500 cyl head on 350

[simonh]

Hello all. My model 16M 1947 has a 500cc cylinder head on it. Was this a performance upgrade in period? I am currently doing a decoke having lost compression and noticed that the valves are the same size. I do have a spare 350 cylinder head, so my question is which cyl head am I better to refurbish. I am going to need a rebore also as I have a good lip at the top of the bore. Currently+20. Bike was running really well until suddenly lost compression. Thanks, Simon

[spookefoote1956]

Should be reet

[clive]

I would expect a 500 cylinderhead on a 350 barrel to result in less compression. But the inlet valves are bigger. If you are going to change back to the 350 head then you will need the smaller bore exhaust and silencer fitted to the 350 unless some adaptation has been carried out to the 500 head and you are still using 350 bore stuff. .

[Expat]

Hello all. My model 16M 1947 has a 500cc cylinder head on it. Was this a performance upgrade in period? I am currently doing a decoke having lost compression and noticed that the valves are the same size. I do have a spare 350 cylinder head, so my question is which cyl head am I better to refurbish. I am going to need a rebore also as I have a good lip at the top of the bore. Currently+20. Bike was running really well until suddenly lost compression. Thanks, Simon

Hi Simon,
My ‘60 350 G3 suddenly lost compression too and a subsequent strip down showed the exhaust valve had excessive burnt oil/carbon buildup on the stem so couldn’t close properly. Piston and bore are standard but well worn with a lip like yours and after getting the bore honed, I’ve pretty much decided to go with reboring 0.020 over with a new piston and rings from the club shop.
Good luck with your 16M.
Steve

[simonh]

Thanks all for comments. It is not a 500cyl head, but at some point it has possibly had a slightly larger valve fitted. The ex valve is a couple of mm smaller than the inlet but looked the same size as it was coked up. The fitted ex valve is 1.2mm bigger than my spare head.The ex valve guide is totally knackered and at the combustion end it is about 1mm larger in diameter than the valve. So I need a new guide and a rebore. Unfortunately the club shop doesn't list a suitable piston so I will need to look around. Does anyone know if a later piston will fit?

[AndrewM]

Does anyone know if a later piston will fit?

I thought the piston changed in 1948 to lower the gudgeon and shorten the connecting rod. I think earlier 16M or G3L pistons are compatible with the 1947 16M

[SPRIDDLER]

I thought the piston changed in 1948 to lower the gudgeon and shorten the connecting rod. I think earlier 16M or G3L pistons are compatible with the 1947 16M

Taken from here:

https://www.jampot.com/article_read.asp?id=28

[simonh]

Well I have deduced that I have a 350 head with a slightly larger exhaust valve with very worn valve guides. My piston bore is 69.72mm at the top ( under the lip) and 69.5 at the bottom. I think the bike has been fitted with a later piston. The part number on it is 10465 +020. Nothing in stock with this part number, but a company in Austria has these pistons in multiple over size variations. They list it a 1950 onwards 16M, another website cross references this part number to a JP0413 piston, the club doesn't have any of these when I search for the number. Can anyone help ID the piston? I will need a+030 of +040, I think. Much appreciated. I have tried to add picture and drawing

[simonh]

IMG_20230422_160800.jpg

Piston picture

[ajscomboman]

Pt No 800313/4 club spares is listed as a 1950 on but I'm certain it is also suitable for 1948 on, plus 40thou JP brand and in stock.