Hi Nathan,

That's not quite how it works. Remember that at the same time the
heads are being baked to 400F the seats are being chilled to single-
digits. The temperature differential uses the expansion of the two
metals to ensure a tight fit.

But whenever the head heats up to 400F the seats are being heated to
the same temp if not hotter (they are closer to the exhaust gases),
so nothing gets loose.

Happy Trails,

Greg Potts
1973/74/77/79 Westfakia "Bob The Tomato
www.pottsfamily.ca

On 24-Jan-07, at 9:19 PM, Nathaniel Poole wrote:

> What I was told by a reputable VW rebuilder is that you are
> supposed to heat the heads to 400 degrees to press in the new valve
> seats; if the engine gets to that temp while running, you are
> risking the seats becoming loose through expansion and softening of
> the aluminum head. And yet many people talk about running at 450+
> degrees (I have yet to break 350, even on the Coquihalla), and you
> mention 550 as a kind of redline. Any machinists out there? Is
> there truth to this threshold temperature?