[mbolan]

I am doing a frame off on a 65 gto. The motor that came in the car when I bought it has a G077 date code and three freeze plugs per side. The cast number is 979?071 and has a WT engine code (the ? is a disfigured digit and could be a 8,0,3,6,or 9). I know the date code tells me that the motor is a 67 but I can't find the cast number in any listings I have found so far. If the ? digit was a 0 the motor would be a 68 400 but the date codes don't jive. I have the motor apart and have measured the bore. It measures 4.10 and the pistons are stamped .040. These numbers indicate a .040 389 bore but they didn't make 389s in 67. I don't think it could be a 326 because it would have had to been bored .381 to get to the current 4.10. It can't be a 400 because 4.10 is not big enough.

Can anyone ID this block?

[Dick Boneske]

The engine is a '68 400 originally installed in an A body. The only casting code similar to the one you listed was 9790071. This engine was used only in '68 and '69. It was cast in July, 1967, so it must have been an early '68. It was a 350 HP, 4 speed, according to the WT code.

The bore started out as 4.120, but now is likely 4.160 if the pistons are marked .040. You must have read your micrometer or venier calipers wrong when you measured the bore. Either that, or the engine was sleeved and bored to .040 over 389 specs.

[mike nixon]

what dick said.

mike

[mbolan]

Thanks for the input! I will check again on the bore. I have had blocks sleeved before and you could definately see the sleeves when you looked closely at the deck surface so I don't think its sleeved. But you think that a 68 block could have a July 67 date code?

[mbolan]

Another question. If you were in my shoes would you look for a correct date code 389 or rebuild the 400 block?

[mbolan]

OK I'm back at my house and have remeasured the bores. They are definately 4.10". I also checked the .040 stamped pistons. They measure 4.073. Iam using a digital caliper to measure and have checked the calibration against a set of Brown & Sharp mikes. THis is an old motor and is still .020 under a stock 400. There are not any sleeves installed. This can't be a 400. If I didn't know anything about date codes this would be a no brainer 389 motor. Something is really goofy here. Take a look at the pics of the casting number I attached in the original post. It sure looks like the 4th digit is an 8. Which does not produce a # know to pontiac man kind.

Please send me a straight jacket. Is there anybody trust worthy in the Dallas Texas area that I could have look at this?

[Dick Boneske]

I suspect you are going to find there is not a mystery with this engine and its bore size, but an error in measuring. You need to measure a known component with your digital calipers to see that it is reading correctly. Mine recently began reading .020" less than the actual measurement.

Engines were cast with three freeze plugs only from 1967 and up. All 389's had two freeze plugs. Something is being missed here. One piece of info that would be helpful is what design pistons are in the engine. Are they the small eyebrows like the '66 and earlier engines used or are they the large eyebrows like the '67 and later engines used? The fact that they are marked .040 suggests that they are 389 pistons if your bore measurement is correct.

One thing you definitely have measured incorrectly is the piston size. They cannot be 4.073" in a 4.100" bore. That would give a wall clearance of .027". Try measuring the pistons near the bottom of the skirt. They will only have a few thousanths clearance in the bore. Measuring diameter at the top of a piston is not a meaningful measurment.

[Dick Boneske]

I suspect you are going to find there is not a mystery with this engine and its bore size, but an error in measuring. You need to measure a known component with your digital calipers to see that it is reading correctly. Mine recently began reading .020" less than the actual measurement.

Engines were cast with three freeze plugs only from 1967 and up. All 389's had two freeze plugs. Something is being missed here. One piece of info that would be helpful is what design pistons are in the engine. Are they the small eyebrows like the '66 and earlier engines used or are they the large eyebrows like the '67 and later engines used? The fact that they are marked .040 suggests that they are 389 pistons if your bore measurement is correct.

One thing you definitely have measured incorrectly is the piston size. They cannot be 4.073" in a 4.100" bore. That would give a wall clearance of .027". Try measuring the pistons near the bottom of the skirt. They will only have a few thousanths clearance in the bore. Measuring diameter at the top of a piston is not a meaningful measurment.

[mbolan]

your right about the pistons. The skirt measurement is 4.0990. Thats a lot closer to the bore. I also have cheacked teh calipers accuracy by comparing it to measurents i am taking on objects with a 1" Brown & Sharp mike. They measure the same. The bore looks like a .040 over 389 but that can't be possible if this is a 400 block.

Does anybody know a good pontiac guy in the Dallas Texas area that I can have look at it?

[mbolan]

Oh ya I forgot about the pistons. They have 4 eybrows. 2 pointing toward the top and 2 pointing toward the bottom.