How would you know how much thickness of the cam tower shims you should be
using after a mill job. You could tighten up the camshaft chain also in
the process of compensating for the mill, if you knew how much play you have
in the chain, vs how much you "should" have in the chain. I think before I
removed the head from the car, I would see how much play I had in the chain.
I would lay a straightedge from the crankshaft gear to the cam gear, then
depress the chain taking all the slack out of the chain, and measuring the
distance from the straightedge to the chain. Armed with that information, I
then would talk to a machinist that was familiar with how much you would
need to shim to come back into a tight tolerance with the chain. Then add
the metal that was removed from the head to that amount, to come up with the
right shims for the cam towers. And be sure not to mix up the cam towers,
nimber them and put them back in the same place.
-----
From: Thomas Walter <walter@omni.sps.mot.com>
To: datsun-roadsters@autox.team.net <datsun-roadsters@autox.team.net>
Date: Tuesday, October 12, 1999 4:18 PM
Subject: Re: u20 startup help
>Rick,
>
>On any OHC engine, it would be a good idea to always check the top
>of the head for warpage.
>
>You're correct that the top sealing area of the gasket and top of
>the head are milled. Should be obvious that the towers, and rocker
>adjusters, are all removed to allow the top of the head to be milled
>flat.
>
>Look at a feeler gauge, check the thickness of a 0.020" blade. A gasket
>could still seal with 0.020" warpage to it. Now look at the camshaft...
>while it MIGHT be able to withstand a 0.020" "bend" in it, it will not
>last for long. I've seen snapped camshafts due to a warp in the top
>of the head.
>
>Once both top and bottom are surfaced (milled flat), you should install
>some cam tower shims. I picked up a set from Stan Chernoff.
>
>This is just one of those "little" things to remember when you are
>having the head redone.
>
>Cheers,
>
>Tom
>
>>I have seen several posts about milling both the top and bottom of the
U-20
>>head. Tom's, point about the posts keeping the cam fairly well aligned got
me
>>to thinking... when the top of the head is milled, is the sealing area for
>>the valve cover and the cam tower area cut? In my twisted world, they both
>>warp the same. If they are, how would know how much was taken off off a
used
>>head? I assume you measure the total thickness from the bottom of the
head
>>to the area where the valve cover seats. Also, I have had other aluminum
>>heads cut (Nissan L20B springs to mind) and never had my machinist mention
>>the top. Should it just be checked, or is milling the top and bottom of an
>>aluminum head a hard and fast rule? Or just the U-20? Can't imagine that.
>>Never know about these things!
>>
>>
>>Rick Stayner
>
>
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