[LPI]

Great Pictures Larry, Thanks!

Just a side note in post #204 the 1967 block clearly shows the "M" stamp on the pan rail.

That is a note to the assembly department saying that journal diameter is different.

Again great pictures showing the block casting changes from year to year.

[LMSRACER]

Originally Posted by LPI

Great Pictures Larry, Thanks!

Just a side note in post #204 the 1967 block clearly shows the "M" stamp on the pan rail.

That is a note to the assembly department saying that journal diameter is different.

Again great pictures showing the block casting changes from year to year.

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

Thanks,
Larry S.

[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 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.