Do the Pontiac 389 and 421 use the same block?

[LDO]

Just curious. I've heard conflicting answers.

[Ace-Garageguy]

If I remember correctly, the 421 uses larger main journals than the 389. The 421 also had 4-bolt main caps. This would necessitate a different block casting, though they are very similar visually (on the outside).

EDIT: Yup. Checked it. Definitely different. The 421 is beefier in other areas as well.

Edited April 21, 2018 by Ace-Garageguy

[TomZ]

1 hour ago, Ace-Garageguy said:

If I remember correctly, the 421 uses larger main journals than the 389. The 421 also had 4-bolt main caps. This would necessitate a different block casting, though they are very similar visually (on the outside).

EDIT: Yup. Checked it. Definitely different. The 421 is beefier in other areas as well.

According to his autobiography, Mickey Thompson used Pontiac in his Bonneville record car because they had bulletproof bottom ends.

[Ace-Garageguy]

Yes, that was the party line at the time.

Also at the time, the inability of the Challenger I to make its return run (to clinch the official Land Speed Record) after clocking 406.6 MPH one way was blamed on a gearbox failure. Now, it looks like it may have actually been an engine failure, but Thompson didn't want to put the blame on a partner he received considerable cooperation from. The gearboxes in the car were essentially junkyard-sourced LaSalle units.

The engines, supplied by Pontiac, may have been test units that had already had a hard life before being delivered to Thompson. 

I had hoped to have met and talked extensively with his son Danny by now, but so far, I haven't been able to due to other commitments.

Whatever the real reason, Mickey Thompson's achievement ranks as one of the most significant events in the history of hot-rodding.

[Carmak]

For the purposes of modeling the 389 and 421 only differ externally in one way (other than part number). The 421 block has a small cast lug (about the size of a piece of Brach's caramel) directly above the top transmission bolt hole on the driver side. It is know as the transfer lug. 

After 1960 ALL Pontiac blocks that were not either 389 or 400 received a transfer lug. Therefore a 421 and 326 block are  externally the same as they both have a transfer lug.

Internally the big difference is larger main journal diameters. This was a blessing and a curse. The crank was stronger but the bearing surface speed was higher for a 421 than a 389 at any given RPM. A stock 421 could spin faster than it's oiling system keep up with. This typically did not end well.

 

[Robberbaron]

Aren't the 428 and 455 the only other blocks with the big journals?