Cam Towers

[Trnelson]

Getting ready to assemble my cylinder head this weekend. On a mock up run the cam didn’t turn what I would call freely, I could turn it with one hand but it did require some effort. This quote from “How to modify your Datsun” has me wondering how precisely “striking the tops of towers” is done?  I understand the towers are align bored but it sounds like there is some percussive finesse potentially needed to get them back in line. 
Any tips on the assembly?

 

thanks

 

Tom

[madkaw]

Question : did this cam turn freely in this head before ? 
Did you mark them before removal in order to get them back in proper placement?

Have you checked the head for flatness ?

[ETI4K]

Start, if you can by measuring the torque required to rotate the cam.  If you don't have anything that measures directly,  clamp a wooden arm of known length to the cam and use any decent kitchen digital scale.  Compute the torque and record it for a baseline.

When dialing in the towers, you may have to adjust one or all, so with each adjustment,  record the result:

Tower 1

R  1 tap right - No effect

L  1 tap left  -0.3 in.lbs

Sorry for crappy example of a table, but trying to do this on a phone pretty well blows.  Try one tap on each tower, then try two, and so on.

As for tap intensity, install one tower and bolt it down snugly to a known and recorded torque, then use a dial indicator to indicate right to left motion.  Tap gently, then harder until you see the indicator hold a new position.  That's your starting tap intensity.  It's hard to measure impact force, but your other recordations should permit you to return to your baseline if it just keeps getting worse.

Eventually, you won't be able to make any improvements,  and hopefully what you've done will be enough.

Tighten everything down and hope like hell the torque doesn't change.  If it does, start over.

Good luck

Edited January 23, 2021 by ETI4K
Typos

[Trnelson]

The towers are marked and only finger tightened for the mock up.  The wipe marks all looked good but I didn’t check how freely the cam rotated. As far as flatness goes all I have is a 28” straight bar, I can’t remember the tolerances off hand but I did check it with a feeler gauge when I took it apart (Per Monroe manual) and I didn’t have any gaps outside of spec. I’m sure that leaves plenty of room for error but that’s what I had. I will say the engine ran pretty well prior to tear down so I honestly wasn’t looking too hard for parts being bent or out of alignment. 
Reading that caption makes me think it’s not uncommon, you just have to finesse everything back together. Maybe that’s why the FSM says to not remove them...🤔

[Captain Obvious]

1 hour ago, Trnelson said:

Getting ready to assemble my cylinder head this weekend. On a mock up run the cam didn’t turn what I would call freely, I could turn it with one hand but it did require some effort. This quote from “How to modify your Datsun” has me wondering how precisely “striking the tops of towers” is done?  I understand the towers are align bored but it sounds like there is some percussive finesse potentially needed to get them back in line. 
Any tips on the assembly?

First thing I would do is make sure the mating surfaces (bottoms of the towers and mounting pads on the head) are clean enough to eat off. And I would (under magnification) make sure there were no burrs or "upset" metal anywhere on any of the mating surfaces. One might wonder how burrs might get kicked up? Well in the case of the head on my car, my PO used a screwdriver between the towers and the head to pry some of the more stubborn the caps off. That screwdriver wedging caused burrs that prevented the towers from sitting properly after that. So if you (or a PO) did anything like that, you need to look into it.

As for the percussive finesse... I would put all the caps in place, lightly tap them with a plastic mallet to seat them, put the bolts in and snug them up to a low torque. Maybe a third of the final torque.

Then put the (well oiled) cam in place and see how it spins. If it spins free, then tighten up the towers to 2/3 spec and see how it spins.

If it still spins free, tighten the towers to spec and see how it spins. If it still spins free at full torque, congrats... You're done.

Let's start there and see what happens. If it DOESN"T spin free, then you need to start the percussive adjustments. But I'm going to start off hopeful and see what happens?

[madkaw]

Good advice Captain .

The finger tight caps can be an issue 

[Captain Obvious]

Right. I should have mentioned that. But once they have seated, things should, or at least "COULD" be good.

And something else I didn't mention... When you're tightening the bolts. the torque should shoot up real quick like. You shouldn't need to use the bolts to jack the towers down onto the head. It should go from nearly zero torque to turn the bolt head to significant torque in a very small amount of rotation. That's the sign that you had every thing clean, had no burrs, have the towers aligned on the dowels and seated properly with the plastic mallet tapping.

if you find that you have to use a wrench and turn the bolt a significant amount*  before the torque shoots up, you should stop... Take it apart again and find out why.

* More than a half turn maybe?

[Trnelson]

Thanks for the advice. No screwdrivers were used by me during disassembly but I suppose the PO may have. I’ll check and those items before I make another attempt. 
I failed to mention that it do I’d spin freely when the cam was in the first 4 towers, it was when I got to the fifth one that things got janky. I’ll start there to see if I’ve overlooked something obvious. 

[Captain Obvious]

On that rearmost tower, I've seen situations where the cam journal was slightly narrower than the bearing in the tower. And that can result in a small lip in the tower bearing surface at the rear edge of the rearmost tower.

Since the front/rear positioning of the entire cam is controlled by the hardware at the front of the cam, it's possible to insert the cam "too far" into the journals when doing a test fit. And if you do that, you can push the cam into and over that lip on the rearmost journal resulting in it being difficult to spin. I don't know if all of the heads work out this way, but in my limited experience I've seen that.

So, the question is... Does the cam slip into the rear most journal almost all the way before it gets harder to turn? Or does the difficulty in turning it start as soon as you start to get that rearmost bearing engaged?

[Trnelson]

Update, I studied the towers more closely and sure enough there was a large burr on the rear most tower. I carefully filed it down flush and checked all the others. You were correct, obviously someone had prayed it off with a screwdriver!  I oiled the cam and towers with the bolts torqued to 7 lbs. It still stopped at the fifth tower. I removed the fifth tower and tried to push it into position but it wouldn’t go any farther. It seems the issue was with the front cam journal entering the tower. I loosened all the bolts back up, reinstalled the fifth tower and when it stopped I lightly tapped the top of the first tower and viola, it slipped right in and turned with ease.  I incrementally torqued all the bolts down and it spun just as freely as with no torque!  I did the happy dance and started laying out the valves for installation. Here’s where it becomes my typical Datsun story...  I realized in all my happy dancing that I had forgotten to put assembly lube on the towers, once again I had to repeat the procedure of loosening torquing etc. on the LAST bolt to torque to 25 lbs with about 1/8 of a turn to go it gets real easy and just spins with little to no resistance. WTHis going on. As it turns out those threads had been Heli-coiled. How they didn’t come apart when I chased the threads is beyond me. Oh well, looks like I’ll be making the 1.5 hour round trip to the closest parts house, damnit!  Better to find it now than at break-in I suppose. 

[Captain Obvious]

Excellent. And then not so excellent.      I guess I'm a little relieved that sort of stuff doesn't happen to just me!

So I'm unclear on the assembly lube you forgot. You don't need it between the towers and the head... You only need the assembly lube on the cam journals where they ride in the bearings. And you don't need to dismount the towers to put that lube on.

[Trnelson]

Correct, I was putting the lube on the journals and bearings but when I slid the cam back in I had the same issue with it not wanting to slide all the way in with the tower bolts at full torque. So I loosened everything back up, lubed the journals/bearings, slid it in and when I was torquing everything down the second time is when the Heli-coil broke loose. 
I finally found a Heli-coil set after 3 stops and decided to check all threads, glad I did because there were two more that appeared sketchy. I’ve replaced them and now have the towers and cam installed. I can easily turn it with my fingers so I’m calling it a success!  I’m now beginning to think that the worn out Heli-coil may have been the or at least another issue with the cam fitment. Oh well, either way it’s in and I can finish the top end today.  
Thanks for the help Captain