[b-man]

Having recently played around with both of these engines I thought I'd post some pictures to illustrate the differences between the 389 and 421 blocks. Both blocks shown are of 1964 vintage.

389 block.



421 block, note the flat machined face on the passenger side just below where the head bolts on.



389 block, no transfer lug.



421 block has the transfer lug. This lug signaled the line workers to transfer the 421 blocks to a different machining center, the 326 blocks had one as well. The far more common 389 blocks didn't have the the transfer lug cast into the block.



389 block with 2-bolt 3.00" diameter mains to accept the factory 3.750" stroke crankshaft. All were equipped with 2-bolt mains with the exception of the rare '59 -'62 420A, 425A and SD 389s. All of the famous '64 -'66 GTO 389 engines were 2-bolt main engines. The somewhat peculiar-looking oil baffle/windage tray and dipstick tube arrangement that attaches to the #4 & #5 main caps is a feature specific to '64 and earlier blocks.



421 block with 4-bolt 3.25" diameter mains to accept the factory 4.00" stroke crankshaft. All 421 blocks without exception were factory equipped with 4-bolt main caps.



389 block casting (4.0625" standard bore size), note the scalloped side of the block at the deck where it mates with the head.



421 block casting (4.09375" standard bore size) is nearly straight across at the deck. There's a lot more metal in the casting where the side of the block at the deck meets up with the head. More iron is cast at the top of the deck near the valley area as well.



389 block, lifter bore area.



421 block lifter bore area. The three stiffening ribs connecting the lifter bores across the center are thicker.



389 lifter bores.



421 lifter bores, considerably thicker and more robust. Pretty surprising how much more metal was used in these 421 blocks compared to the common 389 blocks.

[ho421]

Great info.

[3fastgtos]

Makes me want a 421 block, thanks for sharing
SD

[brucee70]

Great pictures ...thanks for the info. :-)

[Engine-Ear]

Very helpful...I love how easy it is photo-document stuff like this these days.

[tom s]

Bart,389 not made for a block mounted starter?Tom

[tom s]

Sorry,saw it is.Tom

[Ragtop Man]

Great scholarship, B-Man.

Any differences between early and late 421 blocks... and 428's?

[hurryinhoosier62]

Seeing the difference in the wall thickness and reinforcing ribs in the lifter valley makes me realize WHY Nunzi advised me to avoid putting a stroker crank in my '64 389.

[Scarebird]

If they are both bare how about a weight comparison?

[b-man]

Originally Posted by Ragtop Man

Great scholarship, B-Man.

Any differences between early and late 421 blocks... and 428's?

I'm not sure, just happened to have my hands on these two and took the opportunity to take some pics and wrote up this comparison. I do have a 2-bolt main '69 428 block that I may dig out later to document after some cleaning.

Originally Posted by Scarebird

If they are both bare how about a weight comparison?

I'd really like to but the 389 was sold a while back. The 421 is currently at the machine shop, but perhaps it may be possible to get a weight on it while it's still bare before it gets put together.

[Engine-Ear]

Originally Posted by Scarebird

If they are both bare how about a weight comparison?

Interesting question. Also comparing 389 and 421 cranks' weights would be interesting.

[Willshire]

Good write up