[Mike Davis]

Found what is supposed to be a 389 SR block. I have not had a chance to actually go look at it and measure the bore.

Block casting date shows it was cast in 1968 and it has 3 freeze plugs. Owner swears it is a 4.06 bore and was put in a 66 GTO in late 68 at the dealer.

Big question, what was the latest they cast the 389?
Has anyone seen a 389SR block with 3 freeze plugs?
Casting number on rear was ground off and restamped at factory, Does have SR stamp on front.
Thanks

[Rocky Rotella]

I’m not certain that Pontiac would have any issue producing an SR 389 block with the new-type casting for an earlier application as long as if fit the chassis without modification. And it having a 4.06-inch bore would make sense since Pontiac’s SR blocks were always fitted assemblies, which included pistons, wrist pins, and main caps.

I’d love to see pics though!

[Mike Davis]

Thanks Rocky,
It has pistons with it and they look to be the original 66 design. I know the 66 389 castings were thin. I wonder if this 68 casting with the 389 4.06 bore would be extra thick walls like 67 400's, or if they used the internal 389 casting for the cylinders.

[Rocky Rotella]

Sure!

I’ll bet it’s a typical 9786133 or 9790071 casting whose cast number was ground off and stamped with the “correct” ‘66 389 cast number.

It’s probably an excellent foundation. I’d be willing to bet that you’d have a hard time convincing many judges that it’s an authentic SR block because of the visual differences from a 66-and-earlier block.

Since ‘67-68 blocks are slightly different, particularly where the cast numbers are located, where is the cast number on this SR block?

[Baron Von Zeppelin]

Sounds wild

Would be more apt to think it was cast in 1978 instead of 1968.
In 68 would seem like the 389 mold would still be available with 389's still in abundant use and only recently superseded.

Back then they were supposedly required to keep mechanical service parts available for 10 years minimum. Some major components went longer.

[Tom Vaught]

Originally Posted by Rocky Rotella

Sure!

I’ll bet it’s a typical 9786133 or 9790071 casting whose cast number was ground off and stamped with the “correct” ‘66 389 cast number.

It’s probably an excellent foundation. I’d be willing to bet that you’d have a hard time convincing many judges that it’s an authentic SR block because of the visual differences from a 66-and-earlier block.

Since ‘67-68 blocks are slightly different, particularly where the cast numbers are located, where is the cast number on this SR block?

I would agree with Rocky on this one. The 67-68 3 core plug blocks were much stronger vs the earlier 2 core plug blocks.
They made other core changes in 69/70, I believe on the 400 blocks, the 67 and 68 (not drilled for 4 bolt main blocks) seem to have the best features when machined for a 400 splayed main cap. Is the block drilled like a normal 2-bolt main 389 block Mike? Also with a 4.06 bore I would think it would have 389 bore chamfers vs the 400 bore chamfers (which are in a different spot).

Tom V.

[Mike Davis]

Originally Posted by Tom Vaught

I would agree with Rocky on this one. The 67-68 3 core plug blocks were much stronger vs the earlier 2 core plug blocks.
They made other core changes in 69/70, I believe on the 400 blocks, the 67 and 68 (not drilled for 4 bolt main blocks) seem to have the best features when machined for a 400 splayed main cap. Is the block drilled like a normal 2-bolt main 389 block Mike? Also with a 4.06 bore I would think it would have 389 bore chamfers vs the 400 bore chamfers (which are in a different spot).

Tom V.

Tom, it is a std 2 bolt main block, no 4 bolt holes. The chamfers are in the 389 location at the 5 and 7 o'clock positions.

I am wondering how thick the walls would be since it's a .060 under bore!!!!

Here is a link I found to a block similar with the same stamping number and casting date but it is not the same block. This one was run for a short time 6 Months) and then pulled and stored. Owner does have the paperwork where it was changed and the SR numbers match the paperwork.

http://forums.maxperformanceinc.com/...72#post4621372

[Rocky Rotella]

Mike, as I have been collecting data from SR blocks as of late, I'd appreciate any information you can share from the block. Particularly, cast date and SR number. Also, if you're able to get the main cap numbers, I'd be interested in those too!

[Mike Davis]

Rocky,
So far what I have is
Casting/Stamping number 9778785
Cast at the GM12 N
Casting Date G118

Trying to get more info

[Rocky Rotella]

Thanks!

So it was produced on the same date as the example you found. I'm interested to see where its SR8 number falls in line.

[Tom Vaught]

Originally Posted by Mike Davis

Tom, it is a std 2 bolt main block, no 4 bolt holes. The chamfers are in the 389 location at the 5 and 7 o'clock positions.

I am wondering how thick the walls would be since it's a .060 under bore!!!!

Here is a link I found to a block similar with the same stamping number and casting date but it is not the same block. This one was run for a short time 6 Months) and then pulled and stored. Owner does have the paperwork where it was changed and the SR numbers match the paperwork.

http://forums.maxperformanceinc.com/...72#post4621372

Funny thing is the pistons for the engine have 7 of the 389 pistons and 1 of the 421 pistons (top right) with the 421 "deeper valve reliefs".

See my pic attached.

[Mike Davis]

Not sure about that whole deal as the one I am looking at has been run and all the pistons are the same.

[steve25]

To reply to the conjecture in post number 5 here I say this, once 1970 and later came around casting numbers started with a 4 or a 5, not the early 9 used on his SR casting as he has reported the number to be 9778785.

[Gary H]

I must say I was doubtful that this was a 389 block with the later 3 freeze plugs per side. Very cool find! I never knew they existed.

[Baron Von Zeppelin]

Originally Posted by steve25

... once 1970 and later came around casting numbers started with a 4 or a 5, not the early 9 used on his SR casting as he has reported the number to be 9778785.

They grind the original casting number off - like they did with both of these 389's in this thread. 9778785 has been hand stamped on them.
No physical evidence left to know what the original casting was - other than a date code that could be 1968 or 1978.

In Mike's case, the owner still has the dated SR documents with matching SR number. Luckily, makes it pretty easy to nail down.

My suspicion of 1978 was due to the last SR i came in personal contact with - over 10 years ago. (and this one having too many freeze plugs)
It was cast in 1975 - but was ground and stamped as a 1967 400 casting number.
Logic made it impossible to have been cast in 1965 - on that one.
So it had the 60's - 70's fiasco

[Baron Von Zeppelin]

Originally Posted by Mike Davis

Here is a link I found to a block similar with the same stamping number and casting date but it is not the same block.


...
Cast at the GM12 N
Casting Date G118

I remember about the other example, Un-Used 389 SR Fitted Block , that you found in archive search when it was for sale on here.
At that time and up until now - i thought it also was probably cast in 1978 - versus 1968.

Glad it has all been solved and they even have the exact same date code.
Learning experience

#GM 12 was probably a special-use mold blank

[Mike Davis]

Originally Posted by Baron Von Zeppelin

They grind the original casting number off - like they did with both of these 389's in this thread. 9778785 has been hand stamped on them.
No physical evidence left to know what the original casting was - other than a date code that could be 1968 or 1978.

In Mike's case, the owner still has the dated SR documents with matching SR number. Luckily, makes it pretty easy to nail down.

My suspicion of 1978 was due to the last SR i came in personal contact with - over 10 years ago. (and this one having too many freeze plugs)
It was cast in 1975 - but was ground and stamped as a 1967 400 casting number.
Logic made it impossible to have been cast in 1965 - on that one.
So it had the 60's - 70's fiasco

Did that block have 5 motor mount holes? I am guessing the later model block all had both sets.
The one I am looking at has only the early style 2. I have not heard back from the owner yet.

Interesting, I also found one on ebay last night. (see below link)
It has the same casting date as the other 2. It also has the GM 12 N and the same style casting stamp on the ground section.
Interesting all 3 of these were cast on the same date.

https://www.ebay.com/itm/1965-Pontia...53.m1438.l2649

[Rocky Rotella]

Another way to tell the general year of manufacturer is the numbers cast onto the main caps.

[steve25]

I would have a interest in the shift and clock time in terms of the casting on all of these SR389 blocks?

To me it would make sence that once the factory had / saw a need for another block casting and Piston fitting round of SR389 blocks they would have tried to confine such production to no more then 1 or 2 work shifts.

[Baron Von Zeppelin]

Originally Posted by Mike Davis

Did that block have 5 motor mount holes? I am guessing the later model block all had both sets.

I didn't even check about that. But probably had the 5 motor mount holes.
Guy probably still has it but have not visited him since around 2007-2008.

He had already rebuilt it with 48 Heads to go into a 69 Judge.
Then he learned it was a 1967 casting code stamping, and changed his mind.

My sentiment to him at the time was - it ain't a virgin - grind it again and stamp it again.
Don't know what he ended up doing.

Another way to tell the general year of manufacturer is the numbers cast onto the main caps.

Good Info !

Most all of you already know, the 400's also had beefier main caps and larger main cap bolts than the 389's.
So these would be Supreme Replacement 389 SR Blocks.

On a guess, going to doubt they have extra extra thick cylinder walls.
Probably around same as standard issue wall thickness for anything else cast in 1968.
But the opposite would be Cool .