650SS inlet valve size

Sounds a bit like somebody has previously modified this head to take larger inlet valves.

The correct valve heads should be 1.406". The 750 heads used 1.50".

There were 4 versions of head used on the 650 engine but all had the same size inlet and exhaust valve heads. The only differences coming after engine 114870 when the rockers became narrower and after 1966 and engine 125871 when the barrel spigots disappeared, the pushrods were shortened and the valve stems were lengthened by about 0.10".

Even the Mercury heads, which are actually Commando castings (060380), used the same 1.406" inlet valves.

I've got a 650 head with almost the same 'problem'. Bigger valves and bronze valve seats. I thought that the 1.500" inlet valves were correct as that is what is quoted in the Haynes manual; obviously another technical error to add to the list. One of the exhaust seats is also bronze but I plan to have both replaced with hardened seats when funds permit.

Does anybody have any experience with bronze seats for road use or should I have all 4 replaced?

Cheers

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Hi

I agree with the thought that the head has been modified the bronze valve seats are non standard ... well from my experience and Les Emery at Norvil all post war twins had hard valve seats not cast iron like Triumphs. My valve seat cutting tools wont touch Norton seats but quite happily chew away at cast iron ones.

I would think they would probably be the biggest that would fit as the edges must be perilously close to the side of the bore if the block has spigotliners projecting above the block face, have they been relieved ? if you do go for replacement big valves check the clearance as it would be a shame to ruin all your good work first time you start it, Fit the head to the block off the rest of the engine without the pistons and a cardboard gasket of the same thickness as a compressed one. Try the valves in the guides with no springs and checkthe clearances. Also don't forget the pistons have a recess that might need making bigger as the valves are still closing at some top dead centrepositions on the suck and blow parts of the 4 stroke cycle.

Gromit

Hi Trevor,

You can see on the photo that I included, that there is a stamping RTS 126 which I assume was done by whoever modified the head. Does yours have a similar stamp and does anybody know who carried out the work?

Cheers

Previously james_brierley wrote:

I've got a 650 head with almost the same 'problem'. Bigger valves and bronze valve seats. I thought that the 1.500" inlet valves were correct as that is what is quoted in the Haynes manual; obviously another technical error to add to the list. One of the exhaust seats is also bronze but I plan to have both replaced with hardened seats when funds permit.

Does anybody have any experience with bronze seats for road use or should I have all 4 replaced?

Cheers

That photo looks just like my Atlas head with 3/8" head bolts and it looks nothing like my 650 head. Now I have to investigate what parts I have and what modifications.

Previously Jonathan Soons wrote:

Previously james_brierley wrote:

I've got a 650 head with almost the same 'problem'. Bigger valves and bronze valve seats. I thought that the 1.500" inlet valves were correct as that is what is quoted in the Haynes manual; obviously another technical error to add to the list. One of the exhaust seats is also bronze but I plan to have both replaced with hardened seats when funds permit.

Does anybody have any experience with bronze seats for road use or should I have all 4 replaced?

Cheers

That photo looks just like my Atlas head with 3/8" head bolts and it looks nothing like my 650 head. Now I have to investigate what parts I have and what modifications.

The easy way to spot an early spigoted 650 head are the concentric spigot recesses around the combustion chamber. The Atlas recesses are eccentric to take into account the cylinder bores being moved rearwards relative to the combustion chambers.