Valve v's Seat Geometry

[bletch23]

Hello Guy,
I was browsing an engineering catalogue yesterday. One of the sections, dealing with valve seat cutting tools, seemed to suggest that for a valve angle of 45 degrees, the valve seat should be cut to 46 degrees. The idea of introducing the geometry mismatch is to increase pressure at the contact point and therefore improve sealing. I'm perplexed. Does this seem sensible? I thought the key to valve/seat sealing was perfect concentricity and accurately cut parallel valve/seat faces. Or have I completely failed to grasp a fundamental premise?
Your thoughts much appreciated.
Rgds
John B

[SirYun]

Hi,

That is called differential valve angles, idea being that it improves sealing. However it is mostly used for stock overhauling/rebuilding purposes not performance use. I rather just use 45 degrees and lap it in (as i use hand tools).

The whole subject of valve seat cutting (working of the premise that you have actually found a geometry that works air flow wise)is one of many many opinions/beliefs and not too much data. A whole lot of questions can be posed that will give you a headache:

If you cut a perfectly concentric seat on a cold head will it still be like that when torqued down and hot when in use? Some folks have gone as far as cutting the seats while the head is fitted to a very sturdy torque plate while being brought to working temperature by circulating hot coolant through the head. overkill? maybe.

my opinion: try your best with the available means.. a lot of it is down to the operator in the end, so working harder will get better results.

[Guy Croft]

I have used it in the past, some manufacturers do quote 'interference' angles.

I don't use it these days, doesn't seem to make any difference, just makes lapping, er, rather 'unsatisfactory'.

GC

[bletch23]

Hi,
Thank you for the explanations.
Much appreciated.
Rgds
John B