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Crank / Main Bearing Clearance Questions

Hello all.
I have torn down and am going through the engine on my 1978 911SC. As part of this process, I took the case to a local shop recommended to me by a Porsche owner I trust.

The shop cleaned the case, mated the halves and did some preliminary measurments with a dial bore gauge. The ininitial measurements suggested that standard main bearings would be in order. I had them shipped in (Glyco from Pelican), the machinist installed them in the case and measured once again with the dial bore gauge.

Spec for crank / main bearing clearance is .010 mm (.0004 in.) to .072 mm (.0028 in.).

With the new standard size main bearings and the case torqued to spec, the actual measurements ranged from .076 mm (.0030 in) to .089 mm (.0035 in). This exceeds maximum recommended clearance by .004 mm (.0002 in.) up to .017 mm (.0007 in.).

So the question is how to proceed. Options: Run the standard bearings. Buy oversized bearings. Send the case out for shaving and line boring. Maybe other options that I'm not aware of? Please advise.

Thanks, Gary S.


Last edited by brg2290; 04-15-2011 at 08:17 AM..
Old 04-15-2011, 05:44 AM
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Gary

Are your bearing clearance numbers calculated from the difference between the crankshaft main journal diameters and the installed main bearing diameters?

Did you check to see if the crank journals were worn beyond std size?

If you determined the clearances by the difference in diameters, it is more likely that the crank is worn rather than the case. SC Al cases are usually pretty durable as confirmed by your case bore measurements.

Also, the calculated clearances are out of spec by only 0.0006". This is inside the the limit of combined measurement accuracy of each of the measuring tools (micrometer and dial bore gage).

I would recheck all the measurements again very carefully and confirm that the crank is not worn beyond the factory rebuild limit. Also make sure that the dial bore gage is zeroed using the same micrometer used for the crank at the crank diameter so the gage will read bearing clearance directly.
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Tom Butler
1973 RSR Clone
1970 911E Restoration in Progress

Last edited by tom1394racing; 04-15-2011 at 09:56 AM..
Old 04-15-2011, 07:40 AM
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Hello Tom.
Thank you for responding. Yes, I'm quite certain that the numbers were arrived at by subtracting the crankshaft main journal diameters from the installed main bearing diameters.

Are the crank journals worn beyond std size? No. I spoke with the machinist today, and his measurements were 2.3616 inches, which is to the good side of the spec. 2.3610 - 2.3618. I also mic'd them at home tonight, and my measurements show the crank journals to be within spec. as well. While the machinist had it, they micropolished the journals, and they look very nice.

The parts are no longer at the machine shop, I picked them up yesterday, so I currently have no access to a dial bore gauge. But I'd have to say the crank itself is good. As you mentioned, others have commented that the SC Al case is quite robust. Maybe my next step should be to unbolt the case and check the thickness of the new Glyco main bearings?

Gary S.
Old 04-15-2011, 06:05 PM
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Measure with plastigage. Separate the case and lay the plastigage along the bottom of each main bearing. Parallel with the crankshaft. Torque the through bolts to spec and then separate the case. Read the actual clearance.

The measurements you have already are probably close enough. Bore gages are very hard to get an exact inside reading.
Old 04-15-2011, 06:34 PM
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I think the jury is out on plastigage for accurate measurtement of bearing clearances

Measuring bearing clearences
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1973 RSR Clone
1970 911E Restoration in Progress
Old 04-16-2011, 02:27 AM
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ededoad, thanks for the suggestion.

Tom, for some reason your link just takes me to a main forum page. However, your point is well taken - plastigage has its place, but this might not be one of them. Since, as you stated, the clearance excess is within the error range of the two measuring devices, I'm thinking maybe I should send the main and rod bearings out to Calico for coating and call it good.

Gary S.
Old 04-16-2011, 03:17 PM
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try this link

Plastigage Versus Micrometer - Accuracy Test - Car Craft Magazine
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1973 RSR Clone
1970 911E Restoration in Progress
Old 04-16-2011, 04:29 PM
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I wonder whether one article in a popular magazine is a reason to condemn a system used industry wide, and recommended in GM shop manuals among other places. I notice that the test was done without grease, which would help to keep the Plasitgauge from smearing, and better hold it in place. This from the Plastigauge website:

"Perhaps the most widely used application of PLASTIGAUGEŽ is in the measurement of clearance in plain automotive bearings. This application is described in detail:

Remove the engine sump cover to reveal the big-end and its retaining set-screws. Remove surplus oil and release the big-end shells by unscrewing the set-screws. Apply a smear of grease to the journal and small quantity of silicone release agent to the shell.

Trim a length of PLASTIGAUGEŽ to fit across the journal using the grease to hold it in place. Replace the shell and tighten the set-screws to the recommended torque setting - without rotating the journal.
Now remove the shell to reveal the PLASTIGAUGE which will have been spread across the bearing surface as a stripe or band. Match the width of the stripe against the calibrated gauge card supplied and read off the clearance.

Ideally you should remove the PLASTIGAUGE stripe with a clean oily cloth or industrial de-greasing solvent, but users may be assured that any PLASTIGAUGE left behind is oil soluble and cannot harm the engine in any way."

Old 04-17-2011, 06:18 AM
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