[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.

[242177P]

Nice. Those photos provide incredible contrast.

I was under the impression the transfer lug was actually a mechanical feature though, as in it would trigger or catch something while on a conveyor.

[steve25]

I think I recall the reinforcement ribs on the back side of the 421 blocks being thicker than the bread and butter 389 motors also, can you confirm that for us?

[mgarblik]

Pictures are great. If only the 421 block had the 3.0" mains!. That would have made a good block even better. The 389 lifter bores are really scary thin. A .903 lifter would leave a paper thin wall.

[Tom Vaught]

Mike, I have a 64 421 376 HP Block and a few times have considered a 3" main cap and a spacer for the block. Would have to figure out the thrust deal though. Probably not worth the bank for the buck.

Tom V.

[b-man]

Originally Posted by 242177P

Nice. Those photos provide incredible contrast.

I was under the impression the transfer lug was actually a mechanical feature though, as in it would trigger or catch something while on a conveyor.

That makes perfect sense, especially after looking at the slope of the lug on the front side.

We're all learning here.

[mgarblik]

Agree, Tom. My comment was more of a "what if?" Under 7000 RPM with a proper crankshaft, clearance, and balance, not worth it at all. Pontiac engineering must have had experienced some crankshaft failures back in the day prompting the 3.25" main. They probably had little notion of where the hobby would eventually take their basic 348 HP V-8!

[J.C.you]

I believe it all came down to the cost of each assembly for the typical street car 421 and what its rpm design limits were. If they had made a 3" main higher winding version 421; a steel crank, steel rods, improved head/intake package and all hardware would have to be used. Rather than do this they just went with the bigger main cast crank, cast rod, and head that petered out at 5k or so. Cheaper...

[Tom Vaught]

Possibly Mike, I was in college with a student that had worked for GM during the initial testing on the 455 SD engines and they had lots of failures with the engine on the dyno apparently. He would know (he was doing the testing as were other dyno technicians at the time). Some say that is why Martin Caserio (head of Pontiac at the time) had the data to kill the 455 SD program. The only other story I heard was some racers cut down the 421 SD cranks and installed them in 389 425A (3" main) engines when they had race issues.

Tom V.

[steve25]

I have heard that those early 455SD crank issues where before they confirmed the need for a 80psi oil pump to even just turn 6k RPM 24/7?

[J.C.you]

Originally Posted by steve25

I have heard that those early 455SD crank issues where before they confirmed the need for a 80psi oil pump to even just turn 6k RPM 24/7?

I believe the Pontiac faithful of today has proved that is not necessarily the case, rather a oil control issue was the culprit.

[KEN CROCIE]

As Pontiac increased the stroke , they also increased the overlap of the rod throw and main journal. IIrc, that overlap was maintained at about 5/8". The exception is the 455.
The main journal speed of a 3.25" main is 8.33% faster than a 3" main. A 421-455 @ 6000 rpm has the same bearing speed as a 326-400 @ 6500 rpm. Considering we generally turn the long stroke cranks slower , I don't think there is an issue here.