Dave, thanks for your thoughts and for all your other good contributions to
this list.
I am not an expert on engines. All I know is that when I got my head back
from the shop, the valve face was no longer flush with the surface of the
head, as it had been before. I have never even looked at another engine's
valve seating except the one I was shown and my own. I realize there could be
significant differences between a Healey head and a Chevy, but the guy who
built mine has seen a bunch of engines, so I trust his judgement. The cost
of the no-lead mods was insignificant compared to the total cost of the
rebuild, so I was happy to pay it for the peace of mind. I have more than
40,000 miles on a non-leaking engine, so I am very happy with his work and
have just let him do a 1275 Midget engine for me (with no-lead mods).
As I understand the theory, it's the slight but constant transfer of metal
between the valve and seat as the valve slams into the seat that causes "valve
recession". The lead additives in gasoline provided some measure of
cushioning effect between the valve seat and the valve mating surfaces. But
then, I could be wrong.
Cheers!
Steve
----- Original Message -----
From: Dave & Marlene
To: BJ8Healeys
Cc: 'healey list'
Sent: Monday, May 03, 2004 8:29 PM
Subject: Re: bj8 restoration
Steve,
A couple of thoughts;
The height of a valve head above it's surrounding surface is determined
by original machined depth of the seat, & the valve head margin. (extra
distance between the valve seating angled cut & the top of the valve).
Different factories & rebuilders use different dimensions & some valves
have more margin than others to start with. This margin is intended to
allow for a clean up valve reface or two before the margin becomes nil &
the sharp edge starts burning. The seat also has dimensional allowances
for regrinding, which deepens the seat. Without knowing the original
projection (height) of your valve tops before the "test period" it is
impossible to say how much valve seat recession you might have had.
Certainly comparing your head to a different head would give no real
evidence either way.
Valve seat & valve face pitting is a natural result of high temperature
metal oxidation. (Flame erosion) It has no effect on valve sealing
unless it is really severe. Lead has no effect on pitting. Pitting can
be reduced by using a more high temp. oxidation resistant material for
the valves & seats. Some racers use the more exotic valve & seat
materials, but they are running for extended periods at very high engine
loads & exhaust valve temperatures.
It is always to the shops financial advantage to do as much work as
possible.
Dave Russell
BN2
|