classic motorcycle forum

In broad terms an alloy Villiers piston is 1/3 the weight of its cast iron equivalent. Thus the engine runs more smoothly as there is so much less weight moving up and down the cylinder. Further to this it takes much less power to move the lighter piston in the same fashion so the extra power required to move the heavy cast iron piston is now available for the rider to take advantage of. Finally the alloy piston disperses heat better than cast iron so the engine can be worked a little harder with no detriment.

So the upshot is the same engine with an alloy piston runs more smoothly, revs more willingly, is a little nippier and stays a little cooler.

Cast iron has its virtues. Given care a cast iron piston will last indefinitely running in a cast iron cylinder. It is easy to work and requires little if any heat treatment. It is also cheap. In some circumstances an operator may value these virtues above those offered by an alloy piston.

Can’t help with the second Mk XII-C engine. My information is based on records of unique Australian motorcycles, not all motorcycle makes. In general most of these minor makes started to appear here in the mid thirties and quickly disappeared when war was declared. None survived WWII. Numbers start around the 6500  and end around 11000 in 1940.
Calculating the Villiers engine approximate year of production from these records was a serendipitous by product of other efforts. It was not my intention nor as I keep saying, does it serve any practical purpose.

As far as your engine stands it is what in my apprenticeship days we called a ‘short’ engine. Not a complete unit and would be used as an exchange, refitting all the essential fuel, ignition and drive components already present from the receiving machine. On its own it is only one sub assembly of a Villiers power unit.

Curiously, the Mk XII-C cast iron piston is well catered for. F W Thornton of Shrewsbury list all sizes and oversizes on their website. It’s only the favoured alloy versions that are hard to find. It seems I’m not the only one to choose alloy pistons over cast iron. Its not a genius decision.

Would you be able to date this engine as well?

I do have a used cast iron piston at 54mm without rings which is for sale.
Please drop me a message including an offer.

Thanks all!

I already noticed these paired numbers.

Villiers made no distinction between any of the metals used in their flywheel magneto’s. What sort of aluminium alloy it is I have no idea. It certainly made no difference to performance. Personally I suspect them to be more of a zinc/aluminium alloy rather than straight aluminium. Although I’ve never weighed one they don’t feel any lighter. I suspect Villiers were just after a cheaper non-magnetic material for their flywheels and eventually concluded that it was simpler and easier to continue with brass. There’s certainly no logic to fitting a lightweight flywheel to a basic “cooking” model XII-C is there.