> > OK ... now just what the %$#^^*&$%##$^ does "on rock" mean ... can
> > it actually be defined??
I feel like I've explained this many times, so maybe it would help if someone
would tell me which word they don't understand !
"On rock" in it's simplest form means the rocker arm is moving. That's why it's
called a rocker arm, what it does is rock. Not like a gray thing you stub your
toe on, but like a rocking chair. Thus "on rock" means it's doing it's thing
and rocking. (One of those 'separated by a common language' things I guess.)
So technically, what we are talking about is the point where both rocker arms
for a particular cylinder are moving (rocking), thus the entire cylinder is "on
rock". This only happens for a few degrees of crank/cam movement, known as the
overlap period if anyone cares ... when people talk about a "high overlap" cam,
they mean this period is longer.
So, the "on rock" method of setting valve lash is to turn the engine so each
cylinder in turn is "on rock". Doesn't need to be especially precise for
setting valve lash, but the same reference is used for setting valve timing
(when it needs to be very precise). With a cylinder "on rock", you can then set
both valves for the cylinder which is opposite in the firing order. Thus,
assuming you turn the engine forward and start with #1 (I generally turn the
engine backwards and start with whichever cylinder comes on rock first), the
sequence is :
on rock adjust
1 4
3 2
4 1
2 3
The same procedure works for a 6 cylinder engine :
1 6
5 2
3 4
6 1
2 5
4 3
In fact it will work for _any_ in-line 4-stroke engine with an even number of
cylinders ! (Using the firing sequence for that engine, of course.) Note how
the right side is the same as the left side, just shifted by 1/2 the number of
cylinders. Also, it doesn't matter what order you do the cylinders in, as long
as you follow the same pairs (and do them all of course). On a car with a
distributor, one way to get the right pair is to follow the spark plug lead back
to the dizzy cap, then jump to the tower opposite and follow it's wire out to
the cylinder to adjust.
Can you do the same thing with the rule of 9 ? Well, probably so ... except
it's the rule of 13 for a 6 cylinder engine and might not work for all of them.
Plus you have to count twice as high and do it twice as often ... seems more
complicated to me. I don't like having one shoe off so I can count to 13 <G>
Randall
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