CSR High comp pistons worth the trouble?

[Andy G]

Having enjoyed a summer on my lockdown rebuild, a 1962 31CSR, I just couldn't resist taking the top end off to see if everything was behaving itself.
I know...the devil finds work for idle hands!

I bought the bike as an abandoned project, tho some work had been done including +20 rebore and new GPM standard compression pistons 7223.

So, as the engine is already opened up again and with several months of cold weather to come....
Is it worth my while to fit the high comp pistons it should be sporting as a CSR, or would the difference be negligible?

[Groily]

Not 'neglible' I don't think Andy, but not enormous either. I run regularly with several blokes with standard G12s on my Mod 31 with JP 8.5s in it, and although mine is certainly quicker on the acceleration front, for cruising along - or 'bombling contentedly' - doesn't seem to be any real advantage. None of us is in the 'how fast will it go in top' category.
A decent state of tune is probably more important than the cr to be honest, so worth checking jetting etc to see if it's set per spec for what you've got.
So I probably wouldn't do it if the new GPMs you have are a proper fit in the rebored cylinders and all else is well. Rivet counters can't see the pistons after all, but there again . . . you may want the machine to be what it says on the tin! In which case . . . .

[Andy G]

Thanks Groily, I'll confess I'm really in the balance here.

The bike runs very well and arguably the lower tune might be a preservative choice, but I am getting some quite rapid carbon build up and wondering if I am basically "using a racehorse to pull a plough".
And before I start detuning jets to match...

I'll think on. Thanks again.

[Rob Harknett]

I would leave well alone. You could invite problems. I just replaced rings. They nipped up, so put the old rings back again. A G12 CSR has enough power to over take todays traffic, sticking to speed limits.

[Groily]

I think any of us would be in two minds, in the position you're in. Not a terrible dilemma to have, but nevertheless, one to think about certainly!

The carbon build-up could be any of the usual things I suppose - but assuming the rings, valves & guides and seats are good, perhaps worth playing with the carb a bit? Maybe check the pilot jet is per spec, possibly lower the needle a notch, etc etc and maybe, ref large throttle openings, check the main jet? I run on a 450, which has always struck me as gi-normous compared to other machines I have, but is I think right. I don't have an air filter. 380 or 400 come to mind as standard, but could be wrong and can't be wotsitted to look it up just now!
I'm hopeless on detail differences between the versions, but I expect we're running the same later type cams and the valve dimensions and timing are the same AFAIK. With book settings per a CSR (which it isn't of course), my plugs are tan in the middle, blackish on the outer parts, which is as good as I can get them; and the piston crowns are quite clean. Using B7 ES or B8 ES NGKs. (I run my bog standard Mod 20 on B7 or equiv and they last tens of thousands of miles even though that engine doesn't run quite as clean as the 650 for one reason or another.)
Might just also be worth checking ign timing - on both sides? With any luck you'll be within a degree or so of the same on both, but it's not guaranteed.
Be interested to hear what you decide when you've thunk!
Happy New Year.

[Andy G]

A G12 CSR has enough power to over take todays traffic, sticking to speed limits.

Thanks Rob, I'm not actually looking for more speed per se, just felt like it should run with the compression as designed..and I can't resist tinkering!

And thanks Groily, I'm pretty sure the timing is spot on and all jets are as spec, but that doesn't mean they wouldn't both benefit from a spot of adjustment of course.

Sadly club spares has none of the correct +20 pistons in stock. Hypothetically where might I find another supplier.. hypothetically of course!?

[Duncan]

Hi Andy, I am not sure that +20 Hepolite are a direct replacement for +20 GPM as the GMP's usually run with greater clearance.

Red Lizard list JP pistons: https://www.classic-motorcycles.co.uk/c ... b_pid=5970