On Wed, 17 Nov 2004, Jerry C Shaw wrote:
> 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?
When the machine shop installs the valves, they'll have to "cut" the seat
to match the valve anyway, so this process should be "automatic" (meaning
you shouldn't have to ask for it). You should tell the machinist that you
are using an alternate size valve to make his life easier. :-)
They'll then "lap" the valves to the seats as part of the valve job.
> Also, do listers recommend the bronze valve guides?
I recommend them under two circumstances:
1. the old valve guide are worn beyond specs
2. you are going to very high compression
# 1 is obvious. But don't lie to yourself. There's a good chance that the
guides in there are okay. Get them measured for an "out of round"
condition (the valve guides will "oval" form the side-loading of the
rocker action). If the guides are still round and within reasonable specs,
it really is OK to use them as-is.
#2 means > 10:1 which is probably not a good idea for a street motor.
Of note - the available iron "stock" valve guides seem to be of dubious
quality, or at least they were when I did a bunch of valve jobs about 5 -
10 years ago. When we measured them with a bore gauge, they had proper
dimensions at both ends, but the middles were out of spec (too large ID).
This is the result of shoddy machine work and cannot be rectified except
by replacing the parts. I needed the iron guides to be 100% class legal
(for racing), so I had to sort through about 30 guides of each type
(intake and exhaust) to get 6 of each that were "close enough".
Last note - replacing the valve guides is doable by most shops. BUT, you
need to pay attention to stuff here. Make sure that the guides are
properly clearanced. If the machinist follows the book, that spec is wrong
if you switched to bronze (because bronze has a different thermal
coefficient of expansion). There are plenty of folks that have had to pull
the head later because the valves were siezing in the head (or worse). A
good machinist will know this and compensate. A bad machinst will install
the guides and call the job done. With bronze, you MUST ream the holes to
spec. This is time-consuming because you have to properly index the holes
and then ream to get the exact dimensions. But a proper machinist should
have no problem with these operations.
> Thanks
> Jerry Shaw
Have fun!!
rml
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