Old 09-13-2023, 03:19 AM
tom420 tom420 is offline
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Join Date: Jul 2023
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Default 1978 L78 T/A 6.6 Original Block?

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 original casting numbers etc.

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

The build sheet lists the motor as "L78" which some sites say was the 500557 block. Other sites say it was a 568557 block. 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. Granted 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.It also has the "WS6" performance package.

I'll attach the original build sheet. I need to return the car to numbers matching so I would appreciate if an expert could guide me in the right "block" direction at least.
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Old 09-13-2023, 05:03 AM
Held for Ransom's Avatar
Held for Ransom Held for Ransom is offline
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Join Date: Mar 2000
Posts: 8,825

Box 26 on the build sheet say PYU. The block would be stamped YU. It would be a 500557 block.

So long, farewell.
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Old 09-16-2023, 11:11 AM
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nUcLeArEnVoY nUcLeArEnVoY is offline
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Join Date: Sep 2021
Location: Homestead, FL
Posts: 160

Sucks that happened, man. Same thing happened to my car at some point. My '79 started life as a W72 T/A 6.6 car, but the original PWH block is long gone and was replaced at some point with a plain jane XA 500557 block from late '76. I was aggravated to find that out. Can only imagine what happened to the original block - must've been a severe failure, since it takes a lot to condemn a block. Oddly, the 6X-4 heads, correct carb, satin chrome valve covers, intake, exhaust manifolds, emissions, etc. are still on the car. Outwardly, the block itself is all that seems to be unoriginal, though I figure internals have been replaced.

I have a virgin standard bore 481988 PWH block that's been magnufluxed sitting in my sister's old room waiting for a future build, and if I could find that, you can easily find a YU 500557 block for your project. Those blocks aren't that desirable since the webbing in the saddle isn't as thick as the 481988 blocks, but if originality is all you care about, then that's to your benefit since people will give those blocks away like candy on Halloween. There are services out there that can punch in the numbers to match, too. I think Butler does it.

1979 Trans Am W72 400/4-Speed WS6 - Starlight Black Hardtop
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Old 09-18-2023, 11:55 PM
Maxthe222 Maxthe222 is offline
Join Date: Jul 2020
Location: Sydney, Australia
Posts: 34

It's simply a 1978 YU block code 500557 cast code 180hp automatic engine.

It will never be numbers matching without the original block that has the partial VIN engine number stamp that matches the VIN number of the car, it can now just be a "correct" engine for the car.

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Old 10-14-2023, 08:28 AM
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PunchT37 PunchT37 is offline
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Join Date: Dec 2002
Location: Lafayette,LA
Posts: 3,196

You can put a "correct" 557 block in or put a decent 400 block in as it will not be a numbers matching block anyway.

Do you still have all the top stuff off the original motor? As in heads, intake, carb, dist ?

Old 10-14-2023, 08:52 AM
Trevor78 Trevor78 is offline
Join Date: May 2022
Posts: 595

He posted this on several forums, never returned.

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