These
units were left when we stopped rebuilding heads awhile
back. Haven't found time since to finish them so we are
selling them as they are. Heads typically include new
valves, seals, springs and guides. Check photos and
descriptions of each particular head for information
accurate to that head. Bottom and top of head have both
been milled flat to correct previous warpage. It will be
necessary for you or your shop install your old studs,
dowels and cam towers.
A
little background on heads...they don't like being run
low on coolant or anything else that allows them to
overheat; faulty distributor, improper removal of "smog
devices" some of which protect the engine...; lean
mixtures, plugged radiator, bad pressure cap, plugged
exhaust; the normal things that engines don't
like.
When
they overheat and warp; both top and bottom warp; putting
a cam back in a warped head usually results in it either
binding and "blueing" the towers; or breaking if the
stress is severe enough.
We
did some of the earliest "engineering" adventures with
oven straightening heads in the 70s; when most machine
shops had not seen many; if any; aluminum heads. Our
methods were crude and done out of desperation. We
learned in addition to warping the heads can sink, twist
and "shift" themselves. Cool.
When
a head is cut it is important that only the slightest
amount be removed; you don't even need to run the cutter
to the ends of the head to make the top suitably flat.
Removing material top or bottom puts the cam closer to
the jackshaft which creates chain slop and alters the cam
timing. It is uncommon to have a problem with valve to
piston contact because the valves have been moved too
close to the pistons; it is usually due to the valve
timing being altered. (When they open and close in
relation to the piston position) Various methods have
been used to counter this situation. We typically used
cam tower shims. Lowering the cam; or raising it more on
the top than it was originally to make up for bottom
cutting alters the valve train geometry...basically this
means the cam lobes will not be running in the center of
the cam follower (rocker arm) lobes. You can "cheat" and
let them go to the edge; but if they go over that the cam
says bye-bye in short order.
Altering
the placement of the wear pattern on a cut head typically
involves changing the rocker buttons' thicknesses.
(sometimes not all of them). If the valve is sunk more in
the head due to multiple valve jobs; it can help or add
to the problem.
Nissan's
service instructions were to never remove cam towers from
a head. This is due to the fact that you can take a new
head; remove the towers; reinstall them; and find the
camshaft bound up. It can be a rather tedious process to
reinstall cam towers and have the cam turn freely. Your
goal is to get it so the cam will turn with two fingers
with the journals oiled (and of course the rockers out!).
Getting it to turn as easily as possible involves
tightening the towers evenly, unevenly, and/or banging on
some offenders with a plastic mallet. Even with the
extensive use of locating dowels that Nissan provided it
can be work. Using "plastigauge" we were usually able to
locate the problem tower or cap. Normally if you get it
correct on the workbench; it will stay correct when the
head is torqued to the block; but not always. There are
all kinds of tricks like clamping them into the towers
and then bolting the towers on; but this can drive one to
temporary insanity as any other method...
In
a perfect world; the car would not overheat and the head
not need to be milled. In a perfect world you would just
as Nissan suggests get rid of the head when it is cut the
slightest amount. In a perfect world the new castings
would cost $80.00. They were $440 as far back as 1980 so
ways had to be found to make it all work. If everyone
that "needed a new head" in 1980 had only that choice;
there would be a lot less roadsters out there
today.
Once
a head has had it's top milled to true the surface; it
does not matter which towers go back on it. You will
usually have an easier time if you keep them in order;
but they were originally bolted to a flat surface; so
this sometimes doesn't matter except #1 and #5 of course.
You want to get the cam as relaxed as possible in it's
"bed" and if need be you can always have the cam journals
"align-bored". Some shops call this "line boring" but
it's purpose is to ALIGN the towers not to bore a hole.
If it is done properly the tiniest amount of material
will be removed. And again; this must only be done if all
attempts by a proficient individual have been undertaken
beforehand.
The
original thickness of the U20 head is 4.528" (115mm).
They can typically do fine up to about .020" off without
a cam chain problem; assuming the chain and gears are in
good shape. After that point you want to think about
dealing with the situation.
If
need be we have used towers, studs etc.
