JAP prewar 1939 500cc OHV motorcycle engine Good for prewar sports bike or race project – SOLD

JAP prewar 1939 500cc OHV motorcycle engine Good for prewar sports bike or race project – SOLD


JAP prewar 1939 500cc OHV motorcycle engine.

  • Good for prewar sports bike or race project.
  • The engine number begins with KOZ/G.
  • KO is 500cc OHV
  • Z is for Sry Sump
  • /G is 1939.
  • JAP piston.
  • Seems to be STD bore.
  • 5-stud cylinder head (available between 1935-1945 (ish)
  • Good fins
  • All cams and timing gears fitted and clean.


With a little help from some Facebook pals, we have established that this JAP prewar 500cc OHV motorcycle engine is dated to 1939. In JAP’s infinite wisdom, they used the same sequence of date letter codes twice. The first run was from 1920 to 1939. The second run of identical codes ran from 1940 to 1959. 1960 to ’63 was just A,B,C, D.

As this engine design was available between 1939 and 1952, the date code “G” only appeared in 1939, so it’s reasonable to say that is the year of supply.

Additional information has been offered to me since I advertised this. The engine came from a Gloucester (UK) Cotton motorcycle, registered in December 1938. I guess, it must have had a very recently produced engine fitted.

For the most part, the engine is complete. It sucks, squeezes and blows. I haven’t made it bang as it’s missing a few parts such as pushrods, valves and the rocker cap.

I have removed the barrel to show the bore and piston. The piston appears to be STD and the bore looks fairly good. It’s used, but OK. The engine comes with the important original mag chain cover and the magneto mount is intact. The engine case halves have matching numbers and the main engine number is all correct and in the right place. The exhaust threads are OK.

As you can see from the images, the cams and timing gears are all there. The oil pump drive is also there. The parts are in decent condition.

Any issues? Only a little bit of corrosion around one of the lowest case bolts. I have shown this in the pictures. There is also one small chip on the timing side. Again, I have shown it in the images. The head is loose as there are no nuts to fasten it down. There are no valves or pushrods. If you look through the pictures, you will see one of the top of the barrel. There is a small chip on the lip. Luckily, it’s not very deep. Please use the images as part of the description. I might have overlooked something that you spot.

There you have it, one hard-to-find, JAP prewar /1939 500cc OHV motorcycle engine.  It would be good for prewar sports bike or a race project as these JAP engines seem to be used on all sorts of prewar sports bikes. I accept that most prewar motorbikes use the JAP engines with exposed push rod tubes. I think this was a new design for 1939, with the pushrods tubes more enclosed. It also has an air duct at the top of the head. Presumably for cooling as the outlet is at the rear.

Suecia motorcycles, from Sweden, used a JAP 500 OHV engine on the Super Sport models. OK Supreme models were known to use the JAP 496cc OHV engine. Perhaps this engine (with hidden pushrod tubes) came a little too late for that model of motorcycle as I have only seen OK Supreme’s with exposed pushrod tubes. I also understand that the British manufacturer, Montgomery motorcycles, used both 500cc and 600cc motors.

Shipping is charged ON TOP of the price for the item. There are too many variables to include shipping in the checkout, so please contact me before you buy and I will give you a shipping price.

I usually ship to most destinations.

International shipping available. Let me know where you are and I will send you a price for shipping with tracking

Combined shipping available. 

Viewing: Please come and check it over before you buy. It is at PE28, near St Ives in Cambridgeshire.

Collection: If you don’t want it shipped, I have plenty of space. If you can’t collect for a while, it’s not a problem. However, I will need to be paid very soon after the sale. Thanks.

Payment: Good old fashioned money on collection is preferred. I can accept payment through PayPal or by bank transfer as well. I can accept EUROS. Please ask. 

Import Taxes: Please note that IMPORT DUTIES are not something I can control. If you are buying from outside the UK, there is a chance that you will have to pay import taxes. Import taxes vary from country to country. If there are charges to pay, it is the buyer’s responsibility to research the cost and pay the duties. The price that you see on my advert is for the item and not the item plus whatever taxes might be due.

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