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1978 L78 T/A 6.6 Original Block?


New member
Aug 8, 2023
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Vancouver, Canada
Okay, I've been up, down, and around this 1978 Pontiac 400 4BBL Trans Am issue for three weeks now, and I still have no idea what block is supposed to be in my dad's 1978 Trans Am bought new from the dealership.

My dad died a few years ago and left a 1978 Trans Am, which he bought brand new from the dealership to my brother.

Just before my dad died, my brother managed to blow up the original engine.

My sister, worrying that my dad would find out and do unspeakable things to my brother, took the car to a "backyard" mechanic to get it back on the road before my dad found out.

The "backyard" mechanic pulled the original motor and slapped in a worn-out Pontiac 301 with an adapter to bolt up the TH 350 auto, which my brother promptly blew two years later, thankfully after my dad died.

Now, I've been tasked to return the car to the original numbers matching beast it was.

Unfortunately, the original block/motor is long gone, so I don't have the original casting numbers.

I need help identifying what exact block was in the original car.

The build sheet lists the motor as "L78, " but some sites say it was the 500557 block.

Other sites say it was a 568557 block, and to throw massive confusion into the mix, the car has the T/A 6.6 decals on the hood scoop, which I have been told ONLY came on the W72 1978 T/As.

My dad could have added the decals after he re-painted the car in the late '80s, but I don't remember.

My sister is trying to find some original pics from the '70s when he bought it, as it also has the "WS6" performance package.

I'll attach the original build sheet as I need to return the car to matching numbers, so I would appreciate it if an expert could guide me in the right "block" direction.

1978 Trans Am Build Sheet-page-001.jpg


Well-known member
Aug 19, 2023
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What a great story!

I love a good bit of family history in the back story of an old car.?

The truth is, you can't restore the car to matching numbers without the original engine.

The build sheet is the final document on the car assembly, so what that says is usually absolutely correct 99% of the time.

So, if it says L78, I'm pretty sure that's what it had.

So, I would plan to find a similar date-coded 400 block as that would be as close to the period correct as you could reasonably expect.

I'm assuming the Pontiac dealerships of the day had access to replacement engines for any warranty replacements.

They were often referred to as a 'service block' & in some cases, the dealership could stamp the engine number onto the TA machined pad by the oil pan rail.

I hope this helps; it's an exciting topic & I can't wait to see some of the replies.

Good luck.?
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