This probably isn't the best time of the day to ask this question seeing how most of you are asleep, but are 389's and 421's the same block? I ask this question because I am dreaming of building a pontiac powered race car sometime in life. Thanks, John
Same external dimensions, but there is a difference in main bearing journal diameters. 389 & 400 small, 421(?) 428 & 455 large. Other differences through the years are motor mounts.
I've never owned one newer than '57. The 287 , 316 , 347 & 370 are all identical blocks. I know this doesn't help you at all, just bringing it BTTT until Rocky shows. Bil
Hey.. My two cents.... Go with a stroked 400 4 bolt... Am run'n a backyard built 455. Too much torque...A good 400 will rev into the 7000 grand range and with good rods will stay together.. Buzzed mine 6500 a few time an waz luckey she held together KRE out of Ohio builds a real solid motor at good prices Butler out of Tenn. is also good Pontiac Dude out of Fla also good stay away from rock & roll eng out of Cal> BAD NEWS... Pontiac Slim
Yes, they are the same block. I have seen a ton of Poncho motors built and the next one I build will be a stroked 400. They just scream. Although a 428, if you can find one, already starts with 330 hp. A nice 400 with done up 6x heads would be a nice one. There are so many options for Ponchos, all of them will pull trees out of the ground with the torque. Larry
A Pontiac, is a Pontiac, is a Pontiac. They used the same block for everything from a 194-CI 4 with one bank lopped off, to the 455. 421, 428, 455 are stroked and use smaller journals than the others. Can be a little weaker than the large-journal engines, some guys turn down the rod journals on a 400 crank for an instant 455 that has large main journals in it. This engine came in displacements of 287 (55), 316 (56), 347 (57), 370 (58), 389 (59-66), 400 (67-77), 326 (63-66 I think), 350 (67-77), 421 (62-66), 428 (67-69), 455 (70-76), 301 (77-81). I might be off a little on some of the years in the 60's. There was a 303-CI Trans-Am racing version too. If there's not a Pontiac tech post already (and I think there is) I could add a lot more on what bolts where and when, but a 389 and a 421 will physically interchange as a bolt in swap so long as both motors are either 64-back or 65 up - thats the year the starter and trans bolt pattern changes. There is an adaptor made to use a later trans with the early blocks. The 301 is a thin-wall casting smog motor, fwiw - heads are junk, intake is junk, block can't be rebored and they like to throw rods. I have a broken rod out of a 301 and it's mate with the bearing wasted, melted and fused into the rod, that I saved, cleaned up and clear-coated for paperweights as a reminder of the first engine I managed to blow up. Sad thing is I wasn't even trying...
The 421/428/455 blocks have large main bearing journals, not smaller and are more prone to cracking under extreme horsepower conditions. Turn down the crank journals on these large CID versions to stroke the 389/400 blocks to larger cubes.....or just get a new stroker crank from the git go.
Later blocks 1967 and newer are a different casting the have 3 core plugs on each side vs 2 on 66 ond older. The newer blocks have better cooling passages and have fewer overheating problems
389 and 421 are not the same block, yes they are the same size, yes they will fit the same chassis, no you can not make a 389 into a 421. All Pontiac engines are the same size externally. They are bored different sizes, and have two sizes of crank bearings. 421, 428, 455 are grouped together. 301,326, 350, 389, and 400 are all grouped together. Pre '65 engines back to 1955 are all grouped together, but still are the same size externally as the newer engines. Motor mounts, and bell housings changed over the years, but from '65 to '79, the bell housings all interchange and all use side mount motor mounts of some sort. Money wise, 455 and 400 are the cheapest to build. Horse power will about equal in a 455 as a similar built 400, just less torque in the 400. 389 and 421 pistons are expensive. Cams, rods, and cranks cost about what other engines similar parts cost, excluding the Chevrolets. Joe
This thread is from 2005- or 9 years ago. Ill guess OP built his poncho and blew it up twice by now But any ways don't miss the nationals - http://www.pontiacnationals.com/
Oddly enough I think he got into jam bands and bought a Subaru. Occasionally people see him up in the mountains but not poncho powered Posted using the Full Custom H.A.M.B. App!
Digging up the dead. Here are some pictures to illustrate the major differences between the 389 blocks (produced from '59 -'66) and 421 blocks (produced from '61 -'66). 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 Super Duty 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.
The Jalopy Journal
