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To shim or not to shim?


Zatarra

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Happy Thanksgiving Everyone,

I have a 1977 280z that soon after I purchased started leaking coolant into the oil, this took me down the rabbit hole.  I had to replace the cylinder head, I sourced a new N47 and am now rebuilding the engine.  The new cylinder head was checked at the machine shop, they replaced a few valves, springs and resurfaced.  It now measures 4.245” in height.  The machine shop measured it.

To make sure I had a perfect seal I also had the block cleaned, honed, checked for cracks and decked.  They worked on it a few weeks ago and yesterday they confirmed that they usually deck them .004-.005. 

From my research the heads measure 4.252” from factory, with the new head height minus the decked block means .012” or .3048 mm has been removed.  I keep reading different specs for when I need to add tower shims.

Per the machine shops research, 4.232 is the minimum height.  (I am within spec)

The service manual says the combined surface grinding limit is .2mm or .0079” between the head and block.  (Out of spec, .3048mm has been removed)

The criteria of the shims sold at the zstore match the factory manual.

How to rebuild your Nissan has opposing specs, plus the wrong cam tower bolt torque spec fyi.  They quote the service manual but in the Cam-Tower shims section they state that if the head is milled more than .020 in (51mm), I’ll have to restore the geometry with cam tower shims.  This matches the machine shops limit and I am within spec.

Note, I am installing a new timing kit and the camshaft and rocker arms are original and in their original order.  This is a simple rebuild where the block and head have been gone through by the machine shop, and I'm doing new bearings, rings, rod bolts.  Staying fuel injected as well. 

Do I need to add shims, if so what size considering what is available?  .015 would put the towers over the factory height.  Maybe I just answered my own question but this is my first project car/build and I have minimal mechanic experience and I like to do things right so I just want to be sure.  I've learned quite a bit these two years and I still havent gotten the car smogged.  😁

Thank you for your time and feedback!

Saul

 

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I would think if you're right at the limit on chain length associated with cam height you could try cam sprocket hole #2 or #3. You want a tight chain but not too tight. The chain tensioner should not show any of the metal from what I remember.

Here's mine. There's the rubber washer between the "shoe" and the housing that should be snug.

The red circle was something else, disregard that.

Screenshot_20211125-153757_Samsung Internet.jpg

 

 

 

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I’m no expert but my head is shaved .020 and I didn’t shim and my chain is tight. I believe the stock cam sprocket has 4 deg adjustments with 12 total degrees. You can shim or buy a Kameari cam sprocket with 2 deg increments that can account for the timing issue and I think the chain tensioner should be fine.
Here’s my engine chain.

86b24adc9759c63e5196f35ab2155277.jpg

d6450292ab73cb25fb567b635d420299.jpg


Here’s the cam sprocket:
https://www.thezstore.com/page/TZS/PROD/10-2042


Sent from my iPhone using Tapatalk

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Site and Ferg, The valve timing position hole on the upper cam gear has no effect on chain tightness. It's a "relative rotation" change only, not a "distance between two gears" change.

In other words, moving which cam timing hole you're using will not change the chain tightener position at all. If your cam tightener is sticking out too far, moving to a different timing hole won't do anything at all to change that.

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Site and Ferg, The valve timing position hole on the upper cam gear has no effect on chain tightness. It's a "relative rotation" change only, not a "distance between two gears" change.
In other words, moving which cam timing hole you're using will not change the chain tightener position at all. If your cam tightener is sticking out too far, moving to a different timing hole won't do anything at all to change that.

CO, yep that’s how I understand it as you stated. But shaving the head caused the head closer to the crank thus creating minor slack in the chain but I felt the tensioner could compensate for that small amount. Secondly the cam sprocket adjustment is for valve timing as the valve are opening slightly earlier than with a normal head thickness. Again I’m the resident idiot....I know you have a better understanding of all this. When I saw the cam sprocket drawing you made that shows the valve timing adjustment it madero the each position I knew it was playing T-ball with a major league player


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I appologise for not fully comprehending your post, but I will give you my take on what you can do.  I had my P79 shaved 0.050" and I did not shim the towers or change the valves.  I slotted the chain guide holes to get the chain path correct and then I drilled the stock cam gear and used eccentric spacers per the "How To Modify..." book.  With a degree wheel, I was able to dial the timing in to the cam card specs.  The engine ran great and was endurance raced.  The spacers were less than $10 from Summit Racing and the other mods were free.  I started with a new quality Cloyes timing chain kit.

EDIT:  One thing I added was a plug spacer that fit between the cam dowel/eccentric washer and the cam retention washer.  The eccentric washers are about half as thick as the cam gear hole, so the washer could walk off the end of the cam dowel.  The plug keeps the eccentric washer in place.  You can just see it in the last picture.

IMG_4895.JPG

IMG_4946.JPG

IMG_4942.JPG

IMG_4828.JPG

IMG_4949.JPG

Edited by Jeff G 78
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23 hours ago, Zatarra said:

How to rebuild your Nissan has opposing specs, plus the wrong cam tower bolt torque spec fyi.  They quote the service manual but in the Cam-Tower shims section they state that if the head is milled more than .020 in (51mm), I’ll have to restore the geometry with cam tower shims.  This matches the machine shops limit and I am within spec.

I'd go with the How to Rebuild advice, to start.  The cam tower bolt torque spec mistake is well known, but seems to be the only significant one in the whole book.  Put things together without fully torquing, if you have the time, and you'll be able to tell if you're going to have issues.  Most of the shimming and work involved is for the purpose of getting the chain to do its thing without problems.  Jeff G's advice is based on the principles of how things are supposed to be, a lot of advice is based on how somebody else did it and it "worked" (p.s. not directed at anyone on the site just the world in general).  You'll probably end up in between, I'd guess.

I think that 108 mm is the factory spec for head thickness.  It converts to 4.25 inches.  Significant figures and all that stuff.

I think that lash pad height might be another reason to shim.  If the top of the head was shaved, the cam towers and cam will be lower.  So, if my guess is right, that's a whole separate reason for tower shims, after the chain considerations.  I've not rebuilt a complete engine though, my engine rebuilding has been in my head and the internet, so I could be way off.  But these are the things that crop up.

Edited by Zed Head
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Thank you all for your feedback.  I am going to move forward without the shims, the thinnest I could find are .015 but only .012 has been taken off.  This coincides with the two resources that ask for shims only when .02 or more has been removed.  I'll definitely pay attention to the chain tensioner once I put it back together too, thank you!

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