[johnta1]

So, What Does the Letter "M" Stand For?

From what I have gathered, not much to us.

I think the factory used a tighter tolerances for individual measurements and the parts to fit in them compared to other companies like Chevy.

They are on the deck/bores sometimes too.

The line worker would know to use a different piston for an 'M' compared to an 'L'.
(same for crank, use different size main bearing according to the letter)

These were not big numbers.
Something like .0005" different.
(or maybe even less)

I have a bulletin where they changed the letters to 'L' and 'M'.
(possibly around 1971? have to look it up)



Thanks for the pics, they are awesome!

[LMSRACER]

Originally Posted by PAUL K

Good Job Larry! It looks like you might be debunking an old PY myth. Have you noticed any distinct difference in cylinder wall thickness between blocks?

I most Certainly Have..... In Fact, the Last Time I Built a #485428 455, I had a hard time finding one that I was comfortable Boring to 4.215". All of them would go to 4.180", but at 4.215" they were getting pretty thin. Every Block that I Sonic Tested was a #485428.

What I've found is that finding a Block that can go over 4.200" is kinda' rare. Unless you want Bores that are less than 0.095" in Thickness. I try to stay above 0.100" in "Non Thrust" Areas of the Bore and above 0.150" in the Thrust Areas. If you're Racing, then "Hard Block" is Your Friend.

Have a Good One,
Larry S.

[LMSRACER]

Originally Posted by johnta1

From what I have gathered, not much to us.

I think the factory used a tighter tolerances for individual measurements and the parts to fit in them compared to other companies like Chevy.

They are on the deck/bores sometimes too.

The line worker would know to use a different piston for an 'M' compared to an 'L'.
(same for crank, use different size main bearing according to the letter)

These were not big numbers.
Something like .0005" different.
(or maybe even less)

I have a bulletin where they changed the letters to 'L' and 'M'.
(possibly around 1971? have to look it up)



Thanks for the pics, they are awesome!

Thanks John....

I've worked on Engines that were Coded that way and the Factory Shop Manual actually deciphered the Codes for the Technician.
It's all about "Select Fitting" the Bearings, which is still a Common Practice. I just wondered if anyone actually could decipher those codes on a Pontiac Block.

I do realize that it doesn't matter if you're Align Honing or Align Boring the Block and Machining the Crankshaft. I was just curious.

Thanks Again,
Larry S.

[Tom Vaught]

Great Work Larry! Nice documenting the obvious dimension differences.

Did you look at one of the most critical indicators of the strength of the block (where the two casting segments come together).

The 67 400 block comes very close to the actual bores.

The later blocks and the 455 block have the casting split line in the middle of the main webs where they see the most stress. (Typically where cracks start like the ones in the other posters thread).

Food for thought. But everyone has their own thoughts on where the strength come from in a block.

Tom V.

[LMSRACER]

Originally Posted by Tom Vaught

Great Work Larry! Nice documenting the obvious dimension differences.

Did you look at one of the most critical indicators of the strength of the block (where the two casting segments come together).

The 67 400 block comes very close to the actual bores.

The later blocks and the 455 block have the casting split line in the middle of the main webs where they see the most stress. (Typically where cracks start like the ones in the other posters thread).

Food for thought. But everyone has their own thoughts on where the strength come from in a block.

Tom V.

Good Morning Tom,
I briefly made mention of the Casting Line Difference back at Post 217.

You're correct about the Casting/Parting Line Differences and that is Definitely something that needs to be Considered. Especially when the Casting Thickness in that Area is Thin, much the way that most, but not all, Later 70's Block are.

In this Case though, I was pointing out the Differences in what "I Consider" to be some of the Better Blocks that are out there. These Blocks could be Modified for the Intended Application without the Risk of Ruining a "Rare Pontiac" Block that would be best Suited for a "Numbers Matching" Restoration.

Let me say this about the Casting Line Location in the 455 Cast #485428 Main Webbing. I can Honestly say that I've never seen a #485428 Casting break there. Maybe it's due to the Thickness of the Main Webbing and Maybe it's due to the Quality of the Casting. If you'll notice in the photos that I've posted, the (1972) Cast #485428 455 Block is a Nice Looking Casting.

Thanks,
Larry S.

[chuckies76ta]

Thank you Larry for all the info and pics you just posted. Even more reason to sticky.

Charles

[LMSRACER]

Originally Posted by chuckies76ta

Thank you Larry for all the info and pics you just posted. Even more reason to sticky.

Charles

Your Welcome Charles...

I didn't plan to spend My ENTIRE Saturday doing that, but what the Heck...

I'll tell you Guys what I'm going to use for My Next (Personal) "Stock Block" Performance Build.

A Late 1971 to Late 1974 #481988 400 2-Bolt, 2-Bbl. (No Cylinder Bore Chamfers), with Splayed Caps on the Center Three and 7/16" Fasteners on the Outer Bolt Position.

This Block is Very Similar to the (1972) Cast #485428 455 Block that I Illustrated.
(Now I've got to find one with sufficient Cylinder Wall Thickness.)

"Screaming Chief" shows what this Block looks like in His Posts #171 and #172.
(Thanks for Showing that 1977/78 Block with the 1974 Casting. I was Totally Unaware of that. Once again showing that there are No "Set Rules" Pertaining to Casting Numbers and Date Codes.)

Have a Good One,
Larry S.

[taff2]

IMO, you can't beat the early 389 blocks . '58 to '60 are the best but are probably a bit limited on bore size . If you want a smaller cube engine the 2 bolt block can be fitted with splayed 4 bolt caps and they are very thick in almost all dimensions going by the one I sonic'd. I also have a '61 389 block to use in my next n/a build,although this doesn't have the lengthways stiffening rib that the '58 to '60 blocks do, it is also very thick dimensionally and has the 1/4" taller main caps too. Although these blocks need an adapter to fit them to a TH400 pattern gearbox, this isn't too difficult to make. Now if someone could start manufacturing these adapters cheaply in 1/4" steel........ (The ali Wilcap adapters are a bit thick for my liking and quite pricey too)
Thanks go to Steve Barcak for bringing the virtues of these early blocks to the attention of the Pontiac world.

[LMSRACER]

Originally Posted by taff2

IMO, you can't beat the early 389 blocks . '58 to '60 are the best but are probably a bit limited on bore size . If you want a smaller cube engine the 2 bolt block can be fitted with splayed 4 bolt caps and they are very thick in almost all dimensions going by the one I sonic'd. I also have a '61 389 block to use in my next n/a build,although this doesn't have the lengthways stiffening rib that the '58 to '60 blocks do, it is also very thick dimensionally and has the 1/4" taller main caps too. Although these blocks need an adapter to fit them to a TH400 pattern gearbox, this isn't too difficult to make. Now if someone could start manufacturing these adapters cheaply in 1/4" steel........ (The ali Wilcap adapters are a bit thick for my liking and quite pricey too)
Thanks go to Steve Barcak for bringing the virtues of these early blocks to the attention of the Pontiac world.

taff2,
I agree with All of that. As you have mentioned, the Bore Size is the limiting factor. IMO, for a Racing Application, I don't want a Bore Size any Smaller than 4.185". I prefer 4.210" or Larger.
Therefore I believe that the trouble associated with the Early 389 Blocks just isn't worth it. That's My ...
On the other hand, the CV-1 Cylinder Head with the Correct Ports could change all of that. The Bore Size would be less of a factor, for sure........ The Jury is Still Out on that one.

Larry S.

[grandville455]

Larry
Thanks for Putting the time into this! These pictures are awesome!
Im using a 70 400 2 bolt block, that is using the program splayed blocks.

[LMSRACER]

Originally Posted by grandville455

Larry
Thanks for Putting the time into this! These pictures are awesome!
Im using a 70 400 2 bolt block, that is using the program splayed blocks.

Question....
Did your Program Splayed Caps come Drilled for 7/16" Outer Fasteners or 1/2" Outer Fasteners?

I ask because I just received a "Splayed 400" set that were Drilled for 1/2". I've always gotten Caps from Program Engineering that were Drilled for 7/16" when the Caps were Splayed. I haven't called them yet because I just noticed it yesterday.

NOTE:
Of course the Factory Style (Straight not Splayed) Program Caps are Drilled 1/2" for the Outer Fasteners. That matches the factory Drilled Holes in the Block.

Thanks,
Larry S.

[grandville455]

Originally Posted by LMSRACER

Question....
Did your Program Splayed Caps come Drilled for 7/16" Outer Fasteners or 1/2" Outer Fasteners?

I ask because I just received a "Splayed 400" set that were Drilled for 1/2". I've always gotten Caps from Program Engineering that were Drilled for 7/16" when the Caps were Splayed. I haven't called them yet because I just noticed it yesterday.

NOTE:
Of course the Factory Style (Straight not Splayed) Program Caps are Drilled 1/2" for the Outer Fasteners. That matches the factory Drilled Holes in the Block.

Thanks,
Larry S.

They came with 7/16x 2 3/4 Grade 8 Bolts, so Im guessing their drilled for those? Block is at machine shop yet. I bought the caps from Butler.

[LMSRACER]

Originally Posted by LMSRACER

Question....
Did your Program Splayed Caps come Drilled for 7/16" Outer Fasteners or 1/2" Outer Fasteners?

I ask because I just received a "Splayed 400" set that were Drilled for 1/2". I've always gotten Caps from Program Engineering that were Drilled for 7/16" when the Caps were Splayed. I haven't called them yet because I just noticed it yesterday.

NOTE:
Of course the Factory Style (Straight not Splayed) Program Caps are Drilled 1/2" for the Outer Fasteners. That matches the factory Drilled Holes in the Block.

Thanks,
Larry S.

Contacted Pro Gram Engineering.. Their Answer was Woops.....
I sent them Back Already. They'll be sending out a New Set as soon as they receive them.

Check Everything, Every Time....

Larry S.

[Tom Vaught]

Originally Posted by LMSRACER

Good Morning Tom,
I briefly made mention of the Casting Line Difference back at Post 217.

You're correct about the Casting/Parting Line Differences and that is Definitely something that needs to be Considered. Especially when the Casting Thickness in that Area is Thin, much the way that most, but not all, Later 70's Block are.

In this Case though, I was pointing out the Differences in what "I Consider" to be some of the Better Blocks that are out there. These Blocks could be Modified for the Intended Application without the Risk of Ruining a "Rare Pontiac" Block that would be best Suited for a "Numbers Matching" Restoration.

Let me say this about the Casting Line Location in the 455 Cast #485428 Main Webbing. I can Honestly say that I've never seen a #485428 Casting break there. Maybe it's due to the Thickness of the Main Webbing and Maybe it's due to the Quality of the Casting. If you'll notice in the photos that I've posted, the (1972) Cast #485428 455 Block is a Nice Looking Casting.

Thanks,
Larry S.

Thank You Larry, for your efforts.

Tom V.

[JD311]

I checked my program splayed caps and they are drilled for 1/2 in as well. Looks like it would still work for the 59 block I have though. I have had these for awhile and just noticed it. Wonder if I should try to send them back or use them?

[LMSRACER]

Originally Posted by Tom Vaught

Thank You Larry, for your efforts.

Tom V.

Thanks for the Acknowledgement Tom.

I would say that ALL of this information (Not Just what I Posted) is applicable to the Majority of Racers or Fast Street Car Owners. I'd say any Build in the 600ish to 750ish Horsepower Range for sure.

I appreciate the others that have taken their time to post and am just "Paying it Forward".

Larry S.

[LMSRACER]

Originally Posted by JD311

I checked my program splayed caps and they are drilled for 1/2 in as well. Looks like it would still work for the 59 block I have though. I have had these for awhile and just noticed it. Wonder if I should try to send them back or use them?

Personally, I prefer the 7/16" Fasteners. It removes less Material from the Main Webbing to Drill and Tap for the 7/16" Fasteners. I believe that the 7/16" Fasteners also provide sufficient "Clamping Load" to the Main Cap. Especially if you use the ARP Studs.

My

Larry S.

[LMSRACER]

Originally Posted by LMSRACER

What I've found is that finding a Block that can go over 4.200" is kinda' rare. Unless you want Bores that are less than 0.095" in Thickness. I try to stay above 0.100" in "Non Thrust" Areas of the Bore and above 0.150" in the Thrust Areas. If you're Racing, then "Hard Block" is Your Friend.

Have a Good One,
Larry S.

In this Post that I had made previously, I want to Clarify what I said.

While rereading My Posts last night I realized that this information may be a little misleading.

I PREFER to have a minimum of 0.125" of Wall Thickness on the "Non Thrust" (Front and Rear) Portions/Surfaces of the Cylinder Bore and a minimum of 0.185" on the "Piston Skirt Loaded" (Inner and Outer) Portions/Surfaces (Not just the Thrust Surface) of the Cylinder Bores as a MINIMUM.

That's what I PREFER. BUT, what I have found over the years is that I have had a lot of trouble finding Blocks that could be Overbored more than 0.030" with that much Wall Thickness Remaining. So, because of that, I've had to adjust what I would accept as a "BARE MINIMUM". That's where the previous specs. of 0.100" and 0.150" came from.

I hope that makes more sense,
Larry S.

[grandville455]

Originally Posted by LMSRACER

Personally, I prefer the 7/16" Fasteners. It removes less Material from the Main Webbing to Drill and Tap for the 7/16" Fasteners. I believe that the 7/16" Fasteners also provide sufficient "Clamping Load" to the Main Cap. Especially if you use the ARP Studs.

My

Larry S.

I talked to my machinist today and he said same thing, He wants the most straight down clamping force possible, and will sometimes ream out caps .030-.060 so there is absolutely no side pressure on the shoulder of the bolt and its all on the washer!...

[JD311]

I thought 7/16 would be fine as well. So should I send these back and get the 7/16 drilled caps? Or can I use these 1/2 in drilled caps and have the block drilled for 7/16? I think you can get arp bolts with a bigger head.