The Stovebolt Site Forums The Shop Area Engine Shop tight GMC 305 V6

I am in the process of rebuilding the 305 V6 in my 66 GMC 4x4. The problem I hit today was when I torqued the main bearings down the crank wouldn't turn any more by hand. If I loosened the bolts then it would turn. The carnk is still stock diameter and the bearings were not oversized. Any thoughts?

If it were me, I'd do two things:

1 - get my micrometers out and carefully double-check all journal, bearing, and cap dimensions. Parts do get mislabeled.

2 - lay a good straightedge along the bearing saddles, and use a bright light to check for slight misalignment. It *may* need to be line-bored.

I've always checked motors before reassembly, and never seen a problem, so I'm not a complete expert.

George
53 3600

Had this problem on a 351 years back, work was done by a mechanic that was a notch above cro-magnon. He rebuilt the engine, had crank ground and put in new bearings. Not sure if the problem was the main or the rod bearings, but they were way to tight. Truck wouldn't go over 25, after a month we got it up to 40, just before she froze up. :mad: He swore on a stack of bibles that he put in the right bearings and kept telling us the truck would improve once it was broken in.

Have to agree with George, mistakes do happen. Find a set of mikes and snap gauges and double check all of your bearings, something is wrong.

You should be able to nail down which bearing is causing the problem by only tightening one at a time. In the parts industry, parts are returned and exchanged all the time. Somebody could have put one half or both halves of an undersize in one of the boxes. Once you find the culprit bearing, you can add a shim under the cap just to get an idea how far out it is, not to leave in of course.

I always use Plastigage when I assemble an engine. Place the small strip of plastic between the crank journal and the bearing and torque down the bolts. Remove the bearing cap and check the plastigage on the package. This way you can tell if you have too little clearance or not enough.

Just a thought,,,,are all the bearing caps on in the correct place and direction?

I'm thinking your engine uses a rope style rear main seal? If so, when the crank caps are tightened, the new seal adds enough friction so you can't turn the crank by hand.When using a rope seal, I install the crank with no seal, torque the caps one at a time, feeling crank rotation each time.Then if all is well,I disassemble and install the rope seal, remember to lube the area on the crank that contacts the seal.The above procedure is used in addition to miking the crank if you doubt the clearances.Never assume the other guy did his job properly, check everything.