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porscheboy1
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Crankshaft tolerances.
Hi guys. I am restoring a 1978 911 SC and it is time to stare re-assembling the engine. I miked all the journals on the crank shaft and they are right at the bottom of the allowable limit. The Bently manual states for the lower limit 2.3611 for the mains and I have for the six mains +.0003, +.0001, +.0002, .0000, +.0002 and -.0001 above and below that limit. On the rods the Bently manual states for the lower limit 2.0855 for the rods and I have for the six rods .0000, +.0002, -.0001, -.0001. -.0001,.0000 and -.0001 above and below that limit. All the journals are round and have no grooves worn in them that I can see or feel. Could I polish them a bit with some fine crokus cloth and put in a new set of srandard bearings? Are there available beasings a few ten thousands over size? If I need a regrind to whom could I send the shaft that can give it the proper heat treatment to bring back the required hardness. Thanks, John
1964 356 SC 1978 911 SC 1982 911 SC |
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12-06-2012, 02:32 PM
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Join Date: Jan 2004
Location: CT
Posts: 11,538
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Here is a typical inspection report for a 3.0 crank. All dimensions in mm.
Note that the wear limit is the min diameter allowable for rebuild. The limits in the Bentley manual are limits for a new unused crank. 
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Tom Butler 1973 RSR Clone 1970 911E 914-6 GT Recreation in Process Last edited by tom1394racing; 12-07-2012 at 04:13 AM.. |
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12-06-2012, 05:42 PM
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Join Date: Oct 2003
Location: Central Fla
Posts: 1,864
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crank
John, send your crank to Armando at CCR in Ca. he has done all my 911 cranks for years, If you are at the lower limits and round I would have him verify that, pull the galley plugs, clean it and if needed heat treat the crank and it will grow a few tenth's and look at the clearances with factory bearings and also you can coat the bearings and gain .0004 per shell, crossdrill the # 4 journal and groove that bearing as well.
Mike Bruns JBRacing.com CCR 650 342 0568 Tell him I sent you
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The two most useless things to a driver are the braking distance behind you and nine-tenths of a second ago. |
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12-06-2012, 05:43 PM
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porscheboy1
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Gentlemen, Thanks much for the information. I'll convert my readings to metric, my micrometers are in inches, and check it out.
Regards, John |
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12-06-2012, 06:47 PM
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Join Date: May 2003
Location: Detroit (Rock City!)
Posts: 783
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A tenth is pretty damn small; are you verifying your mic against a known-good standard before measuring?
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'90C4 |
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12-07-2012, 05:26 AM
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porscheboy1
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A standard bar came with the micrometer and I check it often and zero the mic if necessary. I practised a lot and am able to ger consistent readings. I also had a machineist double check me on a couple of the journels.
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12-07-2012, 11:25 AM
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Join Date: Apr 2010
Location: Houston, Tx
Posts: 951
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If your mics are of high quality, then you can trust the standards. Otherwise, if questioning a few tenths of a mil, it's worth a few dollars to have someone else inspect it. Heck, even if you have nice mics, it's still worth to get a second hand reading before you spend another chunk of money.
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12-12-2012, 09:26 AM
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