09-11-2018, 08:02 AM
(This post was last modified: 09-11-2018, 08:41 AM by Bob Culver.)
I dont know what current valves are made of but presumably a modern steel. Sevens were fitted with valves of XB steel at best and materials inferior to that (ie James VK+ later used just for inlets. Valve grinds at 5,000 miles were common.) Many common cars had XB valves to the late 50s but very inferior to modern steels which run the life of the car without attention. XB is magnetic. I would not reuse as exhausts.
Can reface in a lathe but although can set without runout 4 jaw chucks invariably have a degree of non parelleism. Can set up between centres but a lot of mucking about.
Cut with a keen tool, definite continuous cut, cutting oil, slowish speed. On older cars I lap block seats with another valve or a neat fit dummy valve. If the good valve is extensively lapped, steps are formed. (In modern practice and for new cars not done at all)
If narrowing seats in the block beware opening the inner dia. It does not wear smaller.There is no recovery
For the BMC A engine with same size valves, books give clearances typically .0015 to .0025 and .002 to .003 new,
Can reface in a lathe but although can set without runout 4 jaw chucks invariably have a degree of non parelleism. Can set up between centres but a lot of mucking about.
Cut with a keen tool, definite continuous cut, cutting oil, slowish speed. On older cars I lap block seats with another valve or a neat fit dummy valve. If the good valve is extensively lapped, steps are formed. (In modern practice and for new cars not done at all)
If narrowing seats in the block beware opening the inner dia. It does not wear smaller.There is no recovery
For the BMC A engine with same size valves, books give clearances typically .0015 to .0025 and .002 to .003 new,