Should valve & seat angles be equal or interference?

[bamfordsgarage]

We have embarked on an in-car valve grind for my '47 D25. Two exhaust valves were burned and needed replacement.

 

In reading up on the procedure, I find that there are two typical 'fits' between the valve and seat. One technique specifies an equal 45° angle for both components, the other specifies a slightly steeper 46° angle on the seat, the result of which is a slight interference fit.

 

The drawing below from a 1970s auto-mechanics textbook illustrates both: A for equal angle, B for interference. Both are deemed OK, and the text instructs to follow the manufacturer's recommendation.

 

I haven't found a Mopar recommendation for this fit. My factory workshop manual specifies the valve angle at 45° but is silent about the seat.

My inclination is to go for the interference fit, but would like to know first what the factory recommended, and to hear learned and other opinions from the forum.  Thanks in advance!

 

 

 

[Dodgeb4ya]

I have always ground the flathead six seats @45 degree's...seat width .060"- .080".

Three way grinding as required depending on how far the seats have been sunk.

Guides need to be checked for wear,

new valve's being installed also.

I use a Sioux seat grinder.

 

[Sniper]

Some say difference in angles tends to narrow the area where the seat and valve touch, limiting heat transfer.  But other say that having that slight difference helps "cut thru" any carbon build up on the seat keeping it working longer.

 

Myself, I wouldn't spend the extra money on another cutter/stone to get a 1 degree difference.  If you are going to get that far into the weeds on valve angles you'd be doing at least a three angle job too and now we are talking about a lot of cutters/stones to do all this.  If you have the stuff already and want to experiment, well you probably know more than I do then, lol.

[kencombs]

IMO, the 1deg difference is the way to go.   Narrow the seat from both directions as needed to place the contact in the middle of the valve face.  After doing that and a light lap to indicate proper seating all around and the difference is usually gone.  The seat is so narrow and the angle so small, the lapping removes almost all of the 'gap'.

 

Using stones as the tool makes it easy as the stone dresser will allow you to change the same stone from 45 to 46 and reverse.  Even 45 to 30 if needed.   Just do that carefully as it easy to knock the diamond off the dresser if you try to remove too much at once.

[Dave72dt]

I've done it both ways and haven't found any discernable reasons to prefer one way over the other on any stock engine and I'm not taking something apart at regular intervals for comparisons out of curiosity.  To ensure valve seating, I always lapped them.  When most of your work is customer based, you do what you can to eliminate comebacks.  There's no profit in do overs.

[bamfordsgarage]

Thanks all for your comments. We’re going to grind the valves at 44 and the seats at 45. And, probably, finish up with a light lap. 
 

The borrowed equipment I’m using has a good selection of stones of various angles.