OK, guys. I have searched these threads for days and have not seen were anyone has had this problem.

I started out just going to replace the rear main seal on my 79 351M. After I removed the rear cap, i discovered the bearing had some wear. I figured since I already had the oil pan off, I would throw in new main and rod bearings. I got the main bearings installed in the 1-4 mains and the motor spins free. When I installed the rear main cap, it locks the motor up. Will not spin at all, i can loosen the bolts and it turns free again.

I've tried for several days to make different adjustments but nothing has worked. I've even installed the old bearing still locks it up. I'm really not wanting to pull the motor out. Hoping someone with more experience with these 351M has dealt with this before and knows a trick.

Thanks in advance.

 

rear main installed wrong?

 

Check the rear main journal very carefully for any knicks or other damage. A very small knick could cause it to lock up.
How did you replace the main bearings? Did you replace the upper half also, if so, how did you roll the top half out?
A distant possibility is the bearing is oversized and marked or packaged wrong but I doubt it. That ones really reaching.

 

If you already double checked to make sure that the bearing was installed correctly, then you might be able to try polishing the main journal. I've read about diesel guys doing this with the crankshaft still in place.

 

Thanks for the responses.

I replaced both the upper and lower bearings, and used a plastic scraper to roll the top bearing out. There were no issues when a removing it, it came right out. Even now I can remove the upper with the same plastic scraper. The locks are aligned and the halves are in the right places. The motor was rebuilt in 87 and the crank was machined down .010. The bearings I removed where marked .010 and the news are as well

The crank is smooth, no marks or nicks that I can find. Tomorrow I will grab some plastigauge and check the clearance.

I have completely racked my brain trying to figure this one out. This will be last
351m I work on...or at the least the last one that I will try to do any bottom end work on...in the frame.