Setting Up Engine Main Shaft Bearings

One key to long engine life is how the main shaft bearings… aka sprocket shaft and pinion shaft…  are set up. The running clearance of the bearings should not be set up until the crankshaft assembly has been built and trued. The truer the flywheels are, the tighter you can set your bearings up. If we lived in a perfect world both shafts would be exactly in line and minimal clearances could be used. However if you have built a few Harley engines you are well aware that the world is nowhere near perfect. The further from true your shafts are, the more clearance you want to put in your bearings, without getting excessive.

The factory specification for sprocket shaft bearing end play is .001″ to .007″. I like to set them up between .002″ and .005″.  I consider .001″ too tight and anything over .005″ seems sloppy. A properly set up sprocket shaft bearing should have no problem going 200 thousand miles. They are a very durable and tough bearing. Of course the rest of the engine won’t last that long, so you will get to inspect that bearing about every 50,000 to 70,000 miles.

Factory specification for the pinion bearing fit is .0004″ to .0008″ new, and up to .002″ as a wear limit. As with any roller bearing I like to see a nominal of .001″ and use a range of .0008″ to .0012″.  I think anything under .0008″ is too tight. I’ve tried different clearances in my own personal motors,  and I can absolutely tell you a little oil clearance goes a long way. Again sloppy clearance isn’t any good, but when set up right it will take a long time for the bearings, shaft, or race to wear out. However when set up too tight the hardness comes right out of the race or shaft.

As a last note if the flywheels don’t true up nice, less than .002″ on each shaft, throw them the fuck out and buy some new ones. Even .002″ on each shaft ( for a total of .004″ between the shafts ) is excessive.