924Board.org

[ideola]

OK, I must be losing it or something.

Trying to get a cam installed on the new 931 motor I'm building.

What I've always done is:
- Assemble short block
- Rotate crank so all pistons are half way down
- Install head gasket and head without cam onto short block
- Install cam
- Rotate cam to TDC
- Carefully rotate crank to TDC
- Install timing belt

The problem I'm having is that when I install the cam and torque the cam bearing caps to spec, I can't get the cam to rotate. It's too stiff, and I'm afraid of using the cam sprocket bolt for fear of breaking it off in the end of the cam. Also, the lifters are not making contact with the short (non-pointy) side of the cam lobes. There is about 2mm of gap.

This is with a standard 931 head, rebuilt by a new-to-me machinist, with stock valves, guides, and springs. OEM lifters with 1-notch grub screws for starters.

I am beginning to suspect that he screwed something up with the valve train installation, but I can't figure out what he might have done. It only had a 3-angle valve job, no new seats or anything. Valve tip height appears to be normal compared to other heads I have lying about. I've tried two different cam with same results.

Before I go to the trouble of taking the head back to the machinist, I want to make sure I'm not forgetting something completely obvious. It's been a couple of years since I built my last motor, and I'm getting old, apparently.
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erstwhile owner of just about every 924 variant ever made

[Fasteddie313]

Are you sure that you re putting all of your cam bearing caps back on the same place they came off and in the right direction?

You know.. Line bore.. They are all unique.. Mixing them up can make your cam bind..

Try putting the caps on with no cam and see if you can feel ledges where the caps meet the head, should be a perfect circle with no step where they meet..
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80 Turbo - Slightly Modified

[ideola]

[Fasteddie313]

Well first I would double triple check those cam caps..

Then I'd put your cam(s) on V-blocks on the 2 outermost journals, and use a dial indicator on a mag base to check to see if your cam(s) is true and straight.. I'd guess somewhere within 5/10,000 idk..

If that checked out I'd start measuring the cam journals on the cam, 2 measurements each 90 degrees apart, write em all down..

Then put your caps on torqued to spec and measure the journals in the head with a bore gauge, 2 each 90 degrees apart, write em all down.. Compare #s..

If you get different measurements on each head journal 90 degrees apart it would probably be those caps..

Compare your #s and see what you got..

You could plastigauge it instead..
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80 Turbo - Slightly Modified

[ideola]

Winner winner chicken dinner.

Cam bearing caps. I swapped on a matched set from another head, and everything rotates easily.

So the bearing caps from this head are suspect. Which confirms my instincts were right in the first place when I asked him to line bore the head.

So looks like that's the next step.

Also, with the other bearing caps in place, once I rotated the cam a few times, with the single notch grub screws dialed in just far enough to fully engage the threaded bore all the way through, tappet clearance as at .75mm pretty much across the board. So that's a good sign, just need to get the correct set of screws swapped in to set clearance.

Whew. Thought I was losing it.
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erstwhile owner of just about every 924 variant ever made

[Mike9311]

Ugh, now I know what will have to be done. I have caps that fell off one head or another. Not trusting those without boring

This was interesting. Glad you solved your issue
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car

[motormouth]

When you line bore the caps/head, how do you take up the slack between the cam and the bores? I thought that the caps were the bearing surface for the cam; if you remove material, what takes the space of the material you machined out?

[ideola]

The amount of material removed should be relatively miniscule, and if only (or mostly) taken from the caps (as opposed to the head) would have negligible impact on cam timing. There will likely be some impact to valve clearance, but this can be accommodated using the OEM grub screws, even if having to switch out to the next size up on the screws.
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

[KDJones2000]

True what Rasta says, but your machinist will likely tell you that you cannot "mix and match", as different caps are going to have different "horizontal" alignment than the head.

We tried that on an old Ford block that had 2 broken caps and we wanted to use ones from another block, and they told me no way, and to just get another block.

About the only time you can remove material and re line-bore the cam or crankshaft is when you have spun a bearing, and are using the same cap, just cleaning it up.
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1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S

[ideola]

I'm not sure I buy that...how do you think they machined the caps in the first place?

In any event, still waiting to hear back from machinist on someone who can do it anyway. I've sunk big dollars into this head, so there's no way I'm abandoning it now because of the caps. I have many sets of caps lying about, and one set or another is bound to work, even if it may not be the ideal solution.

Stay tuned.
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erstwhile owner of just about every 924 variant ever made

[dubrict]

I broke a cap a few years ago, went to the junkyard and pulled 4 or 5 out of old 2-liter volkswagens.. found one that didn't bind and have been running that motor for at least 3 years now no problem. Just sayin'
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1979 924 NA
2009 Chevy Impala
2005 Dodge Dakota

[TJC]

Rasta does have the correct explanation for making the bore "smaller" and then bringing back to specification again.

As for not being able to use different caps, if your machinist said you can't do that then I would suggest that you find a different machinist. I worked at a high performance/race engine machine shop for a number of years and I will give you just one example. We built a ton of small block Chevy Sprint Car engines and invariably just about every one with a cast iron block was fitted with a set of Summers Brothers billet steel main caps and align bored to a rough spec and then align honed to the finished spec. So, is it preferable in a stock application to use the caps that came with the engine/head (for ohc applications), even connecting rods...yes, is it a hard and fast rule that you must...no.
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'95 BMW 318i/5 ..."Pearl"
'87 Porsche 944 NA... "Liebchen"
'02 Porsche Boxster..."Sunbeam"
'04 BMW X3..."Xander"
Still on the Prowl!

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