New 421" SBC just back from the engine builder and run on a dyno. I put all the accessories on and at that point the engine spun freely because I had to keep the flywheel from spinning when I tightened the pulleys and coupler. I put the thing in the boat and now it's locked up. Starter won't turn it over and I can't turn it over using a socket on the crank bolt. I removed the starter and still can't turn it over. The only thing I can think of is that I replaced the coupler bolts with allen head bolts. The heads of these bolts are a little taller than a regular bolt. Is the clearance inside the bell housing that close that they could be hitting the housing? Any other ideas??

Sounds like you are going to have to reverse the installation process until the engine turns again, then you'll know.

Sounds like a good place to start looking.
I don't know what the clearance in the housing is but I can't imagine it's that close.
Is the thread length the same as the old bolts? If they're longer they could be striking the rear of the block and stopping it there.
Good luck.

Thread length is the same. I want to make sure I'm not missing anything obvious before I pull the engine back out.

I had the same problem with mine. Turned out with the new crank pulleys I put on they required shorter bolts. Make sure your crank pulley bolts are not hitting the pan or the timing chaine cover. Otherwise I would pull off the crank pulley and then see if she turns free, If not then it has to be those bolts I would think since you already had her on the dyno.

Jon

Along with the obvious clearance issues, the bolts used to hold the coupler are similar to clutch cover bolts, they need to have a shoulder on them that pilots in a counterbore at each bolt hole in the flywheel.

marc,
Are you saying there definitely is a very close clearance there so that is most likely the problem?

No, sorry I did not mean that's your problem for sure, I just wanted to point out that there is another issue in regard to the coupler bolts. I do seem to rememeber having that problem the last time I used some ARP bolts and hardened washers, they were pretty tall and did hit the flywheel housing.

Real good point Mark belive part number 10-14883 coupler bolts, 10-845757 flyweel bolts.Ordered mine on monday

What I don't understand is that the motor spun free after putting on all the accessories including the coupler. So, the problem shouldn't be in the motor or at any of the accessories. Are you guys thinking that the coupler bolts are hitting the cover??? Can the clearance be that tight in there. After all the difference in the height of a hex bolt and an allen bolt is maybe 1/4".