VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Valve lapping

Is it possible to "over lap" a valve? I lapped my valves on my 28 today, and the #1 exhaust valve was difficult. By the time I achieved a full contact ring, the valve had a reses in the face. The valve appears to have full anular contact, just don't like the fact that it has an annual groove in the face of the valve. Is it safe to put the head on he car like this?

I would say it would be safe for limited use. It would be better to have the valve refaced on a regular valve grinder and then lapped in to make sure its seating in all areas.

Lapping is not doing a valve job. Lapping is just for "fine tuning" the valve and seat after grinding. It is also important to have the seat and valve angles correct for proper sealing. This can not be done by lapping.

I agree that valve needs grinding and the seat should be looked at also.

When lapping a valve you are making the contact point between the valve and seat wider the more you lap it.This is not good.There should be an interference angle between the valve and valve seat of 1 degree.This tightens up the final width of the seat and gives a tight seal between valve and seat.

Thanks all. I could not bring myself to put a compromised head on my car. The head and valves are currently at a top-notch engine shop, specializing in valves, heads cranks and cams. Should be ready in 2 to 3 weeks, giving me plenty of time to prep the engine for assembly. Happy engine, happy owner. The shop is Paul's Custom Grinding, family owned since 1947. Plenty of cranks and cams being custom made, no small task.

Beats me how you could have an interference angle of 1 degree between the valve face and the valve seat(unless you have either the valve face angle or valve seat angle ground different to original specs),when both valve face & seat have the same angle-in the case of a 4cyl Chev,both angles are 45 degrees.

45 degree on seat and face is not out of line on these low RPM engines. It was first used in poppet valve steam engines way back in the 1800s. Most of the original valves of the day were two piece with a tool steel stem and a cast iron head. They like a good surface for heat transfer and they seal just fine under the pressures they are exposed to, in fact the cyl pressure acually helps the valve to close tighter if the stem is a bit on the loose side. Most shops I have found try to do modern type work on old heads and sometimes it can cause problems. One I have encountered is the valve stem to guide clearance. Most shops get them too tight and the valve springs aren't that strong to slam them shut like a modern engine, plus the oiling factor. Modern heads get tons of oil through the pushrods to help carry away heat down to the oil pan and keep also things lubed up top. 28 engines don't have that luxury so heat will build up in those parts and has to be carried off by the water in the head but has to transfer through the other parts. Low RPM engines just need to be built a little looser in the stems than modern engines because the stems will expand more than the cast iron guides. Tight guides will tend to become sticky when hot and end up burning some of the exhaust valves. Most shops just don't get that until it is explained to them. If a valve face has one side that takes more gringing to get straight, it was very hot at one time and was warped slightly, possibly because it didn't close quite all the way and have that moment of heat transfer. Just think of a Bi-Metal strip and what happens when it is heated nice and hot and how it bends. An old valve is made from two kinds of metal and can do the same thing. Different angles were introduced to help the flow through the opening and increase power and were only introduced after technologys improved the valve metals to take more heat. they also helped shuck off carbon off the seat. In 1928 gas engines had already been made for about 40 years so a lot of trial and error had already been done up to that point. They knew more back then than we give them credit for. Now if you are building a 28 full race motor forget everything I said.