Cam won’t rotate when heads torqued
 

Old cam rotates fine in assembly of heads and cam holder. Although it gets a bit sticky when 930 gets to 6 - 9 o’clock.

When the heads are torqued to 15 ft lbs, the cam won’t rotate. Wayne’s book says to try a different torquing pattern if this happens.

Questions:

1.) since I’m getting new cams, should I wait for those to do the head to head stud fastening?
2.) I’m supposed to swing 90 after the initial torque but wanted to test the cam rotation first. Maybe swing 90 and hope this fixes it?
3.) seems the cam should not bind at the 15 ft lbs first step, if this is correct, is there a more methodical correction technique other than just trying a different torquing pattern?

Verify that your old cam is straight, using v-blocks and a dial indicator. Then use a straight edge across the heads to check if one is off. Do those two things first.

Did you change cylinders? Heads resurfaced?

Heads were not resurfaced, cylinders were changed from 3.0 to 3.2 short stroke.

Just curious, where does it say 15 pounds plus 90 degrees? Wayne’s book just lists a torque value...

Does the cam spin before it’s torqued?

The cam spins great prior to torque. I’m using Carrera cylinders with a taper seal torque.
Was told 15 # wait one hour then swing 90. But it’s not in Wayne’s book.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/987414-3-2ss-top-end-rebuild-compulsory-newbie-questions-2.html

Well it appears the Carrera is 15 Nm then swing 90. Units! Doh. Not clear this would cause an issue with the cam turning difficulty though. I see the torque sequence in the bulletin is different than Wayne’s book too.

15nm and then a 90 degree swing. It corresponds to about 40nm.

Cylinders all the same height class? Base gaskets all the same and installed? Or the heads sealing surfaces are not the same?

Thanks for the thoughts Craig,

Checked and base gaskets there, same as all came in kit.
When I check the cylinder sealing surface, I can get a 0.16 to .2 mm feeler gage under the straight edge on both the number 2 and number 4 cylinder?

This tracks with my finding yesterday where torque to number 3 and cam rotates fine torque to outside nuts on number 1 and cam rotates. But torque to number 2 and cam won’t go in. :confused:

That’s enough to cause a problem


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Thanks again Craig, measured again with different edge (shorter) across cyl #2 but only adjacent lips of 1 and 3 and now 0.038 mm is the only feeler blade I can get between #2 and my straight edge.

Wanting to be sure of my issue before I start to correct it.

Cam won’t rotate when heads torqued
 

.038 isn’t bad but .2 is almost 8 thousandths off

If you have a 3-4” mic you can check the heads see if they are off, should be around 84.5mm

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Without seining 90, will I get an accurate enough measure to determine what cylihead gasket I might need to correct number 2?

4,5,6 is okay so I can leave it be while chances are made to 1,2,3 side?

Update Wednesday stardate ...

Some success (I think) I followed protocol on the 4,5,6 side and the right side cam spins like a top after torqued to 11 # and then swing 90 +- 2.00000 degrees. Ha.

Tried 1,2,3 again with protocol of service bulletin and after 11 # (but no swing) the cam won’t enter the journal nearest #3. Loosened nuts and cam goes in and spins like a top.

Time for some corrective action i say!

I doubt my heads are off as I had Bruce’s head guy do them. He (Robert said my heads were in great shape). He did no machining.

Suspect the Carrera cylinders are off and I’ll check gap after 11#. Gap between likely changed as I pulled up my cylinders to verify 101% that my case to cyl gaskets were there.

Simple math/geometry. The depth of the heads have to be the same as well as the depth of the cylinders have to match within .0015-.002'' per head and cylinder combined. Otherwise, the cam tower is bowed and binds up the cam. I just finished machining a set of cylinders to get the deck height I wanted. I used a Starrett depth gauge and leveling block to make and check my measurements. Bob

Interesting, those depth gauges are a bit costly.

Where did you find these tolerance numbers? 0.0015 - 0.002 inches?

based on cam journal clearance. Too much bow and you pinch the clearance.

I have also cut the cylinder base to make sure all are the same height. Don't cut the tops. This will mess up the CE groove or top side taper (year dependent).

Ok so did you install the camshaft retainers and the gears? Sometimes if everything has been resurfaced, it will rub on the side of the camshaft retainer (93010519600). Because now the chain box is slightly too long for the new stack height. the cam will spin fine in the tower but not once the cover and gears attached.

My recall is years ago, struggling with this issue. I think I was out .003'' and the cam freedom was barely acceptable. I then think I opted for .0015'' for safety. A lot has to do with how much the cam and the housing bores are worn. Obviously the more wear, the more you can get away with. The depth gauge I have is a verticle ruler with fine graduations that sits in a base, with a slide that you work up and down the ruler. Drop the slide down to rest on what ever you are measuring, and read the measurement. A knurled wheel allows you to move the slide up and down in tiny increments. On mine, you can read it to .001''. Depending on what you are measuring, a lot has to do with feel. Bob

Working on it, I tested some snips from feeler gauge to prop up the head on the center cylinder.
Doing this to estimate what case gasket might work. So far I’m able to get the cam to spin with no resistance with all cylinder nuts torqued down to 11#.

Only shimed number 2.

If either the heads or cylinders are machined, the chain boxes have to be as well.

Interesting thoughts, so I put the retainer on with the chain box too and no issue, all turned well. Did not do the gears, but it seems the retainer tolerance is key.

Happy so far

If just a few thousandths are taken off the heads or cylinders - a clean-up job - the rubber O ring around the cam retainer part (3 6mm bolts hold it)will accommodate the difference. At some point (like guys running longer rods)it won't.

If more is removed, you can get it back with the cylinder base gaskets.

The issue here seems to be that tightening everything down caused cam bind, and that pretty much has to be due to some unevenness in the heights, however caused.

Just curious, you have your cam housings bolted solid to the heads, before torquing the head studs?

I'm sure there's a 40 page thread on the very subject, with at least 4 different ways of doing it, but I figured it would be better to have the heads tightened to the studs first, then put the sealant on the cam housings and torque to the heads with the 8mm bolts at 18 ft/lbs.

Is it possible you have an uneven amount of sealant between the heads and the cam housing, causing it to distort?

Yes, I’ve gone the assembly route with housing bolted to heads. It’s a preferred method of two gentlemen who have built a lot of engines. So I went this route.

I think there is no chance uneven sealant (between housing and heads) could cause this, why would the cam spin freely with assembly at rest? Also, the sealant does not cause one of the cylinders to be taller than its neighbor.

I put a thicker case/cyl gasket under number 2 today, installed cylinder over rings and bolted up to 11# and the cam spins free as a bird in flight. Thinking I’ve got it so far.

The case is probably warped . #2 cylinder runs the hottest and the coresponding case cylinder deck is usually relatively low . So I guess a thicker cylinder base gasket is one way to fix the problem .

If the sealant was uneven, one head could be farther from the cam carrier and the sealing surfaces of the heads would not all be in the same plane. Given that the clamping force of the cam carrier to heads is relatively low I see this as a potential problem area. Easy enough to check with a straightedge.

I would be surprised if you are able to fully rectify this using varying thickness base gaskets. If you put a .50mm gasket on one cylinder and .25mm on the others, that's 10 thousands of an inch height difference. In relative terms, that's a mile.

Look at it this way - Porsche went to the trouble of grouping cylinders by height, with only .007mm difference between the groups. That's 35 times smaller than a .25mm base gasket. Now perhaps Porsche were putting a finer edge on it than really needed, but there is no way you are going to "make up" more than a thousands of an inch (maybe 2) simply by tightening things down.

In fact, I wouldn't even torque things down like this, you are liable to permanently deform something.

Better to find out where the problem lies and fix it properly, you are much more likely to be successful and will have fewer problems down the road with cylinder to head sealing issues, etc.

Either the case is warped, the cylinders are of varying heights, or the heads have been machined in varying amounts.

Hope this helps!!

Maybe you’ll be surprised, because with 0.5 gasket under number 2 cylinder and torqued per the spec sheet, the cam is spinning freely. Calling it done.

Again I’m ruling out your speculation on the cam housing sealant. Why? Because I could measure a difference with a straight edge between cylinders 2 and 3. Noted about 0.25 mm, part of which was due to the difference in thickness of the VR gaskets, the rest perhaps due to difference in the used 98 mm cylinders.

Maybe the case is warped after 115 miles (an SC with a warped case?)? Don’t know, don’t care. I wasn’t planning on touching this engine because it was fine, but well the wife wanted me to paint it, so I pulled the engine to paint, then do the valve guides and head studs (none broken) and it ballooned from there. So if the case was warped before I started, it did not effect performance any.

Well that's great - glad you got it sorted. Onwards and upwards!