Help identifying engine block - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 12-05-2017, 11:29 PM Thread Starter
 
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Help identifying engine block

Hello, I could not find a thread in the forum for this and nothing I have found in engine code sites have been able to identify my block.

I have a 1969 GTO. It was built in Baltimore in October 1968. I have the dealer order sheet and from what I could decode from that and other items, it seems to have been a down option 2 barrel 8.6 compression 265 horse motor and it was originally an automatic. The block code is XM, but I see little evidence of other 1969 identifying marks on the block. Here is what I can decipher:

1. There is no transfer lug and it has 3 freeze plugs on each side
2. The date code on the distributor pad is L217 (which tells me the block is from December 21, 1967) - I know XM was used in 1968
3. I do not see a year marking next to the distributor (I have a modern GM HEI distributor and it is hard to see in there), BUT if this is a 1968 block, there would be no year noted by the distributor, right?
4. I do not see a casting code near #8 cylinder
5. On the front side of the engine block, on the passenger side - in front of the first freeze plug, there is a casting GM 10
6. Above the XM stamp on the front of the block the number 337397 is stamped. I am sure about 337, but the 397 is questionable
7. On the front of the block, down near the bottom of the timing cover, there is a number 287116193 - I am pretty sure I can see that right

In case anyone asks, the heads are cast 48 and the date code on them identifies them as being cast in August 1968

I suspected that the engine in my car was not the original even though I was told by the prior owner that it was. But, is it possible I have a 1968 block in a 1969 car and it is the correct block? or was Pontiac careful to not cross model years? Is it possible that early in the car's life the block took a vacation and this is a GM replacement block? That may be why the previous owner thought it was the original block.

If it is a 1968 block, that would explain why the normal 1969 identifiers are not present, but the GM 10 stamp concerns me. It's definitely a Pontiac engine - it has a windage tray.

I am getting ready to pull and rebuild the engine. Then I can get a better look at markings, but I thought I would ask you guys what you think first.

Thanks!

Mike
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post #2 of 4 (permalink) Old 12-06-2017, 08:12 AM
 
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The block is a '68 XM coded 400 2bbl. The XM coded blocks typically were stamped with taller font stamps, & usually stamped deep in the face of the block. no mistaking an XM stamped 2 bbl block.

your 48 casting heads, am betting your '69 GTO is a HD 3spd or 4 spd car, the H dated 48's are original to the car, it's matching WT coded block was swapped out years ago. 48's were also used on RAIII WS & YZ coded 400's, but those engines were not released till near the first of the year, Jan of '69. Last, a Pontiac dealer would not have swapped in the XM block, instead they would have used an Service Replacement 400 fitted block.
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post #3 of 4 (permalink) Old 12-06-2017, 10:24 AM Thread Starter
 
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Thanks Jim,

This car was born a low HP 2 barrel car, so I doubt it was born with a WT block. XM was also used in 1969 from what I have found and given that the normal 1969 identifiers are not present, I guess (to your point) someone swapped out a 1968 block in the first years of life. I did want to comment on the stamp, where you said that XM is prominently stamped. Mine is not, the size of "XM" looks normal, but it is faintly stamped. The X looks like what I would expect to see with any other stamp, but the M is fainter. None of the other numbers (the 6 digit code above the XM and the numbers down by the timing cover) match anything on any of the decoder sites that I have found.

I know it is a Pontiac block, but the markings make me think it is a generic block. That said, the car has been tastefully modified (ie, I know it is not a concourse correct car), so should I care? As mentioned, I am going to rebuild it. Sure, I would love to have it at a show and have WS stamped on the block, but that ain't happening.....

Thanks again,
Mike
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post #4 of 4 (permalink) Old 12-06-2017, 11:04 AM
 
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XM coded blocks were used for all '67-69 400 2bbl GTO's. All had a turbo400 & typically high ratio reared ratio. most such blocks had different font taller two letter code XM. Most ive seen, the XM was stamped fairly deep. I've had several of these XM blocks & observed many others in yard cars.

It appears you lucked out with the engine having the Hxx8 dated 48's, a previousowner most likely installed them on the '68 usage XM block. Do know there are verified instances of '69 GTO's being invoiced without the 400 2bbl economy engine option, only with the hydramatic transmission noted on the invoice. Anyone buying such a '69 GTO, just looking at the invoice copy(PHS) would think its original engine was a 400 350 horse YS engine. Am not sure if the same lack of notation on original invoices could have happened on a base transmission 3 speed '69 GTO, but with that type of drivetrain build, one that the body build was in Oct of '68, it would have had August of '68 dated 48's.
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