389 VS 421 Blocks
 

A guy on the GTO forum posted this link and I thought it would be interesting to folks here. It came from the HAMB forum. The structural differences between the 389 and the 421 are quite interesting. The photography is great...enjoy.

https://www.jalopyjournal.com/forum/...blocks.972245/

Also originally posted here in the street forum archives:http://forums.maxperformanceinc.com/...d.php?t=772995

You're welcome. :)

389 vs 421
 

Very interesting! Didn't know that about the lifter valley area, or the deck area of the blocks being different

Thanks for the link, so many more great pics! When I saw it on the GTO forum and wanted to share it here, my senior moments didn't think to check here.:o
Appreciate it!

Yep, Bart "B-Man" (One of our Moderators) has done done very nice documenting of Pontiac engine parts over the years. Thanks again Bart! LOL!

Tom V.

The story goes that Smokey Yunick used 425a blocks in the early 60s NASCAR engines because the 3 1/4" mains in the 421s created too much heat and would not live for 500 miles at the rpms they had to run. 427 Fords and Mopar 426 wedge and hemis only have 2 3/4" mains. Pontiac and Oldsmobile really over engineered their engines back then.

Year ago when Smokey was alive and visited the PRI Shows I had a chance to meet him and ask questions about the Pontiac 61-62 Twin Turbo (M/T Crossram dual AFBs Blow Thru) system he ran on his dyno. I wound up eventually buying all of the parts for that system.

Randy Williams (at one time before his passing) had a goal to build a Pontiac vehicle with that "Smokey" Turbo set-up on it.

He bought all the parts from me and then he passed on without the engine/parts ever being used. That Twin Turbo engine was also based off of the 425a block (mentioned above) vs the 421 block. He was not happy with the engine set up (with the crossram) though.
It backfired one time and puffed one of the AFBs right thru Smokey's tin roof in Florida.
Smokey had some cool cast iron Turbo Manifolds, and of course the Crossram but also some special TRW/Rajay Turbos and special high flow wastegates.

So Smokey was a "Boost Guy" also at one time.

Tom V.

425-A was the only early moter i've been through. Promenant Pan-rail casting feature.

350s, 400s, 428s, & 455s all look to me MUCH closer to the 421 photos, but not so thick on the 3- valley webs, nor so straight along the Head- gasket edge.

But the 389 lifter bores are silly-thin walled. What a shock. Never seen such thin walled lifter bores on any PMD block.

I've always thought of the 428 as a factory over-bored 421. Looking at the pics, I wonder if Pontiac continued the heavy duty features of the 421 block like the thicker lifter webs and bores or was the 428 more like a 400 block with larger crank/main bores?

The 428 blocks were quite different from the very robust 421 blocks. As you have surmised they're quite a lot like the same year 400 blocks just with larger main bearing bores.

Here's a 1969 YH 2-bolt main 428 block that I have stashed under my workbench. Note that the block is scalloped and not straight across at the deck like the 421 block. The lifter bores and bracing aren't quite as thick as on the 421 block. Another difference is 2 core plugs per side on the 421 vs 3 per side on the 428.

http://i137.photobucket.com/albums/q...h/DSC03386.jpg

http://i137.photobucket.com/albums/q...h/DSC03388.jpg

http://i137.photobucket.com/albums/q...h/DSC03391.jpg

Thanks, Bart. Ah, well, one could always hope until reality sets in. Appreciate the pics and info!

This reminded me of when Jeff Koerner went to the PRI show with the flat tappet roller lifter he invented, and wanted to show it to Smokey. Jeff waited in line until it was his turn, then he handed his lifter to Smokey. Smokey looks at it, and spins the foot on it, back and forth, then looks up at Jeff, then looks back down at the lifter, and spins it some more, then looks back at Jeff, and says, "well now, that's a pretty damn good idea there", and hands it back to Jeff.

A shot of the same 421 block that was featured in the 389 vs 421 comparison thread, all built up and coming off the test stand.

With any luck I'll have it installed in my '64 Le Mans convertible within the next few months, backed by a fresh TH350 and 13" Continental converter.

http://i137.photobucket.com/albums/q...O/DSC02402.jpg

One of the most under valued pontiac blocks is the 61-62 389 block casting 538181. The same block is used for the SD engines so it has all the good casting bits that b-man has pointed out and has the 3" mains as a bonus.The two bolt engines also have masive mains and so may not need to be upgraded to four bolt. I may be wrong but I beleive all SD engines were bored to 428 so its a good chance they are a quite thick wall block ( I have not had mine soniced yet).The only down fall is the block mounted starter but a adaptor plates gets round that problem.

67 and up blocks weigh in at a good bunch more then 389 or 421 blocks.

That's a beautiful thang, Bart.

K

Steve you are correct about post 63 blocks but the 61-62 blocks come in at 187 ibs compaired to a 69 400 block at 177 lb and 455 block at 181 lb. They took some meat out after 63. Fast 69 posted that 59-60 blocks were 184 lbs so the 61-62 blocks have more meat than those as well.

Huh? The '59-'60 389 and the '67 400 blocks are known to have the thickest cylinder walls and heaviest bulkheads of any production Pontiac blocks. Mine weighed in at 225lbs when shipped (210 for the block, 15 for the pallet). A member on here bored a '59 389 to 4.250" (+.190 over stk) and still had acceptable cylinder wall thickness. Another member ultrasonically tested a '67 400 block and found it to have cylinder walls at more than .200 thick(non-thrust side) before boring.

Just curious. Maybe I missed it on this thread. What did a typical 421 block weigh? Or did they vary over the production run. I have a '66 9782611 (think that's the right number).

I posted that on average the 3 per side freeze plug blocks weigh in more, not less!