of my pistons?

Slow 1200 · Post by **Slow 1200** » Wed Oct 30, 2002 12:51 am

Hi all,

My 64 is much more livelier than I'd expect from a tired 1200, so I'm suspecting it has big bore p/c's (82mm? 83mm?), is there anyway I can know without splitting the case?
And, if it turns out I do have big cyls, which jetting would be recommended for the stock solex 28? (now I think I have a 117 main, which does feel lean)

cheers

Pete65 · Post by **Pete65** » Wed Oct 30, 2002 5:21 am

you can take the heads off one side and clean off the head of the piston with some solvent. The pistons are stamped on the flat face. There are a couple markings, one will be the size. If you have a bentley manual you can find in there what all the different markings are. Good luck.

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Slow 1200 · Post by **Slow 1200** » Wed Oct 30, 2002 9:40 am

thanks, but taking out the heads is out of the question right now, are the barrels distinguishable from the outside somehow?

Post by **Marc** » Wed Oct 30, 2002 11:08 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Slow 1200:
thanks, but taking out the heads is out of the question right now, are the barrels distinguishable from the outside somehow?<HR></BLOCKQUOTE>

Not always - the fin design varies but it's nothing you can count on. You could jack the car up way high on one side, pull a plug from a cylinder that's at BDC and see how much light oil or solvent it takes to fill the cylinder. If it's under 12 ounces/350cc it's stock, if it's over ~13¼ ounces/390cc it's a big-bore. Or find a racing association that has a "P&G pump" which can measure the diplacement fairly accurately.
Jetting should be based on plug readings. Go find a long upgrade on the highway that you can take at full throttle and pull over at the top - do a "clean shutoff" (hit the clutch and turn off the key, coast to a stop) and pull some plugs. You're looking for a light tan color down deep on the insulator, if it's refrigerator-white you're too lean.

Insaino Dwayno · Post by **Insaino Dwayno** » Wed Oct 30, 2002 11:45 pm

<BLOCKQUOTE>quote:<HR>
Go find a long upgrade on the highway that you can take at full throttle and pull over at the top - do a "clean shutoff" (hit the clutch and turn off the key, coast to a stop) and pull some plugs. You're looking for a light tan color down deep on the insulator, if it's refrigerator-white you're too lean.<HR></BLOCKQUOTE>
This might seem redundant, but if you do this, be careful of burning yourself. I've pulled plugs when the engine has been sitting for a while, and still cooked my digits.

-Dwayne

BugMan53 · Post by **BugMan53** » Sat Nov 02, 2002 10:01 am

Just a stipid thought. If you know the cylinder wall thickness then why could you not meaure the dia. betwine the cooling fins and - the wall thinkness.
X-measured dia.
CWT-clinder wall thickness
X-CWTx2=boresize

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ROBERT H. · Post by **ROBERT H.** » Wed Nov 06, 2002 3:58 am

THE REASON WHY YOU COULDNT DO IT THAT WAY IS BECAUSE YOU WOULD HAVE TO REMOVE THE HEAD TO CHECK THE WALL THICKNESS. THE ONLY WAY IT IS POSSIBLE TO ACCURATELY KNOW PISTON O.D. OR CYLINDER BORE I.D. IS TO REMOVE THE HEAD AND MIC THEM. YOU WOULD HAVE TO KNOW THE COMBUSTION CHAMBER CC TO USE THE CC'ING METHOD.

Post by **Marc** » Wed Nov 06, 2002 5:46 am

<BLOCKQUOTE>quote:<HR>Originally posted by ROBERT H.:
YOU WOULD HAVE TO KNOW THE COMBUSTION CHAMBER CC TO USE THE CC'ING METHOD. <HR></BLOCKQUOTE>

No, you merely assume that the engine has a compression ratio that allows it to burn pump gas and infer what the combustion chamber volume and deck height would be - it's a very crude approximation and would never be able to discern between 82s and 83s, but for a gross difference like that between 77 and 82 or 83 it'll suffice.
BugMan's plan might work if you had a few extra cylinders around of different bore size and manufacture to compare to, but the cylinder wall thickness varies a lot so it'd again only be good for making crude approximations. And what if someone slipped in a stroker kit? Hard to measure a few mm difference in stroke accurately through a sparkplug hole....
There's really no substitute for yankin' a head, although the P&G pump can come pretty close in the hands of an experienced operator. I didn't mention that to use either the pump or the beer-can method you must remove the rocker arms first.