Hi,
When I bought a very sad, abused Mk3 Roadster (white with blue and red striping) two years ago and stripped the engine, I found a compression plate under the cylinders, which was a surprise. The bores are still standard with little wear, and the bottom end also shows surprisingly little wear, and all fasteners are original type, so perhaps through some miracle, it was still has never been apart from new?
The bike was UK registered new in 1976, though the VIN plate was a US & Canada type - a cancelled order sold in the UK perhaps? If it had been originally destined for the US, could it have a compression plate to comply with US regulations?
I had a Mk3 Interpol in 1979-85 and that engine was original and undisturbed, and did not have a compression plate - though I believe some Interpols did?
I managed to acquire new, standard Norton Villiers pistons (from Australia!) and am planning to restore the electric start to working condition (more to avoid laughter for it not working than regular use). I am not planning to refit the plate, as I cannot see why it would be needed? A standard compression Mk3 engine is not hard to kick over - maybe it was to assist electric starting (or US regulations)?
Does anybody know why the compression plate may have been fitted originally?
Regards, Andy
I would check for a step
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Low octane fuel.
Hi Andy,
The most likely reason for the decompression plate would be lower octane fuel. Tetra-ethyl lead was added as standard then, but Canada and/or the USA were probably more environmentally conscious than other countries and their octane rating required a lower compression. This would be a much cheaper method than doing it properly by fitting different pistons!
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... at the top of the piston travel, and if there is one there (which is almost inevitable) have it honed out otherwise you'll be plagued with snapped top rings.