Help identifying pistons

gtiller said:

Back with the bike today, and have had the opportunity to take some proper measurements with some calibrated Mitutoyo kit

Piston measurement at 90 degrees to the gudgeon pin is 73.350mm (2.8880 inch)
Bore measurement (measured fore-aft, about halfway up) is 73.56947mm (2.89305 inch)

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If the piston measurement was taken at the bottom of the skirt, then you have a piston clearance of .005", which should be just fine. Stock recommended clearance is .0037" - .0047" when new, and the pistons collapse a bit (.0005" or so) immediately after breaking in, so you actually have very little wear.

If it was taken a bit higher on the skirt (half way up?) then the clearance is tighter, since clearance is calculated using the diameter near the bottom of the skirt (1/2" up, typically), and the skirt is machined with a taper.

I do have to question how you can measure the bore out to 5 decimal places. Must be a pretty special bore gauge.

All in all, it looks like you have as set of +.020" (or +.5 mm) pistons (hence the 73 and 5 markings) that were slightly undersize, and a bore that was honed for them.

Ken

Thanks Ken, really appreciate the feedback!

Piston measurement was taken at the same height as the centre of the gudgeon pin at 90 degrees to it.

Regards the barrel, it's a Mitutoyo Digital Bore Gauge, I measured once in inches then converted to metric.
Accuracy is rated at ±3µm / ±.00012" - a really nice bit of kit!

your maths is wrong, 0.22mm clearance is nearly 0.009 ''' of an inch, measure the pistons at the biggest part towards the bottom of the skirt front and back

As he measured in inches only the incorrect metric figures give 9 thou and so can be ignored, the 5 thou clearance as per the inch figures are the ones to use.

gtiller said:

Thanks Ken, really appreciate the feedback!

Piston measurement was taken at the same height as the centre of the gudgeon pin at 90 degrees to it.

Regards the barrel, it's a Mitutoyo Digital Bore Gauge, I measured once in inches then converted to metric.
Accuracy is rated at ±3µm / ±.00012" - a really nice bit of kit!

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That's actually pretty good news. If you measure at the proper location, near the bottom of the skirt, 90 degees to the pin, I think you might find that your clearance is within the standard spec. The piston skirt is tapered, and is larger at the bottom than up where you measured it.

Nice bore gauge. I'm used to an old Mitutoyo dial bore gauge that is graduated in .0001" increments, but I've been tempted by the digital ones, particularly after misreading the dial gauge in the wrong direction a couple of times.

Ken