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Stock Crankshaft Limitations 1275

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#1 bigfish


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Posted 11 August 2019 - 02:21 PM

    Look to replace a damaged CrankShaft in a 89 1275.  I have a few mods planned while the engine is out.  Head, cam ect.  Looking for around 80 ft lbs of torque from completed package.  My question is would a stock crank be all that is needed or do i need to step it up to a modified crankshaft such as the tough enuff crank from 

specialist Components or the like. Not looking at the billet cranks just the modified options .   The car will never see track day use.  Just looking for a healthy engine that will hold up at that performace level.


              Thanks so much


#2 carbon


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Posted 11 August 2019 - 04:35 PM

Stock crank will be just fine at this torque output. Just make sure you fit a decent crank damper.

#3 DeadSquare


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Posted 11 August 2019 - 05:29 PM

Nobody can give you a definitive answer to the question "all that is needed", because they do not know to what your crank has already been subjected, but ponder on these things.


The full 80 ft lbs is only going to be produces for part of the time that the engine is working.


80 ft lb is not going to bend a 1300 crank beyond it's elastic limit.


As a matter of good engineering practice, at least a 100% safety margin is built in.


Rolled radius cranks are less likely to initiate a fracture.


Polishing, shot-peening and a "tuftriding process" all help to reduce the initiation of fractures, and when I used to make items for the M.O.D., all three were sometimes specified, in that order.


Broken Mini cranks are so rare that they get spoken about, and in 60 years I think I can count the ones that I have heard about on one hand.


Just as the moon never falls to earth, because it moves on a bit, the torque produced by the pistons is relieved by the flywheel rotating a bit, and the flywheel is there to store the torque.  This is a cushion so that if you drop the clutch when stationary, the full 80 ft lb is not suddenly being transmitted through the rear main journal.


As an aside:-   ...   Helicopters do not have the luxury of a heavy flywheel, so all strain from changing blade pitch is transmitted straight to the crank.  The engines therefore have a recorded "life" of about 1,000 hours before they are stripped and inspected, and commercially their cranks are usually good for a second cycle, ie: the equivalent of 120,000 miles

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