Hi,
I'm trying to find out what piston should be fitted to my 1936 model 9. The reason why I ask this as the previous owner has fitted one from a vincent (I believe?) I am therefore unsure what my compression ratio is (or should be). The barrel is back to std bore and the bike runs well,but when riding I find it underpowered and won't go any faster than 45-50mph on full advance.
I would like to source a correct piston for this (or suitable replacement or modified part) that will make my beam go more like it should. Can anyone help with the technical specs / types available, or suggest a suitable supplier?
Jim
My Hepolite piston lists part number 5809 for 500cc models 7,9,12,25,27 and light solo sports covering 1929 to 1938.
CR is given as 6.5:1 and it is domed and 80mm diameter. Compression height is 1 1/2 inches, length 3 1/8 ins, and it uses two 2mm and one 3mm ring. Gudgeon pin is 7/8 ins diameter and length 69.25mm
Vincents use an 84mm piston by the way.
Have you tried Le Hobbs for a piston? I think new ones are being manufactured in Australia.
Vic
The only remanufactured piston that may fit is listed here. Higher compression ratio, it is really for a Model 90.
http://vintagemotorcyclepistons.co.uk/shop/category_12/Pistons---Sunbeam.html?shop_param=cid%3D%26
I understand that a Norton 16H piston can be made to fit, you will need a +.040" to fit the 'Beams 80MM bore. Another possibility is a domed Ariel VH piston, although another small end bush needs to be made (11/16") and the bore taken to 81.8MM which may leave the sleeve a bit thin.
Before doing anything drastic I would ensure there isn't something else preventing your bike from performing.
Graham
I have a NOS Hepolite 5809 piston (+30) in my spares stock if anyone needs any dimensions. My M9 has been fitted with an 84mm Vincent Comet piston which takes the engine out to 544cc. I've measured the combustion chamber volume and calculated the CR to be 6.6 : 1.
Just hope it will do more than 45 - 50mph when it's finished and back on the road.
Cheers
Colin