8BA flathead valve lapping

D

diiulio

Junior Member
I started lapping my valves and as usual, I have a question. How much of an area should have the grind mark on it for the valve seat? Please note my machine shop is flathead savvy and they ground the valve seats, I have hardened seats only in the exhaust. I have new SS valves, stock intake 30 degree and exhaust 45 degree, I gave the shop the valves so they knew what I had. I lapped one side of the engine, intake and exhaust and they all seem to have the same small ring on the valve seat. How much is typical for having grinding marks on it? Also, one of the valves seems to have a little gouge in it that may or may not leak, how do I check this?

This is what it looks like after lapping
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It is an about a 0.111" ring on the valve seat from lapping, about half the area
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Ring on the valve from lapping
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Unlapped vs. lapped valve
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The exhaust is 45 and the intake is 30.

Do I have enough area that is lapped? I want to make sure there is enough contact for a good seal before I start sticking the pistons in.
 
A goal to shoot for is to have the lap marks near as wide as the seat was cut and somewhere close to centered with contact all the way around. The seat angle should match the valve face angle. I can't tell for sure but it looks like there may be a problem with the angles.
 
Some don't lap at all. There are different schools of thought on that.Neway cutters as an example don't lap at all and they work well. Three angle mira cutters fall into that same setup.The lap only tells you where the seat is and really doesn't enhance the sealing if the job was done with great precision. Bluing will tell you where the seat is as well as a felt tip marker.

R
 
Ronnie, can you explain the blurring or marker process so I can check it? Why wouldn't those processes wouldn't lap?
 
The int should be around .060" and the exhaust .075 and be near the top of the valve seat for cooling. Valve faces should have their edges rounded by blendine with 400 grit paper, Sharp edges burn
 
The only reason to lap a valve is to see where the sealing surface is on the valve... Lapping doesn't remove enough material to correct an inaccurately cut seat. I can put a newly ground valve to a seat with finger pressure spin it and see where the outer sealing part of the seat is on the valve.
Most people doing valve work cut the 45 degree seat and a 44 degree valve, so that the initial contact is a knife like edge and seals perfectly and immediately.
Karl
 
Many of the late manufacture valves are stainless steel. The OEM valves for the 8BA were only corrosion resistant on the exhaust valve heads. The stems were generally polished steel but some vendors may have chrome plated them. Some of the newer manufactore valves have chrome plating on the stems but I don't know if they are stainless or not. 45 degrees is the common valve angle on both intake and exhaust valves.

For a long time, jobbers were using exhaust valves for both exhaust and intake so there was no difference. They did the same thing with guides. It was just easier to use all intake guides that to make two different parts. This lead to mechanics putting the seals on the exhaust guide and that will vulcanize the guide in the bore where it makes removal very difficult.

If your valves are 100% stainless steel, I would get the guides machined and fitted with bronze liners to prevent galling of the valve stems. Stainless is bad about galling against other surfaces. This can lead to sticky valves.

Valve seats will quickly get hammered as soon as the first run on the engine so they could even start out with a convex radius on thier faces but that would be flattened out pretty quick. Some British motorcycle engine were made with radiused faces but I always figured just cutting/grinding them to the same angle as the valve face was good enough. Lapping just ensures that the parts fit together well and that the fit is concentric.
 
It depends on who you ordered them from. Speedway sells valve that are .050" larger and have 45 on the ex and 30 on the intake for better breathing. They are probably Stainles steel. I don't like to disagree with Rotor wrench, but. Bronze guids in a flathead is gliding the lilly. Just Knurell them. The knurel holds plenty of oil and if fit tight will seal better.
 
In most modern well equipped shops, valves seats are done on a large valve machine that MACHINES them rather than grind them. When the process is done,
the valve that will go in that hole is inserted and a vacuum is pulled on it. It has to hold for a certain amount of time while observing a gauge.
If you lap that seat afterward, the perfect seal will be destroyed.
If you want to check the location and width of the seat, use prussian blue.
 
Ron beat me to it. I was going to reply this morning but got busy on something else. They probably got the valves from Speedway, int .060 oversize with a 30* seat and the exh are 45* Walt
 
Just got my 8BA running today, I lapped the vales to make sure they were seating. Glad I did because the shop had one that was seat that was oval. I borrowed a stone and finished the job myself. She sure runs sweet.
 
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