Cam spin in towers after head work HELP!

[Ben's Z]

Ben, I feel your frustration. I'm not an engine mechanic, but I know my way around a machine shop. Perhaps I can give you some places to look.

You are assuming the binding began when the head warped and wasn't corrected when the machine shop tried to warp is back to flat. You are looking for a tower that is not aligned with the others when they are not torqued down and you are using the cam shaft as a go-nogo gauge.

To do anything other than hunt and peck, you are going to need a weightless indicator or at least a drop indicator mounted on a magnetic base. The first objective would be to qualify your go-nogo gauge (cam shaft) by verifying it was not bent. To do this, mount the numbers 1 and 5 towers properly torqued and then mount the cam shaft. Place the indicator to ride on the bearing surfaces for towers 2, 3, and 4. With the indicator riding on each bearing, slowly rotate the camshaft and note the runout (highest reading minus the lowest reading). If the cam shaft is straight and true, the runout should be minimal and simlar from bearing surface to bearing surface. If the runout get progressively larger towards one end, the cam shaft might be bent towards that end. To verify, remove the offending out board tower and mount the next tower inboard and again observe the runout.

Assuming your cam shaft is straight and true and assuming the rotation is smooth using only towers 1 and 5, add tower 2, 3, or 4 one at a time to find which tower causes the binding. (From your last post, I doubt it is number 3 since you removed it and there was no change.) Don't stop when you find the first binding tower because it may be ok and an end tower is out of kilter. Once you have identified the culprit, mount all the other towers and verify the cam is happy.

With the other towers inplace, again measure the runout on the problem bearing surface. Note the location of the high spot and mark near it with a magic marker in a place you can see with the last tower installed. Re-install the last tower and find the point of rotation where the binding is the worst. Locate the mark for the high spot and this is where the contact is being made. (Remember the tower may be too high, too low, leaning left or right, or twisted.)

If these steps haven't isolated your problem, you may have one tower marginally shifted one way and the adjacent tower marginally shifted the other.

I concur about everything you are saying but let me say a couple things first and let me know if I should proceed with your tests.

1)The cam I am intending to use worked fine and spins fine my old motor I pulled out. It is out of my 77.

2)I have 2 other P90 cams and they too spin fine in my old head and towers which is still bolted to my old block. They do not spin any better than my N/A cam in the head I am trying to fix.

[djwarner]

Hi Ben,

Yes I would proceed with the tests. Every cam with multiple tower bearings has some degree of runout - it was made by humans - not God. If your cam had zero runout, it would either bind all the way around, or not bind at all. The towers aren't perfect either. At some point in the installation, the cam imperfection and tower imperfection are making contact. Our job is to find out where.

Once we can find out where, we can determine what needs to be done to fix it. We know you head was warped, machined, and intentionally re-warped. There is no reason to believe that the other machined surfaces on the head magically fell back into perfect alignment after all that.

Even without warping, the engine manual states:

"Note: Never remove the camshaft bearings. If you once remove camshaft bearings, the bearing centers will be out of alignment and the recondition is difficult without center borings."

Center boring is taking the cylinder head with towers, cam bearings, and caps installed and running a very rigid boring tool down the axis of the cam shaft thereby cutting perfect circles in the bearings all on that common axis. So without the ability to center bore, you're really trying to pull a rabbit out of your hat. You are lucky to have only a single point of binding.

Thinking about what you have gone through, I have a couple of other points. I have assumed that the head is currently mounted on the block and torqued to spec. If not, any work you do now may evaporate like smoke once the head is stressed during installation.

Second, are you sure that you have maintained the order and orientation of each tower, bearing cap, and bearing? Reversing a cap is like putting a new part and all the previous alignment work is gone by the wayside.

So lets double check your order and orientation and then verify which tower/bearing is the culprit first.

Edited December 10, 2012 by djwarner

[FastWoman]

Engine machine work isn't my forte, but let's just think about this thing a second with some basic geometry in mind. Let's assume the cam towers ARE out of whack, and that the sleeve bearings for the cam are misaligned. Then let's assume you put a PERFECTLY STRAIGHT cam in that assembly, and it can spin with light pressure. Even with the misalignment in the towers, that PERFECTLY STRAIGHT cam shaft is going to spin under the same pressure throughout the full 360 degrees. Or if it binds just a bit, it's going to bind to the same extent throughout the full 360 deg. Geometrically it has to be that way.

Now consider the opposite: Your cam shaft is a bit out of whack, but the cam towers are dead on. The result would be the same. The crooked cam would turn in the perfectly straight towers the same throughout the full 360 deg of rotation.

My unavoidable conclusion is that BOTH your towers and your cam shaft are a bit out of align. Correct either one, and the shaft should turn uniformly throughout the full 360 deg.

Maybe you just need a bit more play in the bearings. Maybe the tolerances are just a bit too tight.

[EuroDat]

Ben, IMO After reading all this. I would check the run out on the cam to see if it is within spec and if so tunnel bore the PITA head and more on to your next problem:bulb:

BTW, Fastwoman is spot on. Both must be out to cause hard spots otherwise the tnsion would be always the same.

Chas

[djwarner]

It just occurred to ask, "Have you replaced the cam bearing with new units and not center bored them?" These bearing were designed to be somewhat undersized when new anticipating that they will be center bored.

To give you some idea of the precision required, here are some tolerances:

Head flatness - typically less than 0.002", Max less than .004" (after machining and before re-warping this was greater than 0.012")

Cam Bearing Clearance (difference in ID of the Bearing and OD of the cam surface) Typical 0.0015" to 0.0026" Max 0.0039"

This means the center of the cam bearings in the towers must be within 0.0015" of the theoretical common axis to assure no contact. I can guarantee you that no cylinder heads or towers or bearing were mass produced in the early 1970's to within these tolerances. This is why center boring was done after the towers and bearings were bolted to the head.

If Ben's head had been milled flat during the overhaul, I'm sure a quality shop would have center bored new bearings as described. However Ben's head was still out of flat (read warped excessively) after milling, and he is in a salvage situation here. The service manual solution here is to replace the head and not attempt to repair. If he follows my suggestions, he might be able to save it this time if the problem is a single tower out of position.

Considering the out of pocket cost to isolate the problem is low, he may save the head with the cost of his time and avoiding the cost of a new head.

[Ben's Z]

As posted at hybridz

Ok. Took back to second machine shop and they took .012 off of the top. They did say it was uneven. I get the head home and set on top of the block, alignment dowels in and ARP head studs in the block. This cam is still not spinning like it should. I set the towers on with the bolts finger tight and put the cam in, tapped and torqued in different and multiple sequences. I have the suspicion now that the head is bent like a banana with the center exhaust ports being the outside apex. Like this looking from the sprocket ")" I believe this because when I was getting ready to put the cam in to the last tower it simply does not slide home and is offset from the tower hole. I walked around the back of the motor and looked at the back of the cam as it was about to enter the tower and it is clear as day that the cam is not going in on center. I tried my other cams and it is the same case. All of the cams go in my old head and towers bolted to my old block like butter. My question is why would my head fit down on the alignment dowels if it has this curve to it? I believe now I have over $300 bucks in a large alumnum paper weight. I even tried an extra rear tower I had, it was worse. I also tried retorquing the head to the block and took it up to 60 ft lbs. Unlike before it never got tighter to turn the cam as the torque load increased, no better no worse.

[Ben's Z]

Ok. Took back to second machine shop and they took .012 off of the top. They did say it was uneven. I get the head home and set on top of the block, alignment dowels in and ARP head studs in the block. This cam is still not spinning like it should. I set the towers on with the bolts finger tight and put the cam in, tapped and torqued in different and multiple sequences. I have the suspicion now that the head is bent like a banana with the center exhaust ports being the inside apex. Like this looking from the sprocket "(" I believe this because when I was getting ready to put the cam in to the last tower it simply does not slide home and is offset from the tower hole. I walked around the back of the motor and looked at the back of the cam as it was about to enter the tower and it is clear as day that the cam is not going in on center. I tried my other cams and it is the same case. All of the cams go in my old head and towers bolted to my old block like butter. My question is why would my head fit down on the alignment dowels if it has this curve to it? I believe now I have over $300 bucks in a large alumnum paper weight. I even tried an extra rear tower I had, it was worse. I also tried retorquing the head to the block and took it up to 60 ft lbs. Unlike before it never got tighter to turn the cam as the torque load increased, no better no worse.

Ok I just thought about this again and then went out and checked my thoughts. The head is actually curved in the middle with inside apex being the center exhaust ports. I once again put my Stanley level across the intake surface on the head and indeed their is a gap at the center exhaust ports.

[EuroDat]

Well Ben, I would go back to the second machine shop and ask them to heat it and pull it down straight using the manifold edge as a guide. You dont mention how much gap it has. This will bend the cam in the middle like a bananna and fatigue will end up breaking it. The bigger the gap the sooner this will happen.

The head was in pretty bad shape when you started rebuilding it, but now you know where its coming from, I would spend the extra money to get the vertical line straight enough so the cam turns freely.

Chas