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Several months ago I signed up for emails from "Engine Builder". Their focus is primarily for cars, but they often have some universal wisdom to share. Here is a link to one of their stories about the importance of valve seat angles. Fascinating! http://www.enginebuildermag.com/Article ... _flow.aspx

Thanks for that, it is extremely valuable. It is something I'd forgotten about from years ago. A mate of mine used three angles to cut the seats in the head of my short stroke Triumph, and it seemed to go quite a bit quicker. It might have been due to the better sealing, but I think it was OK before the reseating job. I'm going to read the article again and think about the cam and torque characteristics of the 850 commando engine.

I believe Jim does the same thing to the heads he rebuilds.

Interesting article. Axtell used 37.5/45/62.5 degree angles for Norton heads, and blended the top and bottom cuts into the chamber and throat by hand. He did a huge amount of flow bench and dyno testing to come up with those values.

The Norton head is a little difficult to do that sort of valve job on because the edges of the seats are so close to the outside of the combustion chamber that you can't use a single radius machine cutter (Serdi, etc.) to shape the top, particularly on big valve heads, because the combustion chamber curves up steeply as you get to the outside edge of the seat. To get a smooth blend into the combustion chamber requires some hand grinding after the basic angles are cut. You can use a single cutter to get the bottom cut radiused smoothly into the throat, with maybe just a little work with a porting tool to blend in any edge left by the cutter.

Ken