Fiat TC valve seat angles

[dafnos]

Hi guy
i'll try to be more precise this time.
when make a reference on angle cut e.g 20-45-70
from which side of the valve you name the cuts first,from the stem side or the side that is in the com.chamber?
and
with 1.5 mm contact cut how much the other cuts must be?

rgrds
dafnos

[Guy Croft]

Hi, thanks.

The first job is always grind or cut the seat to valve angle 45 deg, right out to true valve diameter, then you give the seat a top dress with 15 or 20 deg to make sure the top is concentric. If you don't do this, the contact face can be slightly elliptical and when you are measuring the diameter of the new contact face with the vernier you won't get a true diameter reading. Always measure the diameter at two places 90 deg apart to be sure.

After that it's necessary to go back and do the 45 again because the top cut will have have reduced the diameter. Of course you can do the top cut first but I don't generally because in truing up you can lose the contact face altogether. Then you do the throat correction at 70 deg. The seat should be 1.5-2mm on 43.5mm inlet and 36mm ex, its width is NOT hyper-critical except that if you go too thin it will flow badly.

This extreme limit approach to valve seat work requires a very high level of precision, I always do it by hand with Neway and Peg, and it is quite beyond most shops to do it well.

I have found on some heads that the thing flows better on inlet with sharp edges on the 70 deg grind, but to prove it out each time you need a flowbench.

The OE inlet valve should always have a back grind of 30deg to bring the contact face down to the same width as the the seat or fractionally smaller. Wide seat on the valve seat on narrow seat in the head is a bad flow combo.

You have to remember that opening out the valve seats to true valve diameter, whilst good for valve flow, drops the valves deeper into the head and makes shimming er, less easy..

GC

[dafnos]

thanks guy now i understand the photo helps a lot....
so exept the angle jop,how much importand is the bowl porting
around the valve guide?is it better to give space in that area?should the valve guide be as close to the port roof as possible?
and:
the short radius is it better to open it and smooth it, or just open the port and smooth the s.t.r area?

rgrds
dafnos

[Guy Croft]

Hi,

I won't be giving exact porting info like that on request on the site, you will have to work it out as best you can from the photos and sequences that I post.

GC