[SD421]

Hi,
It was pointed out to me that my 421 block doesn’t have the straight across machined pad under the cylinder head but is scalloped like the 389.

It’s a 1966 421, YH code, has the transfer lug, 2 freeze plugs on each side, has factory 4 bolt w/ 3.25” mains and a factory 4” stroke crank.

Any reason why the machined area on the passenger side is scalloped and not straight across?

[tom s]

I have learned to never say never with anything out of the foundry.I posted on here that I owned a 400 RA V block with a transfer lug.Tom

[Mike Davis]

I have 3 421 Blocks. None of them have the straight deck or straight front timing cover. 2 Blocks are 1966 castings one with YH and one YK, 1 block is a SR block cast in 1967 YH (428 block) but has 3 freeze plugs and is restamped the 421 casting number and has the 421 bore size.

[ANDYA]

A friend of mine has a 66 421 and it is scalloped just like yours . I have a 65 421 and it is straight across.

[johnta1]

What's the block casting code and date code on it?

[Tom Vaught]

Originally Posted by SD421

Hi,
It was pointed out to me that my 421 block doesn’t have the straight across machined pad under the cylinder head but is scalloped like the 389.

It’s a 1966 421, YH code, has the transfer lug, 2 freeze plugs on each side, has factory 4 bolt w/ 3.25” mains and a factory 4” stroke crank.

Any reason why the machined area on the passenger side is scalloped and not straight across?

Looks to me like your block was machined to have the scalloped look vs the normal straight across surface.
I have never seen a scalloped block with the radius you have on your block. "scalloped surface" should be cast not machined.

Tom V.

[b-man]

I’m seeing a cast finish on the scalloped area.

Enlarge the picture and look closer.

[63gpman]

My 66 YH 421 and my 66 YK are scalloped, but my 65 WG is straight. I just figured it was a 66 thing.

[Tom Vaught]

I am going by the extremely sharp edge of the scallop in Post #1 vs the 4th picture in post #8. The scallop & texture looks "created" to me in Post #1 vs the rough cast (Factory casting) shape/texture in Post #8.

Tom V.

[63gpman]

are you saying the factory created these scallops?

[Tom Vaught]

If you are replying to my post, the factory cast up many many 421 blocks with a straight across block surface next to where the head gasket is.

389 engines had the cast "scallop" in the passenger side of the blocks and were not straight across.
Years ago people would cut off the Transfer Lug on the back of the 421 engines and tell people that the engine
was a 389 engine, (no transfer lug visible) but they missed the straight across block surface detail which still
allowed knowledgeable people to easily spot the 421 block being used.

I know little about the 1966 421 blocks but I do know that if GM would remove 24 inches of heater hose to save
money, they were not about to throw-away 1000s of good 389 castings vs having the raw castings machined .030"
overbore and removing additional metal in the crankshaft main bearings bores and putting on 4 bolt main caps and
machining the blocks for those 4 bolt main caps and bore size.

So my opinion is some of the last 389 blocks were used to fill the 421 orders as GM would be using the new
428 blocks on the 428 engines and occasionally a 3 freeze plug 428 block casting on a 421 build.

Tom V.

[63gpman]

Here is a 66 WJ 421 with correct 421 casting on the pad and side that also has the scallop. So they either had to change the casting numbers when the block was poured or change the scallop.

They do list 2 casting numbers for the 66 421 but the numbers cast into the side would have had to be done at the foundry and is different than any 389. Why would they pour a 389 block with new casting numbers vs just pouring a 421 casting?

421 cid 9778791(E)

421 cid 9782611(L)


https://www.ebay.com/itm/1966-PONTIA...5.c10#viTabs_0

[John Milner]

My '66 YH 421 is scalloped and my '65 YH 421 is straight across.

[Tom Vaught]

Originally Posted by 63gpman

Here is a 66 WJ 421 with correct 421 casting on the pad and side that also has the scallop. So they either had to change the casting numbers when the block was poured or change the scallop.

They do list 2 casting numbers for the 66 421 but the numbers cast into the side would have had to be done at the foundry and is different than any 389. Why would they pour a 389 block with new casting numbers vs just pouring a 421 casting?

Hard to say, but not that hard to change casting identification at the foundry
on parts cast. They did that deal every year on the cylinder heads.

New interesting history for the Pontiac Engines.

Tom V.

[242177P]

The 66 421 might be a couple of pounds lighter, but they weren't using 389 blocks.
Unfortunately, Tom is highly suspicious of anything outside of his comfort zone.

I worked for Holley during the time the Street Dominator was designed.

I believe that we have some "skilled metal removal" going on here.

I personally have seen hundreds of the things over the years and that it the first "open" manifold I have seen. Someone did some nice work.

[johnta1]

Interesting thread!
Need some examples of the straight across block for 1966.

Any of these have the small 'x' before the engine code? (WJ, YH etc)

Also looks like the block codes are all 9782611 in this thread I think.

Maybe need a poll of the passenger side straight edge or the 'v' cut.

[Tom Vaught]

I gave away two 421 blocks to a Pontiac friend, at the end of last year but one was for sure a 1964 370 HP "45B" block and the other one was a 1965 356 HP WH block (if I remember correctly). Both had the flat deck, no scallop. Again I know little about 1966 421 blocks.

Tom V.

[63gpman]

I'll throw an assumption out there. The late 9782611 casting is a 428 that was bored and fitted for the 421 with the scalloped passenger side in preparation for the next model year and the 9778791 was basically the same 421 as done in 65 with the flat area on the passenger side. Would be interesting to compare a 9782611 to a 9786135 428. Wonder if the walls on the late 66 421 are thicker? Interesting for sure.

[Tom Vaught]

Originally Posted by 63gpman

Would be interesting to compare a 9782611 to a 9786135 428. Wonder if the walls on the late 66 421 are thicker? Interesting for sure.

My understanding, and a reason why we chose a 1967 400 2-bolt main block for Marty Palbykin's 1600 hp turbo motor, (prior to splayed steel caps, etc) was that the 1967 blocks were a bunch stronger vs the 389 blocks. Same deal for the 1967 428 blocks vs later blocks.

The bore walls might have been thicker, maybe not.
I do know the overall block structure of the 1967 400 engine is a bunch stronger vs any typical (non SD 389 engine) block.
You install steel splayed 4 BOLT caps on a basic 1967 2 bolt (non drilled for 4 bolt main bolts) block and have it cryogenically treated and you have a very good block. Just replying to your post.

But back on the scallop vs non scallop.

Tom V.

[Dragncar]

Originally Posted by Tom Vaught

My understanding, and a reason why we chose a 1967 400 2-bolt main block for Marty Palbykin's 1600 hp turbo motor, (prior to splayed steel caps, etc) was that the 1967 blocks were a bunch stronger vs the 389 blocks. Same deal for the 1967 428 blocks vs later blocks.

The bore walls might have been thicker, maybe not.
I do know the overall block structure of the 1967 400 engine is a bunch stronger vs any typical (non SD 389 engine) block.
You install steel splayed 4 BOLT caps on a basic 1967 2 bolt (non drilled for 4 bolt main bolts) block and have it cryogenically treated and you have a very good block. Just replying to your post.

But back on the scallop vs non scallop.

Tom V.

I picked up one for 50$. But its .060 over with a few pits. Need to have it sonic checked but I hear guys could take them .090 + sometimes.