At one time in the long ago past we used Phosphor, Aluminum- bronze. We found
it very hard, and also very abrasive. It had a tendency to wear through the
chrome surface of the valve stems very quickly. For the past 15 years or so we
have been using a Manganese- bronze alloy. We had our guides made by
Precision Engine Parts to our specifications. The inlet valve guides are
tapered where they hang out into the inlet port & the taper actually goes back
up into the head so that the contact point between the head and the guide is a
straight line around the guide, which it will not be if the guide has a
straight od. (because it comes out of the hole in the head into the port
through the wall of the port where it is at an angle to the guide. As the
guide gets hot and tries to expand, it only touches the head on one side as it
projects into the port. This will cause the guide to bend slightly to one
side, away from the long side of the port.
The exhaust guides we have made with an internal counter-bore on the
bottom end. This is copied from the air cooled VW exhaust valve guides. A
certain amount of head carbon will accumulate on the part of the exhaust valve
stem that hangs out into the port when the exhaust valve is open. When the
valve closes, this carbon is pulled back up into the space at the bottom of
the guide and will eventually abrade the valve stem and the end of the guide
at this point. The counter-bore in the bottom of the guide makes a space for
this carbon to go where it will do no damage!
Some of the best guides we have used were a process from Winona-Van
Norman, an automotive machine shop machinery and tool supplies. They called it
"Bronze-wall" It was a bronze alloy "heli-coil" like insert that was installed
into a standard iron guide, then "broached" into place and honed to size. When
the insert wore out, it could be removed and a new one installed without
changing the centerline of the guide. They were very durable and held oil in
between the coils of the bronze liner. Kas is familiar with this setup and
used it at the Comp Dept., I believe. This only issue is finding someone with
the equipment to do the installation. There are no machine shops in my area
that can do it. i do not know if Winona Van Norman still supplies the tooling
and parts.
The higher the cam lift, the faster the lift rates, the more important the
guide material, clearance and installation becomes.
The same of true of having the correct rocker arm geometry. If this is
not correct, there is to much side load placed on the top of the valve stem
and the guides will not last, no matter what the material they are made of.
There is a Web site called www.mid-lift.com that talks about geometry in
great detail. We have found that with the top of the valves at the stock
hight above the head, the rocker shaft needs to be lowered about .090" from
its stock hight to maintain correct geometry when using a cam that gives
around .465" actual lift at the valve. As the lift goes up higher, the rocker
shaft needs to be lowered, (or lash caps put on the tips of the valve stems to
raise the contact point between the rocker arm and the valve tip.
Greg Solow
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