I'd thought that the standard valve seat angles were 30-45-60 but looking at a Subaru workshop manual it appears that the top-cut is 15 degrees, not 30 and there are more than three discrete angles there i.e. two different throat cuts?

My STi3/4 heads are off to have the seats and valves cut to spec, but should I ask for them to be kept at 15-45 for the top cut and contact angles, or get it recut to 30-45-60 if possible? Is there any advantage in one way or the other?

 

I've never bothered with that. I've always just reground the valves into current seats.

 

Well I think they've done the cutting now, so we'll soon see, when I go to collect them. Next problem is equalising the chamber volumes and getting the skim depth right, but that's for another thread!

 

Is the technique of hairline valve seats still used on inlet valves? (higher seating pressure for a given spring force)

JohnD

 

Well the contact widths are 1mm inlet and 1.5mm exhaust so no, not hairline. I suppose they go as narrow as they can whilst still being able to shed valve heat effectively.