Main cap bolt/stud thread sizes 66 389
 

What is the thread size of the bolt holes for the rear main caps? On a related note, Milodon part # 81223 is listed by Summit as the main studs for Pontiac V8 w windage tray. What I received is 10 identical 1/2 thick studs that appear to be way too long to allow the tray to bolt up. Somethin' ain't right if ya know what I'm sayin'.

Do you still have the original bolts for the main caps?

If so send back those studs and reuse the original bolts.

The thread size is 1/2-13 for the first 4 caps and 9/16-12 for the rear main cap.

Is there a reason why the factory main cap hardware can’t be used?

This engine was disassembled about 30 years ago hastily and sloppily. Since then it's been through 6 or 7 moves and inherited pieces and lost pieces and never been in any state of organization. The cap bolts are part of a pile of head bolts and cap bolts from 2 different engines and I have no way to be sure the history or condition or which ones are the correct ones. It seemed like new studs was a simple fix that is represented as an upgrade.

So sort through your pile of hardware, identify the main cap bolts which should be pretty straightforward and if they’re not rusted or otherwise damaged your problem is solved.

The correct bolt lengths will be easy to ascertain, the four front caps and the one rear cap which takes the only 9/16” hardware in the entire engine will all have roughly the same amount of thread length protruding out the bottom of the caps.

What difference does the history of a few bolts really matter in the scheme of things? If they’re looking okay then use them.