Seat OD in relation to Valve size

[inline__engine]

when installing new seats? what is relationship between seat insert OD to the valve OD for an alloy head?

e.g. if i want to install a 1.5" valve what is the min seat insert OD i should use?
does it depend on seat material? .e.g. bronze, copper, iron, powder steel?

[modok]

If, and, but
I guess the easiest way to answer that, is discuss what can go wrong.
if you run the valve all the way to the edge, (on intake valves sometimes you can) one possible problem is some heads the seat/guide relationship tends to warp regularly.
It depends very much on the particular head and how hard it's running, but those that do will need some additional margin to allow future valve jobs.

The edge of the seatring will be a sharp corner, sometimes that corner can chip. Iron & sintered seats can be brittle.
Tougher materials like mild stainless, solid cast alloys, bronze, not likely to chip. If around the seatring is supported by iron, that's good, might help that, but If the intake and exhaust seats are touching eachother, or nearly so, that's probably where it's going to chip.

If using a large throat % AND seating on the edge, well that means your seatring must be THIN. A concern in itself. How thin is too thin? I don't really know, but, you don't see many testing that limit with....what would be .075" wall thickness seats.

On the exhuast side, I'd rather NOT be on the edge in any case, especially since I don't believe in small throat % have any benefit on exhaust valves.
.040-.060 larger than the valve seems right IMO, for 1.5" valve

[inline__engine]

thanks, it seems like there is some potential benefit in running it only slightly larger as you don't cut as much of the chamber away so pressure recovery could be better

[modok]

i suppose, but, not necessarily. The seat insert can be match the shape of whatever material you removed exactly, although sometimes it may take tools you don't have...yet

[inline__engine]

i suppose, but, not necessarily. The seat insert can be match the shape of whatever material you removed exactly, although sometimes it may take tools you don't have...yet

take an extreme case if you use a 2" insert for a 1.5" valve. how do you add back the chamber that gets cut away? if you sink the valve far enough then sure theoretically but causes other issues

[modok]

the reason seat inserts are flat on the top, is so you can drive them in with a flat driver.
They do not have to be flat on the top, leave it sticking up, cut the seat angles for the right valve depth, and shape to fit the chamber after it is installed.