Exhaust valves

[Tiger]

Hi People
I have two valves which have burned faces.
These are too deep to grind using the normal paste method.
What about refacing on a lathe ??
Regards
            Tim

[bob46320]

Why not ? Just check the stems for diameter and ovality and if within limits (I don't know how much wear is acceptable) re-face on a lathe or grinder.

[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,

[Stuart Giles]

The seating area on an exhaust valve is quite hard, so the usual method of machining is with a valve grinding machine. Most companies who do re-boring crank grinding etc. will have a valve grinding machine. There needs to be enough margin (i.e. thickness) above the re-machined seat on the valve head to prevent the valve distorting in service. In other words if the seat of the valve would make a 45 degree sharp edge with the face of the valve once its machined it really needs to go in the bin.

[Ruairidh Dunford]

I have one of these machines designed to regrind valves. However, new valves are so cheap and readily avaialable, I have never actually used it.

[Chris KC]

Was just going to say, unless you have the means to diy it's probably cheaper / better to buy new valves.

[Stuart Giles]

I have one of these machines designed to regrind valves.  However, new valves are so cheap and readily avaialable, I have never actually used it.

Hi Ruairidh, You're missing out on loads of fun with that machine. Here's mine:



I restored it last winter, the cabinet drawer has all the accessories for the Vibro Centric valve seat grinder which lives in the bottom of the cabinet.

[Ruairidh Dunford]

That looks fabulous Stuart (I would expect nothing less from you)!

My friend has one with the base and attachments like yours - I am saving up for a bigger workshop!

[Zetomagneto]

Why not ?  Just check the stems for diameter and ovality and if within limits (I don't know how much wear is acceptable) re-face on a lathe or grinder.

Why not buy 2 new ones from Tony for a tenner

[Bob Culver]

The valve grind machines have a more sophisticated method of centring the valve and keeping it parallel and steady. Every garage had one but now not wanted.
If valves are seriously burned probably an old style material.