400 Block
 

I have just purchased a 78 T/A with a 400 engine and noticed on the top of the block when you take off the valley pan, the letters X X. On others like 455 it has 5 5 and 400's it has 0 0, any idea what XX is , the car is numbers matching so its not a replacement? thanks

W72 ??? good question, I would like to know as well

The XX means it's a W72 / TA 6.6 Block. The 78 and 79 TA 6.6 blocks had the XX code I have read it is a thicker web block but I really couldn't say but it was the end of the run for the 400


Dave

they did make the 78 79 w72 Block web a little thick. So if you wanted to work the motor up, it could handle more power.

It's a keeper block.

Thanks guys. I like the sound of that. The cars numbers matching so yeah, definenatly a keeper. I have just bought it and on the weekend i was looking through old photos i took of Pontiac Trans am's when i was a kid and found this same car i took a picture of when i was 15, some 22 years ago, originally brown, then white, now red and now mine..

Yes,that is the W-72 motor,and DEFINITELY a keeper.
It was a thicker block,better oiling(starting in the later half of it's production),starting in 78 the bigger cam putting out 220 hp,better valve springs,whole bunch of better stuff than just a L-78.
These cars with the "Z" code are getting harder to find in any condition,I know,I looked for a good one for a while.;)
Nice find:beerchug:

"Z" being a code for Pontiac motor right? I have two Z code 78s and neither is a W72 block car--both are 180 hp Pontiac 400s though.

You are correct,from 77-78 the "Z" in your VIN denoted a Pontiac motor,which could be the 180hp L-78,which is what you have or the 200hp W-72 in 77 or in 78 220Hp W-72.
However in 79 a "Z" code car could ONLY be the W-72,220hp,for it was the only Pontiac motor available in a Firebird,and it had to be a 4spd,otherwise it was an auto with the 180hp 403.