Okay, Daniel brought up several questions about the relationship of cams,
milled heads cam tower shims, rockers, valves and valve adjustment.
To start off with, here's the cross-section of the U20 engine. I'd been
meaning to scan this for a while anyway.
http://www.gordon-glasgow.org/images/U20_Crosssection.jpg
Daniel asked:
Took the head over to Mike Young and this is what he said. The head has
been shaved and it's about 40 thou's to thin. The intake valves are all
pretty sunk into the head. He says that the intake valves are sunk in so
much that the valve clearance adjusters are at the limit of their range. He
says I should 'do a valve job' and replace all the intake seats. If I do
that, will that bring the valves out more so I can get that adjustment back?
Answer:
As the valves beat on the seats, things wear and the valves move deeper into
the head. That's actually in the "up" direction as the engine sits in the
car. As the valves move up, the valve adjusters have to be screwed down
further into the head in order to keep the valve lash correct. Here's a
closeup of what happens:
http://www.gordon-glasgow.org/images/U20_XSect_valves.jpg. Eventually you
will run out of adjustment range when the seats are really beat up. Just
doing a valve grind will move the valves even further into the head unless
you replace the valve seats. Yes, Mike is correct, and it will get you your
adjustment range back.
BTW, that's one tip when you are looking at an engine - look at how much the
adjusters stick up out of the head. Here's how they should look on a
completely fresh head:
http://www.gordon-glasgow.org/Images/Bracket_closeup2_sm.jpg.
Daniel asked:
His recommendation is to also put in cam tower shims. He wants to put in 40
thou shims to make up for the loss of head thickness. Is this a good idea??
If I put in cam tower shims won't that effect how the cam hits the
followers? Won't it decrease the valve lift by that amount?
Answer:
As Keith pointed out in his reply, cam tower shims are usually necessary
unless the head has been straightened and not just milled. This is a
delicate and more expensive process that few machine shops are equipped to
do, but it's the best way to save a head. The purpose of installing cam
tower shims is to get the height of the cam back up to where it should be
and take the slack out of the chain after milling.
Yes, installing cam tower shims will affect how the cam hits the rubbing
face on the rockers (good observation, incidentally), but the difference is
taken up by running the adjuster up farther. With the adjustment set
correctly, there will be no change in valve lift.
Okay, for the anal-retentives among us (and I include myself in that group),
there will be a miniscule increase in valve lift because the contact point
between the cam and the rocker will be very slightly closer to the pivot
point of the rocker, resulting in a tiny increase in rocker arm ratio. This
will also retard the cam timing just a hair because any given point on the
cam lobe will have to go around a little further before it makes contact
with the rocker. More on cam advance/retard later.
Daniel asked:
Is there a limit to how high the shims can go?
Answer:
I've seen cam tower shims advertised that are as thick as .125" so .040" is
only a third of that. No problem.
Daniel asked:
My idea was to put on a thicker head gasket to make up for the thinner head
and to reduce my compression ratio.
Answer:
Yeah, you can do that, but before you do that, make sure of the combustion
chamber size. As Keith pointed out, warped heads are quite often milled on
both top and bottom, so a thickness reduction of .040" might not all come
from the bottom of the head. If you just throw a thicker head gasket in
there, you might reduce the compression ratio more than is necessary. Have
the chambers cc'd first and calculate the compression ratio.
I've heard reports of sealing problems sometimes when people tried to use
two head gaskets. I suspect this is because the gaskets are designed to sit
between two machined hard metal surfaces, so the interface between the
gaskets might not crush and seal properly. Just a guess.
Daniel asked:
I don't understand why a too thin head messes with the valve timing? If the
timing chain is not overly slack I don't see how the valve timing is
disturbed?
Answer:
The key is to understand that the chain that drives the cam is pulling on
it, so the "down" side of the chain, or the right side of the chain as you
are looking at the front of the engine, has a fixed length (ignoring chain
stretch for the moment). If the head is milled, the center of the cam moves
closer to the crank. Since the links on the chain are hooked into the teeth
on the gears, the cam will essentially "walk" down the chain when the head
is milled. See http://www.gordon-glasgow.org/images/U20_Cam_retard2.jpg for
a terrible diagram of what happens (I apologize for my lack of graphic
skills when it goes beyond scanning pics).
(For the record, here's the whole chain alignment diagram:
http://www.gordon-glasgow.org/images/U20_Cam_chains_setup.jpg and a picture
of the whole mess assembled:
http://www.gordon-glasgow.org/images/U20_Cam_chains_pic.jpg)
The effect of this is rotate the cam counterclockwise a bit, which means
that all the valve events will happen a few degrees later than they would
otherwise. Remember, the valve event happens when the cam hits the rocker,
and the relationship between the cam and the rocker is the same as it was
(unless you have installed cam tower shims, which retards it still further).
Notice also that stretch in the chain has the same effect.
So what does retarded cam timing do? In general, it reduces bottom-end power
and increases top-end power. I won't try to explain why it does that. The
Racer Brown writeup that Ronnie Day pointed us to covers all that and more.
That is by far the best explanation of cams I have ever read anywhere. I'm
glad it's finally available on the Web. It's so good that I'll post the link
again here: http://www.datsport.com/Racer_Brown_Menu.html
At this point, someone is probably thinking, "Hey, I had my head milled and
never used a thicker head gasket or shimmed the cam towers, and it ran just
fine. Does any of this really matter?" It's ironic that retarding the cam
timing kills off bottom end power, but increasing the compression ratio will
probably get it back, so yeah, you might never notice. But there would still
be more slack in the chain (unless you shimmed the upper tensioner) so you
run more risk of getting contact between the chain and the upper chain
guide. It's best to put everything back to the correct relationship.
Andy Cost said:
On top of the valves are shims that adjust for this. You will probably have
to put thicker shims on top of the valves.
Question for Andy:
Do you have a source for different thickness lash pads for U20 motors? I've
never been able to find any. Nissan made a big range of lash pads for the
L-series motors but never made any for the U20. You can have the valve stems
re-cut to use L-series retainers and lash pads, but that's more work and
expense than most people want to put up with.
Andy also said:
You will probably have to get the cam towers line-bored if they are removed
from the head and then reinstalled.
Answer:
I've had cam towers off several times and never had to have them line-bored.
Sometimes I can't get them lined up myself and have to take it to a
machinist to get it set up, but even then they didn't have to line-bore it.
I agree it's a touchy procedure, because the clearances on the cam are
pretty tight - .001-.002" - and there's no soft-shell bearing in there, just
steel sitting in aluminum. Alignment is critical. Line-boring might be
needed, but not necessarily.
Thanks to all who chose to put up with my babbling and actually read this
far. ;-)
Gordon Glasgow
Renton, WA
www.gordon-glasgow.org
"In Germany they came first for the Communists and I didn't speak up
because I wasn't a Communist. Then they came for the Jews and I didn't
speak up because I wasn't a Jew. Then they came for the trade unionists
and I didn't speak up because I wasn't a trade unionist. Then they
came for me--and by that time no one was left to speak up."
- Martin NiemVller 1892-1984
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