piston ajs 250 csr

[maxim jurgens]

A few years ago I fitted a new piston in my , then just-bought, 250 CSR ( engine nnr CSR 14432,what year is that ?).It ran more or less fine but tended to get rather hot . Since then I have hardly used the bike but as i now have a lot of time due to Covid lockdown i'd like to make her properly ok.
Some research shows that there are two types of piston , one for low compression and one for high(er) compression. The latter presumably for the CSR, the former presumably for the non CSR models.At the time I ordered one for a CSR but as compression feels rather low I have a suspicion what I was sent was an "ordinary" one. The two pistons apparently each require a different ignition timing .Questions :
1 How does one recognise the two different pistons ?
2 Is there a way I an determine which one I have without taking the head off ?

many thanks in advance for your reply
Maxim Jurgens
Phuket, Thailand

[56G80S]

The one for the 250 CSR is really quite domed, isn't it something like 10.5:1, with significant cut outs for the valves. I should remember as I dropped a valve on it and fortunately was quick enough of the clutch lever to minimise damage which was welded and then a lot of work by me working on the dome with file, needle files and scraper. I only had a 14CSR but it's piston was a completely different look to my G80S.

I'm sure that someone will be able to give some better advice.

Johnny B

[clanger9]

There are 3 different ones! Normal, late CSR (9.5:1) and CS (10.5:1). Here are the normal and CSR ones side by side:

IMG_2242.jpg

Timing is 1/4" BTDC for the normal piston and 3/16" for the CSR one.
I have a CSR piston in mine and it runs great on super unleaded.
The CS one has a really, really high top and would probably need special fuel.

[56G80S]

Nope, no special fuel although I did use the more expensive petrol. Mine was 1966 (and I wish I'd kept it) and was indeed higher in the crown than the one on the right of your picture.

It was much nippier than they're given credit for and took me and my wife to near Cheltenham (from Teesside) and back. It was additionally burdened by a pair of Roadark panniers full of stuff with Linda hefting an Outwardbound II large rucksack on her back.

Johnny B

[dada]

I have CSR Engine without Piston which piston should I look for ?

[clanger9]

I believe the only piston currently available is the low compression one.

[dada]

Is the cylinder head different than normal ones can u send a picture

[Martin.S]

Cylinder heads are all the same profile - some had bronze coloured seats

[dodger]

I think the CSR cylinder heads have a bigger inlet valve and inlet tract to match a Amal 389 as opposed to a 376 fitted to a standard G2.

[clanger9]

There are approximately two heads: the normal 250 one and the CSR/350.
The CSR/350 one has the bigger inlet valve and port.