"..the opposite sides of the seat are at very different temperatures (which doesn't happen in an OHV), and distort when hot. One side is released - guess what happens."
You mean it "shouldn't happen", but it can, especially when Pans have hardened seats crushed into the intake. There isn't enough matrix casting left beneath them to draw away heat in the event of advanced timing, vacuum leak, etc. Even Shovel seats suffer, as evidenced by the point of most distortion of a seat that has seen extended duty is nearly always over the port.
Yet another heat transfer problem can exist with a threaded seat, as there is no way to get absolute contact of the two threads, although a conductive potting compound such as "Seal-Lock" can help.
The way to keep a seat insert in place (beyond "clinching",etc.) is to pick a seat material of a similar thermal expansion coefficient as the casting.
Cast iron seats used to be common.
Now even bronze inserts are extinct.
Only "tool steel" seats (Martite, etc.) are readily available, requiring extreme press-fits to stay tight in the bore for heat transfer. And their metallurgy make them the most sensitive to temperature spikes, collapsing with disastrous results.
Last edited by Cotten
on Wed Dec 03, 2008 5:10 am, edited 1 time in total.