Valve length

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Mr Paul
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Joined: Sun Oct 05, 2014 10:44 am
Location: Devon, UK

Valve length

Post by Mr Paul »

Hello, my first posting on this forum, and hopefully an easy question.
I'm replacing the valves and guides in the cylinder head on my 1959 G12 as they are overly worn. All was OK until I compared the length of the new valves I just bought with the old ones which were taken out today.
length of new valves = 4.155 inch (105.5mm)
length of old valves = 4.490 inch (114mm)
I know the engines changed a lot about this time - my engine is stamped 59 and has eight fins on the barrel. Do I need to 1960 valves to get the same length as the ones that came out - or has someone put too long valves in and I should use the new short ones?
I've searched this forum for an answer but found none. Pic of the valves below, new ones in the middle.
Cheers, Paul
IMG_2465.JPG
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1608
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Location: ESSEX UK

Re: Valve length

Post by 1608 »

I believe the 8 fin barrel you have is the later type with the additional cooling fins below the exh port, yes? in that case you've probably been given the shorter valves for the 600 or even 500 twin. Get the same as the old ones came out. You can't go wrong then. The 59 barrels are the same in height as the 60 but without the additional fins. The change over was 59/60. It is possible that a po has put later barrels and heads on an earlier 59 engine.
in case you are wondering the early 650 barrels have a different size spigot that wont fit a later head, and the later head had modified induction ports and combustion chamber etc. Head and barrels can only be swapped as a complete set.
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ajscomboman
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Location: HAMPSHIRE UK

Re: Valve length

Post by ajscomboman »

1959 650's use the same valves as the later 600's and the barrels of any 650's have no extra fins only the top spigot differs from 59 models to the 60 models which are larger. The heads however on the 60 models do have the extra fins.
g80csp11
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Joined: Mon Jan 01, 1990 12:00 am
Location: WEST SUSSEX UK

Re: Valve length

Post by g80csp11 »

Check out the G&S valve catalogue , they manufacture many if not all the aftermarket valves and include in the catalogue details of the sizes
not looked up for yours but worth checking

http://www.gsvalves.co.uk/assets/vintag ... alogue.pdf
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Stuoyb
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Location: BEDFORDSHIRE UK

Re: Valve length

Post by Stuoyb »

Parts in stock at Kettering would indicate the shorter valve is the '59 only and the longer one is the '60 onwards.
Mr Paul
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Posts: 3
Joined: Sun Oct 05, 2014 10:44 am
Location: Devon, UK

Re: Valve length

Post by Mr Paul »

Thanks guys, as you rightly diagnosed it turns out i have a 1959 engine with the 1960 barrels and heads. Correct parts ordered, and i'm now a wiser person. :)
Cheers, Paul.
Mr Paul
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Posts: 3
Joined: Sun Oct 05, 2014 10:44 am
Location: Devon, UK

Re: Valve length

Post by Mr Paul »

Bike now running well, and back on the road, so I just want to say thanks to club spares who were excellent in supplying and taking bits back, and always super fast in dispatching parts.
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