[John Milner]

What would cause a main cap to bust like this? This #2 cap was on a non running 400 in a project car of mine. I didn’t notice much wrong other than the harmonic balancer weight ring had slipped back on the hub portion of the harmonic balancer. The block was otherwise a nice standard bore block. I really don’t know much about the history of the engine. It had cast rods and standard bore trw forged pistons in it so it had clearly been rebuilt once.

[hurryinhoosier62]

Originally Posted by John Milner

What would cause a main cap to bust like this? This #2 cap was on a non running 400 in a project car of mine. I didn’t notice much wrong other than the harmonic balancer weight ring had slipped back on the hub portion of the harmonic balancer. The block was otherwise a nice standard bore block. I really don’t know much about the history of the engine. It had cast rods and standard bore trw forged pistons in it so it had clearly been rebuilt once.

Engine harmonics would do this. Perfect time to go with aftermarket four bolt main caps…

[Dragncar]

Originally Posted by John Milner

What would cause a main cap to bust like this? This #2 cap was on a non running 400 in a project car of mine. I didn’t notice much wrong other than the harmonic balancer weight ring had slipped back on the hub portion of the harmonic balancer. The block was otherwise a nice standard bore block. I really don’t know much about the history of the engine. It had cast rods and standard bore trw forged pistons in it so it had clearly been rebuilt once.

Need a cap ? On the house ?

[Geoff]

Maybe the bolts for the windage tray were too long, bottomed out, & created a stress point in the cap.

[steve25]

The two bolt caps in all Pontiac motors are just plain gray iron.
The caps on the 4 bolt motors are the higher grade Armasteel.

The balancer failing could have done that, but I would be concerned with reusing that crank now if I where you.

[Schurkey]

Originally Posted by John Milner

What would cause a main cap to bust like this? This #2 cap was on a non running 400 in a project car of mine... ...I really don’t know much about the history of the engine.

Did it break when the engine was running? Or did it break when something fell on it, after the engine was removed from the vehicle?

[PontiacHO]

Missed a gear and blasted the RPMs?

[steve25]

Not at all likely Rpm related as the stock rods let go long before a main cap will and the 3” main caps have more meat to them then the 3.250” main caps which start life as 3.00”.

[Half-Inch Stud]

That front damper is a key clue.
The Cap could be a wishbone from a #2 Main Block split. Check Main Web near oil hole.

Verify both dowels are in place and figure if loose-or-snug to Cap hole.

Otherwise check for spun Main Bearing, and scorched rod bearings.

[turbo69bird]

I’d bet the engine or block was dropped on that cap . I have a 4 bolt block that’s the same and the owner told me they dropped it when transporting it. The block was rare so I kept it but have yet to replace be caps. Everything else about the block seems fine but this hairline crack in a cap if you didn’t look for it you wouldn’t notice it at all you’d probably run it and it’d break like that .

[John Milner]

Originally Posted by Dragncar

Need a cap ? On the house ?

Appreciate that offer but I will be using a different block.

[John Milner]

Originally Posted by Schurkey

Did it break when the engine was running? Or did it break when something fell on it, after the engine was removed from the vehicle?

I am not sure when or how it broke. I bought a car and pulled/disassembled the engine to find this. I first saw a flat cam lobe while the engine was on the stand so I decided to just disassemble everything to check it out.

[John Milner]

Thanks for the replies everyone. I actually was hoping to turn and reuse the crankshaft but now I’m not so sure that it would be a good idea. The main and rod bearings looked ok other than some scratches from the chewed up lifter and cam lobe running through the oil.

[Jay S]

It should have fretting at the crack from the two haves rubbing together if it ran that way. If there is no fretting it must have gotten hit by something or the block was dropped and broke it.

[mgarblik]

Lots and lots of possible reasons listed for why/how that cap is broken. Just for fun, I will add one more. Look at the bolts protruding through the cap. It;s possible the bolts bottomed out and there was little clamp load on the cap. Pontiac blocks are counterbored in the block so the threads don't pull on the surface. But those bolts look to have 2-3 full threads exposed below the cap? Hard to say from one pic. Look at the broken area and see if it's grainy through the entire break. If it is, likely an impact break from dropping it at some point. If any rubbed areas or smooth areas in the break, the two pieces were working or rubbing together while running.

[Half-Inch Stud]

That 400 crank makes for a great Mailbox post.

[ponjohn]

Check the block.

[hurryinhoosier62]

John, if the harmonics were heavy enough to crack a main cap in two EVERY THING ( block, crank) need to be subjected to a thorough NDT inspection. I’m with HIS; there could be hidden stress fracturing in the block AND the crank.

[Joe's Garage]

Originally Posted by mgarblik

Lots and lots of possible reasons listed for why/how that cap is broken. Just for fun, I will add one more. Look at the bolts protruding through the cap. It;s possible the bolts bottomed out and there was little clamp load on the cap. Pontiac blocks are counterbored in the block so the threads don't pull on the surface. But those bolts look to have 2-3 full threads exposed below the cap? Hard to say from one pic. Look at the broken area and see if it's grainy through the entire break. If it is, likely an impact break from dropping it at some point. If any rubbed areas or smooth areas in the break, the two pieces were working or rubbing together while running.

The bolts not applying any clamping force could definitely do it.

AND then the harmonics would kill the balancer.

And probably trash the crank and block, too.

[Half-Inch Stud]

I've run split 455 block (s) on the Street just fine. Even ran 1 after a loose damper cracked, and cracked the Lower timing gear ( during a 3-4 upshift).

Block ought be re-useable, Or worth the ride to find out. Ugm wait, well i never split a Main Cap, helps to get eyes on the #2 Saddle!