cylinder bore measurement

[RichardL]

First, thanks to Drew for starting this topic that I am about to hijack.

Second, another story of my embarrassing screw-ups. (Ya know, after I do everything possible wrong at least once, I might actually become a good A10 mechanic.) After my last crankcase splitting, which was brought on by finding shim bits in the sump, the bike was running great until I got it up to 80 mph. After limping home (not repeating that story here) enough was weird that I pulled the top off again to find some new black rub zones under the right piston oil scraper, signs of pinging on the top of the right piston and stiffness in the unbushed small end of the right billet rod. I filed down the raised ridges in the piston rub marks, honed the cylinders a bit (the right a bit more than left, trying to get to 0.005" clearance) and gave the rod small end a very mild hone. Went to put the barrels back on and here, I believe, is where the latest problem arose. I broke the top compression ring on the right  piston. Well, no problem, me thinks, having a spare set of rings, albeit cheap ones, that originally came with the pistons. So, I'm thinking, I'll just replace the top compression ring on both pistons, not wanting to abandon the remaining NOS Hepolites. Bad idea! Hello! You just honed out the bores to a larger diameter than the oil scraper and second compression ring were formed to. This was realized after putting it all back together with really marginal compression and oil obviously getting in the right bore.

Third, pull it apart again to see if the 60-over bores are salvageable. Obviously, this is preferable to me over sleeves and new pistons, even if I wanted and could find 80-over pistons. This brought me to needing accurate bore measurements, not wanting to throw away money on new rings if slap was going to be horrible. Having now used my new and cheap bore gauge I can tell you that I really like it (but not setting myself up as the ultimate judge of such things). I find it very easy to calibrate against my micrometer held in a vise, then find the measurements in the bore very easy and quick to take, and with repeatable results. 

Fourth, below are the results with the clearances calculated. This brings me to my question. From these measurements, it seems to me that for most of the stroke the bottom of the skirt is below ring travel and is, therefore, maintaining clearance (talking about the right piston) of 0.0047" until it gets into the ring travel area where it jumps to 0.0059" (Of course, my accuracy is not +/- 0.0001", but let's say, for laughs,  +/- 0.00025") With the snug clearance above the rings providing stability for the top of the piston, and with the small zone where the bottom of the skirt enters the ring travel area (resulting in than 0.0059" at the bottom), it seems to me that slap should be fairly well controlled or, at least, still within a tolerable amount. Now, my friends, any objections to putting it back together with a nice new set of rings and the mildest possible touchup honing? 

Richard L.

[muskrat]

"micrometer held in a vise"

[RichardL]

Between a thick towel and a very gentle tightening of the vise (like I saw on YouTube). So, Muskrat, I was hoping you would chime in so that I can hold you completely responsible (hey, there is whole continent and a large ocean providing cover.) I was doing some searching in the forum for opinions on maximum clearance and found a post where you said 0.006". I'm pretty close to that and inclined to run with a new set of rings, rather than jumping into a much bigger and more expensive fix, for now. It seems to me that what I have for clearance is certainly not enough to shake the engine into additional damage.

What d'you say?

Richard L.

[chaterlea25]

Hi Richard,
Could you measure the bore again about 3/4in. down from the top?
This is usually where maximum wear occurs
A quick and dirty method to check bore wear can often be to place a piston ring at different places in the bore
and check the end gap's (keep the ring square to the bore)
divide difference between smallest and biggest readings by pi (3.142) = wear

If you are still at the 0.006 max piston to bore gap I would run wih new rings and see how it goes

Regards
John

[muskrat]

 G'day Richard.
 As John said, go with new rings. Gaped correctly at the least worn part of the bore and remove the ridge at the top of the bore.
For my race motors 6-8thou new piston/new bore clearance, but they only had to do 100 miles very quickly.
John, I'd forgotten that old method. Needs good "feel" of the feeler gauges.
Cheers

[RichardL]

John and Muskrat,

Oh my God! (Maybe too old for "OMG!") I must be delusional. When reading my micrometer for OD at the top of the piston I read what I wanted, not what was on the mic. Specifically, I ignored the fact that I was one graduation on the main scale closer to the jaws than I thought. So the piston is 0.025" smaller than I said. This makes so much more sense, considering the obvious gap between piston and cylinder wall at the top. I think this is normal and I can blow it off on the basis that rings provide stability for the top of the piston and skirt provides stability at the bottom. Right?

So, I tried John's ring gap method and it was very interesting. It turned out that 3/4" down the bore was not the largest diameter, but this is probably because there really was not a valid break-in period since the last honing.  Anyway, by John's method. the worst case clearance calculates to 0.0055". Assuming reality is somewhere between 0.0055" and 0.006" (or, even, 0.00625", allowing for measuring tolerance) I feel like I'm good to go, but interested in anything you might add. 

Thanks, very much, for your help and for letting me blather on about this.

Richard L.

[chaterlea25]

Hi Richard,
The top lands of the piston are stepped down in diamater purposely as the top of the piston gets so much hotter than the skirt, ignore this size for any meaningful measurements
I would build the engine with the new rings

HTH
John