G 12 CSR Pistons help..please

[maurizio66]

Ciao a tutti,

after the blockage of a stupid non retourn oil valve, fitted from the previous owner on my Matchless G 12 CSR 650 year 1966, as in one of my last post I decided to check the pistons as I have some smoke and oil leakage trough left cylinder and head, once removed cylinders I found a couple of JP 0633 + 020 they are like new but the left one is sized and the valve touch the piston as I have some slight signs, probably I have a copper gasket too thin. The cylinder bore seems to be made recently, no signs of consumption apart the sized area with the piston. I put some gasoline in the removed head and it pass trough the valves.

I read that are available Gandini or JP pistons but both are heavier than original and need a generous clearance, I would like your suggestions about this pistons and if there are other brand on the market.

Please can you let me know which is the recommended clearance between cylinder and pistons as I want to check if the bore of my cylinder was right for the JP I mounted?

One more question, is possible to fit on the valve stem an o ring or somethingh like to reduce the possibility of oil cooling down the pistons?

In my area there are no mechanical for vintage engines, I'm well equipped with tools and I will try to do the job with a friend, we do the same with his Triumph T 120 650 and is running great, but I hope in your help for this adventure



Thank you all, Kind regards, Maurizio

[SPRIDDLER]

Hi Maurizio
I'm surprised that no-one has replied so far.
You have many questions and I have never had a twin cylinder motorbike so I am not really qualified to give you perfect information, but just to help you get started, and this is my opinion only.........

1. I assume you mean the piston has at some time in the past seized (you wrote sized) . This does not mean that you must replace it as you can (depending upon how much the piston is damaged) just clean up any scuff marks. Slight marking on the piston crown can be ignored.
I would clean (polish) up the piston skirt (sides), fit new rings and have the cylinder bores honed.

I believe that when JP pistons are supplied they do not come with any recommendations about the clearance necessary. My generous rule of thumb (in Imperial measurement) is to allow 1.5 thou (thousandths of an inch) multiplied by the bore diameter (in BREXIT inches ), plus 0.5 thou. This perhaps more than some would say but I prefer to risk a bit of piston slap when the engine is cold than to have the engine seize when many miles from home and in any case I have only had unstressed single cylinder engines, not high performance CSR twins.

You will have to decide by observation and measurement if you need a thicker cylinder head gasket.

Obviously the valves must be ground to a leak-free fit. Check if they have been damaged by contact with the piston

If the valve guides are in good condition it should not be necessary to fit oil seals to the stems. I do not know how the valves in a twin engine are lubricated but they must have some lubrication.

Hopefully my comments will trigger others to agree, disagree or help.

Oh, and get rid of the non-return valve!

[maurizio66]

Thank you Spriddler for your helpful informations, meabe someone bougth pistons on the this site and can provide some more info about their weight, kind of gasket to use etc..

regards, Maurizio

[JEAN-NOEL]

Hi Maurizio,
I fitted 20 years ago JP autralian pistons on my G12 CSR. A lot heavier than original, and gudgeon pins were tight in the conrods. But I think that JP now are better and lighter than the old ones. T & L sells now JP pistons and they have no problems. But I have fitted new GP (GANDINI) for my last major overhaul. (From AMOC SPARES) Clearance : Derrick, from T & L applied 6 thou (15/100°). I have not yet ran this engine. Just a thought : if one of your pistons had seized, how are the plain bearings ? You can buy head gaskets and others from AMOC SPARES. Copper ones are not available for 1964-on engines, with 3/8 head studs. Interference valve/piston : your valve could be out of true.
Best regards.
J.N.

[maurizio66]

Hi Jean Noel,

probably I will go with GPM even if 6 thou seem to be a lot of clearance but meabe take out other problems, I read about other people from fresh rebuild had problem with GPM pistons too. Do you know if JP produce new lighter pistons for the twins as I read that they are available for single. I would like to see T & L site but I can't find, do you have a link?

Kind Regards, Maurizio

[JEAN-NOEL]

www.tandlenginnering.co.uk
They advertise in Jampot Magazine
Regards

[SPRIDDLER]

This is the T&L website address below, but I don't think it will help much, except perhaps to email them your question.

http://vintage-engine.net/

[JEAN-NOEL]

About of pistons weight :

Original ALCAN Std 650 60-on 8.5 CR = 386 gr (with pins, clips, and rings).
J.P. AUSTRALIAN + 0.40 = 461 gr.

I don't remember the weight of my GPM 8.5 CR +0.40 : they are in the engine, now !

[Groily]

And my JP +20s were +/-468g with pins etc. (Which was within 30g of the previously fitted (but std bore) Hepolites as I recall.)
T&L supplied the JPs, bored the cylinders to suit and dynamically balanced on a prototype new Club crank with new rods, and all has been very well for 3 years + so far . . . Long may it continue!

[moley]

JP's do come with a recommended clearance, big a4 sheet in box !! nAndy