Date: Sat, 8 Feb 1997 21:14:05 -0800
From: Mike West <mwest@cdsnet.net>
Subject: VTG and the Rocker Assembly II

Valve train geometry II

 This one is the continuing saga of the untrained delving into
the unknown with wild abandon.

 The rocker box is a can of worms of the first order.

I've spoken on several occasions on tolerances.
This little box is full of them and it's not all good.

Tolerances is a word for allowance you give a machinist in making
a part.
It tends to be based on the type of equipment he has to work with.

For instance, there is no use asking a machinist to make a part
circular within .00002 if the lathe will only give you a repeatablilty
of .0004. Actually that's a pretty good lathe.

Then there is setup.

Say you want the top of the spigot faces cut and parallel to the crank
and inline with each other.

The inline part can be pretty close because they will both be cut at the
same time and the same setup.

The parallel to the crank tho. . . . how much time do you want
to give this guy to set this up at $50 an hour?

So he gets say a plus or minus .0010, ten thousandths, he can
do that without too much trouble with a simple enough setup.

So cost is the other factor here, actually the biggest factor.

Getting back to the valves, you can see a half dozen machine
operations that all affect the height and location of the valve
stems and then about four more that affect where the rocker arm
is going to come out.

Now we want to have these rockers built with untrained labor
and all the parts be interchangeable.

Even as flawed as I am about to declare them, they have run for
over 50 years.  I'm a low-down ingrate.  :-)

Without further ado, lets look at one head, the right one and
see what happens to the rockers as they push the valves up and
down and spin them at the same time.

The rockers on the right head are all offset to the front of the
car and this is what is supposed to give them the spin as they
go up and down.

#1 exhaust spins alright but to do so it moves along the rocker
shaft against the spring loading and then bounces back.

#1 intake having the same offset tries to do the same thing but
is resting against the rocker assy post. Can't move along the
shaft so all the offset just gets burnt up as friction into the
various parts involved.

The only turning the #1 intake can get is based on the clearance
between the valve and the valve guide. Tiny.

#2 intake has the spring on the same side as the offset so it
can rotate ok as it goes up and down.

#2 exhaust has the same condition as the #1 intake, ie., it rests
against the post and has no spring loading on that side, so it
can't do anything but burn the forces as friction.

All of the above is based on the adjusting screw at least passing
thru the center-line of the valve stem. If it doesn't then it won't
turn at all.

So what all the above says is that 2 of them work ok and 2 don't.

It also says that even the best of them is side-loading the
valve stems into the valve guides.

Now if we take this head and move it around to the other side,
it means the #3 exhaust is not a good spinner.
It's the same one as the #2 exhaust.

Many of, if not all of the aftermarket outfits remove those springs
on the rocker shaft and replace with solid spacers.

That puts the 2 good spinners into the same small spin state as the
poor spinners.

Another item that gets changed is the valve springs themselves.

The greater the the spring load there, the greater the
friction/galling you're going to have.

The two items above are addressed to the hi-performance crowd.

OK , we have excessive wear on the valve guides, rocker shafts,and
the spacer springs on the rocker shaft.

Also there is wear on the end of the valve stem and the adjusting screw itself.

The items listed under "Also" are not excessive because the valve is
catching hell down in the combustion chamber anyway and won't last
long enough to call wear on the other end "excessive.

The Bentley "Official Service Manual" says 25,000 miles on a head.

Learn to live with it, if you get more you're golden.

I'll post the "how to do a dinkum valve setup" in my next post.

I will say this tho, as always, Bob Hoover is on the mark.
The swivel foot adjusters is the way to help aleviate some of
these galling/friction problems.

The CB Performance setup with it's big foot probably isn't too
bad either.

west