Exhaust valve seat again G11CS

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Exhaust valve seat again G11CS

Postby clive » Mon Dec 01, 2014 8:52 pm

Well I seem to have rotated the exhaust valve seat again, although on the other cylinderhead (58 G11CS) :headbang: . I'm wondering what might be causing this, other than a heavy throttle hand. The engine does seem to run a bit hot with just a touch of bluing on the exhaust pipes just as they leave the head (no more than a 20 pence size blue on either side). I have set the ignition by the book (7/16 before TDC if I remember correctly) using a electronic gizmo so I am pretty sure its accurate. However should I be advancing or retarding the ignition slightly to stop it over heating? (I am hoping a fellow member will be able to do the same trick he did on the other valve seat and to back on the road again soon). Advice please
clive
if it ain't broke don't fix

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Re: Exhaust valve seat again G11CS

Postby g80csp11 » Mon Dec 01, 2014 9:21 pm

advance will cool down the exhaust , but risk of detonation if running too weak
if the mixture is weak with the correct timing that will also run hot

timing should be 3/8" for magneto twin according to my book for road models but 0.5" fro sports twin in 57
perhaps too retarded after all - worth checking do you have high compression ratio ?
not very clear in the comp supplement on Christians archieves
Last edited by g80csp11 on Mon Dec 01, 2014 9:32 pm, edited 1 time in total.

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Re: Exhaust valve seat again G11CS

Postby REW » Mon Dec 01, 2014 9:29 pm

To avoid bluing of exhausts due to running a touch warm, some of my chums with other makes of twins richen their carb settings slightly. This also, if not overdone, might avoid other nasties (heads getting too hot and loosing grip on a valve seat perhaps?). I see many twins of all makes that get ridden (as opposed to shown off) and do not have blue exhausts so I suppose it can be avoided without other harm being caused.
Ron

1951 Matchless G3L, (thumping round the Durham Dales) isolating in the garage 2m from the Ajay

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Re: Exhaust valve seat again G11CS

Postby Duncan » Mon Dec 01, 2014 10:29 pm

Clive

I have had seats loosen on both singles and twins, I did think it may be a bit of an age thing although if you look at the valve seats in late twins they have about 5 “castellation’s” that stop them from turning so perhaps it has been a problem for a while?

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Re: Exhaust valve seat again G11CS

Postby Rob Harknett » Mon Dec 01, 2014 11:11 pm

I recall Jampot at Pateley Bridge, a Herts member limped in on his G80 combo with compression loss. It was discovered valve seats he put in had rotated. Chris Saggers asked me where he put his head, when he was away from his tent. I found the head in his tent, Chris put it on the Bar - B - Q . when hot enough Chris got the valve seats correct by pouring in water, then rotating the valve seats until they did not leak. He then peened all round with a punch. They never rotated again.

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Re: Exhaust valve seat again G11CS

Postby Duncan » Mon Dec 01, 2014 11:45 pm

Rob

One of my failures was down to a previous owner peening the seats in, personally I would only ever recommend it as a get you home fix as in my experience it will eventually fail again.

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Re: Exhaust valve seat again G11CS

Postby clive » Mon Dec 01, 2014 11:46 pm

Thanks for the replies. We considered peening round the valve seat on the other head, but the 58CS head has an enlarged inlet track (or at least mine does compared to a standard 600 head) which leaves the thickness of the valve seat rather reduced making peening look a bit risky. A fellow West London member sorted out the other head by turning the seat to realign it and then drilling and tapping into the head through the side of the valve seat. a 2BA screw was then turned into the head and the end cut off when really tight then peened the stud. I will be asking his help on this one too.
I just wondered with the bluing of the pipe (most of the curve is a nice straw colour) if I was getting the timing wrong. The pistons are I think possibly higher compression. If this happens regularly it may simply be age in which case I hope the fix will sort it. I do realise that I give my bike some stick but prefer to do that and fix it than plod around. I will check what advance I have it set on, and the main jet (I think it is a 270 rather than 280 as I have an air filter fitted maybe I should change it back to 280). Many thanks
clive
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Re: Exhaust valve seat again G11CS

Postby Les P H » Tue Dec 02, 2014 12:19 am

Would it be possible to get an oversize seat made up for you they are usually made of aluminium bronze? Otherwise it is possible to copper plate the outer diameter and machine back to give the required interference fit...pressed in with the head heated. Centre punching or screws is not going to ensure a gas tight seal around the outer diameter of the seat and these methods seem absolute bodges to me....Les

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Re: Exhaust valve seat again G11CS

Postby Duncan » Tue Dec 02, 2014 12:30 am

Les

The seats are not straight along their back but look a bit like a straight sided hour glass (at least the one I have removed from a single head was), this prevents them from falling so removal means overcutting and making a new seat which can drop if not secured, you are also reliant on there being enough meat left in the head to hold it steady.

Coincidentally some people who have lead free seats fitted do not realise that some companies put an insert in the original seat as opposed to fitting completely new seats so they could still suffer this type of failure.

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Re: Exhaust valve seat again G11CS

Postby Les P H » Tue Dec 02, 2014 8:48 am

Thanks for that information Duncan, I didn't know that. Perhaps a taper threaded version needs to me made up. It would need to have say 4 small holes in the outer facing ring to be driven in by a peg type tubular driver...If this new seat was driven in hard with the head heated to 250 C I reckon it would stay put....Les

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