Oversize valves in Twin Cam heads

[mr.vman]

We are in the process of converting a standard head to the large Dave Bean, so called "D production" valves. Intake is, 1.625", the exhaust is, 1.375". Question, will the standard size valve seats work with these large valves? The seats measure, intake; 1.625" Outside diameter (this is the same OD as the large intake valve). Exhaust seat; 1.500" outside diameter, the exhaust valve is, 1.375" OD ( I think this fit will work). The intake valve OD will be close to the the, OD of the seat. Is this going to lead to a problem? In the old David Vizard book, Tuning twin cam Fords, he writes to install this large valve on the orginal seat. I know we will have to open the seat throats (ID's) and blend the ports in. Installing larger, OD seats might be an option but, there appears to be little space between the seats and spark plug threads/hole (in the combustion chamber). The area between the seats and spark plug hole is an easy area where cracks can begin. Also, is there material enough to support the seats? Boring the intake side for a larger seat could break into the spark plug threads/hole, leaving little material to hold the seat, 360 degrees. I had a twin cam with this combination that ran well. I never recorded what valve seat OD's were.
Thanks in Advance, Steve Vukobratovich

[rgh0]

Hi Steve

You can seat the 1.625 valves on the original or a 5 or 10 thou oversize seats OK. You just end up with the seating face on the very outside edge with a 1.625 valve head.

You can actually fit seats up to around 1.7 OD to accomodate the 1.7 inch valve used in a Hart head. This may just break through into the spark plug thread depending on the exact tolerances of the head dimensions you are modifying, I have one of my spare race heads here with exactly that situation in a couple of the cylinders. However due to the angle of the seat versus the spark plug this breakthrough only occurs at the very lip of the seat and plug thread intersection so not a problem. You need to recess the 1.7 inch valves anyhow to avoid a clash on overlap with the exhaust so you can play with the details to avoid a serious clash between the valve seat and plug thread.

The new heads being made now in the US use a 10mm plug to help with this problem and I have also seen an original Lotus head with the plug holes welded up and redrilled for a 10mm plug and the plug hole relocated to clear the valve seats better - dont know what they had inside the head but it must have been big!!!

cheers

Rohan

[john.p.clegg]

Steve

I have 1.625 inlets and 1.4 exhaust valves fitted,they do work,machine the seat insides to the bore you want and make up a mandrel to help with the fitting of them...you can then blend the throats to the seats.

John

[bcmc33]

This is my QED head with 1.625" inlet & 1.4" exhaust valves.