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Discussion Starter · #1 ·
I just measured my cylinder bores with a dial bore gage and I'm getting 2.831" left cylinder and +.0005" larger on the right.
I'm not concerned about the difference but rather the size of the bore themselves.

According to the Clymer manual a .040" over std. bore for a 650cc should be 2.8348"-2.8353"

Note that I have not yet pulled the block off yet to measure the pistons themselves but should I be concerned with the measurement I'm getting? The bores look nice, no scrapes or scuff marks so I'm reluctent to pull the block if I don't have to.
Could use some clarification from you pro's out there. :)
Thanks,
Al
 

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?

that's interesting.

coupla things come to mind:

have you compared your Mikes to a check block?

the best machineists i've had want the pistons before they bore, could you have had some strange pistons that had to be fitted?
 

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Discussion Starter · #3 ·
That's the first thing that came to mind is that I set up the dial bore gage incorrectly so I double checked the setting master and triple checked the bores again with a telescopeing gage.
Same size as with the bore gage.

I plan on pulling the cylinder block off and measure the O.D. of the pistons and check ring gap to see if everything is within spec.
If not I'll just put a 750cc kit on it.
 

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yup!

I see. You'd see if one or the other was off by comparing the two.
They gave you the same readings is what you mean or close enough at any rate.

Bet it's an off stock piston set when you get them out. Makes you wonder how many piston sets are that far off the manual boring sizes. A little time with a new set of pistons and a sunnen hone sould have ya smiling.
 

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The pistons vary in size. A machinist that knows what he's doing won't just bore the cylinder without having the pistons to insure getting the right skirt clearance. The rings come a bit big to allow them to be trimmed to fit after a bore job .
 

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Half a thousandth between bores is not uncommon, especially if the last work done was a simple honing with new rings.

I sure wouldn't be worried, unless the pistons are marred, you are experiencing low compression, and/or smoking.
 

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Discussion Starter · #8 ·
Update

I pulled the block off and took it to work and again measured 2.831
with a SUNNEN dial bore gage in the bores. (no need to put it on the CMM) To make matters even worse the piston O.D.'s are 2.825 max. on one and .8195 on the other. Also found out the tappet od's are out of tolerance too.

No doubt I will rebore/hone to the the proper tolerance and buy a new set of pistons/rings/pins.
What is a good set of pistons to use for these bikes? :confused:
 

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I use JCC which are re-branded as Emgo, Cycle Craft and other names.

Use .0045 clearance on the cylinders, and .010 end gap on the rings. (Take the new pistons to the machine shop BEFORE boring and final honing).
 

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Well... you just did what I was going to post that you do... check piston 'diameter'.
I just rebuilt a Massey Ferguson diesel tractor (also British) :), new pistons, new sleeves etc. The bore dia was a half thousanth under tolerance according to the book .... but the pistons were about a half thousanth under too so I knew I still had proper skirt clearance.

It looks like you are going the right way though... good luck with this...
 

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Discussion Starter · #11 ·
Thanks for all of the input guys! I guess I'm in the "Pay me later" with my bike since I only paid $1550 for her.

Seems like everything I check is out of tolerance or just plain done incorrectly, but I'm confident I'll get it done right and on the road. :)
After all, part of the fun is working on her!

Al
 

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Seems like everything I check is out of tolerance or just plain done incorrectly,
Anything that measures up within the tolerances in General Data in the manual,is as-new.
A part is only new once and the manufacturing tolerances are not wear limits.You can have quite a lot of wear before it causes any problem.No wear limits are given.

A new cylinder bore has a tolerance of 0.0005" on diameter.It will still work fine when it has 0.006" wear/taper.It will measure at standard size at the bottom ,but 0.006" bigger at the top.The upper part of the piston will have about twice as much clearance near TDC as it did when it was new,but it will still work OK.

A new piston ring has 0.010"-0.014" gap,according to the manual.You could file that gap out to 0.060",and for all practical purposes it would run the same.
You'll actually get better results if the second ring has about 30% bigger gap than the top ring.Better power and oil control.
 
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