classic motorcycle forum

Hi Ralf,

I'm sure you're right. The "Y" in this context probably means "no oil box" = "cheaper version". I had a look through the Show Issues of the Motor Cycle in 1927-28, and very few British makers offered sv JAP singles in this era. When they were "serious" bikes, like to 500cc sv HRD, they used engines with oil boxes. I couldn't find a single illustration of a 500+cc JAP single engine without an oil box, but that doesn't mean there weren't any, instead that they were cheaper models, and who wants to see an illustration of a cheap model!!

In the VMMC machine register I found a 1929 Sun with a KY/S engine, here's an loose LY/D (1934) engine https://cars.bonhams.com/auction/23600/lot/131/a-1934-jap-550cc-sidevalve-motorcycle-engine/ , but overall the big oil-box-less sv JAP singles were not common in British bikes.

Fun to learn something new! Do you have a plan for the engine?

Cheers

Leon

Hi Ralf,

The really unusual thing about your engine is that it doesn't have an "oil box" on the crankcase underneath the timing chest. JAP introduced the oil box in 1914 - it uses crankcase pressure and flapper valves to distribute oil around the engine. All large JAP engines after 1914 used the oil box, but not yours. Yet it has the "sports" style cylinder with the separate spark plug hole.

Without an oil box, my guess would be that the engine was for an industrial application (a trolley towing luggage on a railway platform, a concrete mixer, or whatever), but it could also have been supplied to order as a "cheap" motorcycle engine? Sorry I can't be more definitive.

Cheers

Leon

Hi Keith,

No I don't think the "inertia ring" (see collected guff below) has anything to do with it. Sounds like the compression is unnecessarily high? Maybe the head has been shaved, or if it's new perhaps it has a smaller combustion chamber than the original. I bet it would run better and last longer with lower compression... try a thick copper head gasket?

Fitting a decompressor would work too, but I suspect the compression needs to come down a bit too. The Brooklands Villiers in the photo higher up had pretty high compression, but was not hard to kick over. Ditto various Bultacos in my younger days.

Cheers

Leon

"We may mention the provision of a single thin ring, called the inertia ring, above the top of the first pressure ring on the piston. This ring is free to rotate and to move up and down slightly, and it has been found to be very effective in keeping the rings free from gumming deposits."

"The Villiers 346cc long-stroke engine was fitted with a patented inertia ring, which was one of the most important improvements made in two-stroke engine design for many years. The object of the inertia ring was to prevent the gumming up of the ordinary piston rings, and it achieved this object in a very simple manner. In practice, it was found that a film of oil formed above the piston rings gradually became burnt and carboned, eventually fixing the rings solid in their grooves. The inertia ring, which was fitted above the top piston ring, was designed so that it could not touch the cylinder walls. It was permitted to have a slight up and down movement and to rotate freely. This movement prevented any film of oil forming above the piston ring and so kept the rings quite free."

"It is interesting to note that the latest 148 cc. Villiers engine has an inertia ring in the same groove as the upper piston ring. Because of its “floating” the inertia ring scours the top ring and groove and prevents it sticking up for long periods."

"The piston is of cast iron and carries two compression rings; in the upper groove is a special device patented by the makers of this engine, known as an inertia ring. The object of this ring is to prevent the piston rings from sticking in their grooves. This inertia ring is clear of both cylinder and piston walls, and is free to move within its own clearance limits, thus tending to keep the ring and groove clar of carbon deposit."

The drilling in the cylinder between the exhaust ports is for an oil feed.

Or perhaps to get some pressure to drive the weird Villliers oiling system? That would make more sense.

There's another survivor, powered by a 5hp JAP v twin, and with a Jardine gearbox. It's a nice original bike. This is the only one with Mr Whiting's own V4 engine, and his own 2-speed gearbox.

Leon

I'm only slow because I'm spending too much time in the shed doing a 50-year service on my recent acquisition... the clue is that it has more grease points that any bike I've ever encountered.

Leon

Hope your friend wasn't expecting more... a bitza can be fun but very disappointing if it's a surprise.

Leon

Hi Pete,

What a ripper! The main frame is certainly BSA, very likely from a vertical-engined model with the oil in a "sump" out the front, early-mid 1930s. The best plan is to decipher the frame number - on the forged steel backbone somewhere - then have a look at a list of BSA frame numbers to see what comes up. https://www.bsa1930s.nl/engine-and-frame-numbers/

The rest of the bike has been "added" to the older part of the frame. Worth leaving as an interesting mix.

Cheers

Leon

[Ed: took me a couple of goes to spell decipher!]