I don't think that is the problem but should be on the list of things to check! I do think the proper bolts should be used there.If you use bolts that have the thread all the way up then it could take less to shear!

I am pulling the flywheel housng off a new engine tommorrow. I will try and check the clearance for you. Checking it is not a problem, remembering is the issue! My memory is toast.

Here's an update - Pulled the flywheel housing and sure enough, the coupler bolt heads were hitting the housing. Put hex head bolts in and everything is great. There is probably less than 3/16" clearance in there from the head of the bolt to the housing. Live and learn, but it always seems to be the hard way.

Thanks for the update

So how does she run???

It sounds great but it's still in my driveway! Hope to splash it this weekend.

Let us know how it works. Is a 421 a 400 with a stroker crank??

That's made with a 3.875 stroke crank. I think it's similiar to the new cranks beings used in the GM 383HT crate motors. You use it with a 400 block or aftermarket (Merlin Motown, etc.).

A close friend runs one of these monsters on the jug... Motown block, 421 CID, all roller, 9.0's @ 150+/- in a 3000 lbs. (with driver) Nova. I like it!

It's a DART Little M block with a SCAT steel 3.875 crank. Last year it was a 415, this year it's 030 over so a 421. Had it out on Saturday and it ran real good but still needs a little tweaking. The weather's been so great here, I just want to enjoy it for a while. I'm tired of wrenching.