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MG MGA - Valve clearance on block

Wonder if someone can offer some advise.
I have a MGA with an old Riley 1622 engine and MGB head. Just had the exhaust valve seats unleaded. From what I calculate, there will be say 10 thou clearance between the exhaust valve head and the top of the block, when the valve is fully open. With a head gasket in place this will increase by the thickness of the gasket, to say 50 thou.
Is this acceptable, or do I need to grind small eyebrows in the top of the block. If so what depth of eyebrow.
The situation on the inlet valve would appear better with say 40 thou clearance without a head gasket.
Interested to hear what people think.
Regards
Henry
H BUCKELDEE

Too close. If valve lash disappears for any reason you're screwed. You need eyebrows in the block for the exhaust vales.

When measuring clearance of moving parts, always do it with zero valve lash. Allow .060" min clearance for intake valve, .080" min clearance for exhaust valve, and .100" min clearance between any valve and piston at point of closest approach. For an MG pushrod engine the piston never gets close to a valve at full lift. But the exhaust valves do overlap above the side of the cylinder bores (intake valves too on the 1500 engine).
Barney Gaylord

Henry, this should not be necessary unless the head has been planed too much, or you have a higher lift cam than standard MGA, or you have one particular model of MGB head - I forget which. How have you calculated the clearance?
Art Pearse

The head I believe is from an Austin 1800S or 18V engine. Casting number 12H2595. Combustion chamber volume 38cc approx. But the Riley engine still has its original cam with 0.312" lift at valve, if my measurements are correct.
Barney-By valve lash do you mean valve clearance at the tappets.
The clearance figures you quote(.060 and .080")I assume are with a head gasket in place.
H BUCKELDEE

The MGB 18V engine in North America 1972 to 1974 had the "L" head, 38cc chamber where other MGB had 45cc. The physical configuration of the "L" head is similar to a standard "18" head that has been shaved enough to reduce the chamber volume (and it has one size larger intake valves). That's why the valves are so close to the block. For this engine the factory installed eyebrows in the block as standard issue.

Yes, adjust valve clearance to zero before you measure valve lift. It is possible for this condition to occur in a running engine, and you don't want to bend the valves if it does. And yes, measure with head gasket in place.
Barney Gaylord

Many thanks Barney for that. This site always comes up with the answers for my MGA or TD.
All I need now is a suitable router and grinding wheel, unless there is a easier way.
The engine by the way has been running like this, with no eyebrows, for years by PO's. More luck than good judgement perhaps.
H BUCKELDEE

This thread was discussed between 26/02/2011 and 27/02/2011

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