Cutting Valve Seats

[AHM]

Hi All,

Having put new valve seats in the head I now need to cut and lap them. The question is how much do I need to cut?

With the exhaust I believe that I need to cut the seat so that I have full contact with the valve and then lap - is that correct?

For the inlet the new seat has a blended radius rather than chamfer. I have a 45 deg cutter - what should I do?

It is for a road car, so durability and reliability are important.

Thanks,

Simon

[ardee_selby]

Simon,

Have you seen this article?

http://www.lotuselan.net/publish/twink_rebuild_2.shtml

The section on the valve grinding operation is worth a read. Hopefully other folks will be along to add more first hand experience shortly...

1.0 Mine was done by "others" as part of a head rebuild
2.0 I don't think my (early) edition of Miles Wilkins book covers it in any more detail.

HIH

Cheers - Richard

[ardee_selby]

FWIW, have just stumbled across this company and their re-working of a Stromberg head describing the angular transitions they aimed for:

http://www.ikengineering.co.uk/cylinder ... pment.html

Cheers - Richard

[rgh0]

For a standard road engine getting a wide seating contact face on the valve and seat is good for durability as it minimize wear rates and maximise heat transfer from valve to cylinder head. On race engines you aim for the narrowest seat contact possible on the extreme outer edge of the valve and maximum throat size and then cut additional angles to maximise flow. In race engines the seat contact face may be very narrow at 1mm wide for the inlet and and around 2 mm wide on exhaust to handle the extra heat transfer need of the exhaust and the less crtical nature of exhaust port flow. In road engines the contact width will typically be around 3 to 4mm for both.

When cutting the seats in a twin cam the key is to get the valve height correct with the distance of valve tip from cam centre line correct and spring installed height also correct. This takes care these days as head and seats will probably have been modified and you may not also have a standard cam base circle or buckets or spring pockets or springs or valves. This all means that valve seat cutting needs to be done by someone who understand the target you are aiming for and there is no simple single answer that works in every situation.

cheers
Rohan