Friday 5 June 2020

Piston, Rings and Cylinder

Amazingly, after nearly 90 years the bore showed little wear and still had a nominal diameter of 74mm – it had never been rebored. I suspect that the piston was as originally fitted to the engine.

However, there was sufficient corrosion on the surface of the cylinder to warrant a rebore and so the search was on for a suitable replacement piston.

The critical dimensions of the original piston are:

Weight (piston + pin + circlips): 330 gm

Centre of pin to top of crown: 1.625”

Pin diameter: 0.625” (but up to 0.75” would be possible in the connecting rod)

Centre of pin to top of top ring: 15/16”

Centre of pin to top land: 1 5/64" 

At the time of the rebuild, FW Thornton had an AE-manufactured New Imperial piston that would have been just about OK but a further search found a 75mm diameter (equivalent to a +.040” oversize on the original 74mm bore) quality forged Omega piston for a Ducati that fitted the bill perfectly. It has a slightly larger pin diameter and the same weight to within 12 gm The Omega and original piston are shown below for comparison.

The barrel was bored and honed to give 0.0055” clearance on the thrust/antithrust face of the piston at the bottom of the skirt and the rings were gapped at 0.012”

On the subject of setting ring gaps, some years ago I discovered a very useful little jig for removing a few "thou" from the ends of piston rings in a controlled way.

The ring is supported to be at a right angle to the abrasive  wheel and a few turns on the handle is much safer and easier than the alternatives. These are still for sale on ebay under the name “Piston Ring Filer” available from suppliers in the US as of the date of this posting.

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