Jerry:
I can share my experience here. I asked the machine shop to
check my head, and rebuild as necessary. In addition I asked for
crack checking, shaving the head 0.120", and three angle valve seats.
The machinist said the intake valves and guides were all fine,
so he simply recut the valves and cut 3 angle seats. The exhaust
valves and guides were all out of spec, so the valves were scrapped
and he pressed out the guides and installed bronze guides, and cut
3 angle seats. I supplied new (stock) exhaust valves. I replaced
all of the springs with PI springs, and replaced the entire rocker
assembly.
The valve bronze guides allow tighter tolerances, and
therefore conduct heat away from the exhaust valve more efficiently
and allow the valve to run cooler. For street this is probably
overkill, so if your budget is tight don't feel bad if you
use stock iron guides.
I did NOT install hardened exhaust seats, nor go for larger
exhaust valves. I had no signs of valve seat recession, and I was
trying not to go too crazy on machine work (it adds up fast). If you
have valve seat recession, you will absolutely need to install the
hardened seats. I feel (this is controversial) that you can skip them
if your valve seats do not have any recession.
The larger exhaust valve can definitely add some performance,
but it seems to me that if you do not open up the valve pocket to
match the larger valve, much of the potential will not be realized.
See if the machinist can open up the area immediately under
the valve while he is at it. I suspect he will know what to do, since
installing larger valves is a relatively common trick.
Another thought - I don't know if installing the earlier valve
in the later head results in the valve being too close to the side
of the combustion chamber or not. You should measure your stock valve
carefully, and compare where the edge of the exhaust valve will
end up with the larger valve. The rule of thumb is that you want
3/16" between the edge of the valve and the combustion chamber wall.
If it is less than this, you will want to have the machinist
shave the chamber wall (or you can do it with a Dremel). If you do
not clearance the valve in this manner, you can end up with less
performance because the combustion chamber wall 'shrouds' the
valve (in effect blocks off a portion of the valve) and reduces the
flow.
If you are going to all this trouble, why not get yourself
a Dremel, and pocket port your heads? It is good for 5-10% more
power, which may well be more than adding the larger exhaust valve
will get you.... Just a thought.
If you need any more ideas on how to spend copious quantities
of money during your rebuild, drop me a note. I am always *happy*
to help other people spend their money =;-o
Cheers,
Vance
------------------------------
Cogito Ergo Zoom
(I think, therefore I go fast)
TR6 Web page: http://home.comcast.net/~v.navarrette/
-----Original Message-----
From: owner-6pack@autox.team.net [mailto:owner-6pack@autox.team.net] On
Behalf Of Jerry C Shaw
Sent: Wednesday, November 17, 2004 6:00 AM
To: 6pack@autox.team.net
Subject: Larger Exhaust Valves in Late Head
I have a '74 and noted that many have recommended larger exhaust valves.
BPN
sells 32.5 mm (1.28 ") 21 4N SS valves, which is substantially larger
than
my stock 1.195" valve diameter.
Do I just ask the machine shop to grind a larger seat?
Also, do listers recommend the bronze valve guides?
Thanks
Jerry Shaw
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