Crankshaft Size

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Crankshaft Size

Postby Dragster_Racer » Mon Nov 27, 2017 4:58 pm

The new motors being built with 3 1/2” bores are using 1” diameter crankshafts. Some older ones out there used the 7/8” crankshafts. Is there is big difference in reliability? Does the 7/8” crankshafts have much lower value in the used market?
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Re: Crankshaft Size

Postby tanners » Tue Nov 28, 2017 10:37 am

The 1" crank makes the engines a little more bulletproof, however I have not had issues with a 7/8' journal crank. When I run a 7/8" journal crank, I swap the cranks out at 750-800 passes. I ran a 7/8" journal crank to failure at approximately 1200 passes a long time ago just to see how many passes it could go. The 7/8" journal cranks are typically used in engines with smaller timing gear sets which put the crank and cam closer to each other. By running the smaller journal, the big end of the rod is smaller and allows the cam not to be cut down so much.

I have never had an issue running either 7/8" or 1" ARC cranks as long as I kept my refresh interval under 250 passes and replaced a 7/8" journal crank at 750-800 passes. The only engines that have blown up on me turned out to have cheaper Chromoly cranks installed. While the cranks didn't break, I lost 3 motors in a row with low passes. The rods broke and the clutch post on the crank twisted. Once I found out what had changed and went back to ARC standard cranks, I have not lost another rod or crank since.
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Re: Crankshaft Size

Postby sprintb2 » Wed Nov 29, 2017 3:54 pm

The only 7/8" cranks I've had break are the ARC 6499, one was about 550 passes, other was in a used engine bought by a friend that was supposed to have less than 100 outlaw passes and then got 36 bracket passes. Another guy we race with broke two in about three years. I won't put one in an engine any more. I've never had any other ARC cranks break.
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Re: Crankshaft Size

Postby Pool Man » Thu Nov 30, 2017 9:53 am

Out of all the years we have raced I have had only one ARC 7/8" crank break. I wish I had a photo of one of the cranks I had to cut a clutch from. It was butchered. Continued to run the crank for a couple of more years with no problem.
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