Britbike forum Forums British Motorcycle specific Brands Ajs & Matchless forum '58 AJS Model 31 650

Hi. Does anyone know of pistons for the above model ? The UK club claims there was no such model, but there definitely was (A friend has one). The club sent him later model 650 pistons, which are entirely different, and could not possibly be modified to fit. They were a US only model, along with the 20 B 550 (I have one of these)
Cheers, Dale.

Hi Dale, I must admit to not hearing of a ‘58 AJS M31, however more than once have heard of the ‘58 Matchless G12, so no reason why the AJS didn’t exist.
I am fairly certain the pistons are the same as the 1959 model which is a domed top piston, this changed in 1960 with the redesigned head. I have seen these earlier pistons come up NOS at times on eBay, though I believe JP Engineering in Australia manufactures both types and they know the difference.
I believe this is their website https://www.norndaautomotive.com.au/jp-pistons/product-information.html .
Goodluck
Adrian

Hi Dale,years ago I also got the same claim from the UK club that this model doesn't exist. I was very lucky a few years ago and acquired one of these "no such model" bikes. It is unrestored and had very low mileage when I got it.I've ridden it about 30,000 miles and it is just great.
The pistons are special as I believe that to make the model 30 a 650(or 31) they stroked it ,so the piston skirts are very short ,and these pistons are hard to get . The Hepolite part number for this series is 15242,with a bore size of 2.8346 or 72mm.
I found a set on Ebay that no one was bidding on as they didn't know what they were for .The 59 pistons are not the same ..
If you want you can email me at [email protected] for more info.I'd post a picture of the bike but I don't know how to do it on this site.
As Matchman says JP might be able to help you out ,maybe by sending them a sample ..

Thanks for the replies. I'll pass the suggestions on to my friend.
Cheers, Dale.