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Slippery Slope Expert
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Mike Main Bearing Bores
Is it necessary to torque the case together (prior to assembly with bearings and crank, et al) and mike the main bearing bores (w/o bearings)? I've heard of this being done but I don't think it's in Wayne's book. Case is aluminum SC.
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07-06-2014, 07:44 AM
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Registered
Join Date: Oct 2008
Location: Nash County, NC.
Posts: 8,500
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Maybe if there bearings didn't fit or the main is spun but didn't you start with a running engine.
Don't try to make things more complicated than what they are Bruce |
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07-06-2014, 09:30 AM
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Registered
Join Date: Jan 2009
Location: maryland
Posts: 271
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If you want to be sure that the lower end stays together, do it. High mileage engines do beat out the bores. Unbalanced flywheel/clutchs will open up the first main bore. Track cars will oval out the middle main bores. (The early T engines of the 70's with the non counterbalanced crank would beat the crap out of the case after 70 k miles) They seem to oval out on an axis 45 degrees to the hoizontal. You only are allowed a thou to a thou and a half for crush on the main bearings. The bores started their life round and if they are not completely round, the clearance between the crank and the bearing will vary according to the crush on the main bearing. More clearance, less oil delivered to the next bearing in line.
Same theory applies to the rod big end bore. |
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07-06-2014, 10:52 AM
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Registered
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
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Matt is spot-on on this. Mag cases are far worse than aluminum ones, but its worth checking on any SC/Carrera/Turbo cases that have a lot of miles on them.
These issues can be properly repaired by machining the case parting lines, then re-boring the main bearing bores back to standard (to prevent using OS bearings). Naturally, the rods should be reconditioned anytime they are removed from the crank by re-sizing the big ends and re-bushing the pin bores.
__________________
Steve Weiner Rennsport Systems Portland Oregon (503) 244-0990 porsche@rennsportsystems.com www.rennsportsystems.com |
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07-06-2014, 11:15 AM
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Slippery Slope Expert
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Thanks; I had planned on doing it, but was trying to simplify things. I knew we did it on my 914-6 engine long ago. It turned out to be fine, which was surprising considering I had an S rev limiter in it and used it a lot!
One thing I didn't know was the 45 degree thing; I would have measured across and vertically. |
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07-07-2014, 03:41 AM
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Slippery Slope Expert
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When doing this is it necessary to install and torque the perimeter nuts/bolts also, or would just the through-bolts at the main bearing saddles suffice?
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07-08-2014, 03:08 AM
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