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Hello All, please correct me if Iḿ wrong; I have a convertable 66 4spd sitting on the back burner for about 10 years now. Had a machine shop redo the engine, and they informed me the car has a 421 I asked the mechanic who was doing the rebuild if the engine ever looked like it had been changed, and he told me not that he could tell. (that was about 15 years ago) I later replaced the clutch and had problems with Summit Racing (I think) because they had no listing for a gto with 421. I then started to do research, and found some literature about Pontiac running NASCAR Catalinas with 421s (I believe there is one or two valuable aluminum body models floating around somewhere). Anyway what I further discovered is that the 421 and 389 blocks are the same in that, intake, exhaust, etc can be interchanged. My car (here in Hawaii) was ordered from the dealer (i have the build sheet) with factory air. Now why would you need factory air in a convertable? Because at the same time the car was ordered, a 421 shortblock was ordered. GM would not allow an A body car to ship from the factory with a engine larger than 389 in 1966. (which may explain why they relented a litlle in 1967 due to competion with other car models, and shipped GTOs from the factory with a 400) Anyway, from what I have gathered, you walked into the Pontiac dealer, ordered your car with AC because an AC car has an oversized radiator, and at the same time ordered the 421 shortblock and paid the dealer for the swith out, so when the car arrived, the engine was yanked, exhaust intake accessories etc were switched to the 421 block and reinstalled in the car. When your car was delivered to you, you drove off the lot with a "dealer option" GTO with a 421, but the emblem badges were not changed, fakeing out any competitor in a stop light off the line challange. (unfortunatly the previous owner did not order power brakes which makes stopping interesting). Please update me if I have any of this wrong because I did spend hours of research trying to get to the bottom of why my car has a 421; btw the casting numbers on the block, exhaust & intake of of the same year series making unlikely the idea that someone (my car was a one owner) could go to the junkyard in later years to buy a 421 & find a matching casting series.
I don’t know t this has been a long thread blame me and Taylor, he started another post on the ringer lol
 

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Hello All, please correct me if Iḿ wrong; I have a convertable 66 4spd sitting on the back burner for about 10 years now. Had a machine shop redo the engine, and they informed me the car has a 421 I asked the mechanic who was doing the rebuild if the engine ever looked like it had been changed, and he told me not that he could tell. (that was about 15 years ago) I later replaced the clutch and had problems with Summit Racing (I think) because they had no listing for a gto with 421. I then started to do research, and found some literature about Pontiac running NASCAR Catalinas with 421s (I believe there is one or two valuable aluminum body models floating around somewhere). Anyway what I further discovered is that the 421 and 389 blocks are the same in that, intake, exhaust, etc can be interchanged. My car (here in Hawaii) was ordered from the dealer (i have the build sheet) with factory air. Now why would you need factory air in a convertable? Because at the same time the car was ordered, a 421 shortblock was ordered. GM would not allow an A body car to ship from the factory with a engine larger than 389 in 1966. (which may explain why they relented a litlle in 1967 due to competion with other car models, and shipped GTOs from the factory with a 400) Anyway, from what I have gathered, you walked into the Pontiac dealer, ordered your car with AC because an AC car has an oversized radiator, and at the same time ordered the 421 shortblock and paid the dealer for the swith out, so when the car arrived, the engine was yanked, exhaust intake accessories etc were switched to the 421 block and reinstalled in the car. When your car was delivered to you, you drove off the lot with a "dealer option" GTO with a 421, but the emblem badges were not changed, fakeing out any competitor in a stop light off the line challange. (unfortunatly the previous owner did not order power brakes which makes stopping interesting). Please update me if I have any of this wrong because I did spend hours of research trying to get to the bottom of why my car has a 421; btw the casting numbers on the block, exhaust & intake of of the same year series making unlikely the idea that someone (my car was a one owner) could go to the junkyard in later years to buy a 421 & find a matching casting series.
OK, I'll bite on this one.

The 1966 was the last year for the 389CI and the engine was upped to 400CI in 1967, so all GTO's came with the 400CI as factory engines beginning in 1967 and on up, with the 455CI being offered in 1970-73.

The 421CI, for quick ID, will have a large triangle shaped lug at the top/back of the block on the passenger side.

The A/C was an option and some people did install them on convertibles for those hot summer days. The A/C option did have the heavy duty radiator.

Not knowing what you documents state with regards to the 421CI, the engine could have been purchased as a complete 376HP tri-power engine, part # 9782545 for $826.00 plus a Completion Package part # 9784024 for $122.70 having all the needed parts to complete the engine such as pulleys, belts, fan, coil, starter, etc.. So the total package would cost $948.70.

But, from what you ask, yes, the 389CI and 421CI do share many parts. So, the next option is to purchase a 421CI block which would have been fitted with the pistons, pins, and rings, part # 97804022 for $245.45. Then the needed crankshaft, part # 9782279 for $99.10. Cost would be $344.55 for the needed purchase parts. The remainder of the parts needed to complete the 421CI would have come from the 389CI engine as it would have been stripped down and the parts transferred to the 421CI to create the complete and running engine. So the additional costs would have been labor for the disassembly of the 389, the assembly of the 421, removal/installation, and probably a few miscellaneous parts. Heads should be "093", with the heads and intake casting dates close to the date of the car's manufacture - which most say is typically within a month of the car's assembly date.

So this would be considered a dealer installed, or 421CI conversion, and not a factory option or factory installed engine as the 421 was not an option nor factory installed in a GTO. Your 421 should have a casting number and a casting date on it near the distributor with the casting date being very close to the date the car shows being sold by the dealer.

Does the engine have an engine code or Engine Unit Number (EUN) above the engine code. If the block was purchased through Pontiac, then my guess is that there will be no 2-letter engine code or EUN. You may find the letter "P" stamped on the front of the block where the 2-letter engine code would have been. I enclosed a photo of my brother's 1967 400CI replacement block with the letter "P" stamped on it.

If in you documents you still have the metal Protect-O-Plate, it will have the engine's EUN as will the original billing history.

So hope this helps. Very interesting car and with all your documents, makes this a unique documented dealer installed 421CI conversion of which is the first one I have heard of documented.

01  1967.JPG
 

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Per the article it was a 80 Grand Am with a Trans Am 301 block (Stronger than a regular 301). And it was coffee not donuts that the supervisor bought everyone on the line. Great story in the March issue of Smoke signals.
 

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OK, I'll bite on this one.

The 1966 was the last year for the 389CI and the engine was upped to 400CI in 1967, so all GTO's came with the 400CI as factory engines beginning in 1967 and on up, with the 455CI being offered in 1970-73.

The 421CI, for quick ID, will have a large triangle shaped lug at the top/back of the block on the passenger side.

The A/C was an option and some people did install them on convertibles for those hot summer days. The A/C option did have the heavy duty radiator.

Not knowing what you documents state with regards to the 421CI, the engine could have been purchased as a complete 376HP tri-power engine, part # 9782545 for $826.00 plus a Completion Package part # 9784024 for $122.70 having all the needed parts to complete the engine such as pulleys, belts, fan, coil, starter, etc.. So the total package would cost $948.70.

But, from what you ask, yes, the 389CI and 421CI do share many parts. So, the next option is to purchase a 421CI block which would have been fitted with the pistons, pins, and rings, part # 97804022 for $245.45. Then the needed crankshaft, part # 9782279 for $99.10. Cost would be $344.55 for the needed purchase parts. The remainder of the parts needed to complete the 421CI would have come from the 389CI engine as it would have been stripped down and the parts transferred to the 421CI to create the complete and running engine. So the additional costs would have been labor for the disassembly of the 389, the assembly of the 421, removal/installation, and probably a few miscellaneous parts. Heads should be "093", with the heads and intake casting dates close to the date of the car's manufacture - which most say is typically within a month of the car's assembly date.

So this would be considered a dealer installed, or 421CI conversion, and not a factory option or factory installed engine as the 421 was not an option nor factory installed in a GTO. Your 421 should have a casting number and a casting date on it near the distributor with the casting date being very close to the date the car shows being sold by the dealer.

Does the engine have an engine code or Engine Unit Number (EUN) above the engine code. If the block was purchased through Pontiac, then my guess is that there will be no 2-letter engine code or EUN. You may find the letter "P" stamped on the front of the block where the 2-letter engine code would have been. I enclosed a photo of my brother's 1967 400CI replacement block with the letter "P" stamped on it.

If in you documents you still have the metal Protect-O-Plate, it will have the engine's EUN as will the original billing history.

So hope this helps. Very interesting car and with all your documents, makes this a unique documented dealer installed 421CI conversion of which is the first one I have heard of documented.

View attachment 135131
Thanks so much for all that info especially location of stamps & casting #; I need to check If I have the protect o plate and follow up on checking around the block for casting ID. I do have the owner manual, so maybe protecto is together. Back story on the aquistion, I had gone to a local body shop for some other business, and while waiting for an estimate, I noticed a car in the back of the lot with the hood up, and thought, gee that looks like the LeMans I was restoring, so I walked ovr to look at it and there sat the GTO, with primered hood and fender. So when talking to the owner I asked about it, he told me it was sitting there for 5 years, and before that was in the shop for 10 years. I told him I was restoring a 67 LeMans and could maybe use some parts. I asked why he had it so long. He told me the car was in a minor fender bender, and repaired. I asked if the owner owed him money; he said no he came in to pay for the damage, then never returned to pick up the car. I asked if I did all the paperwork if he would put a mechanic lein on the car, and then sell it to me; he thought about it and told it was taking up space and would like to see it gone. I asked him how much he would like, he responded I dont know. I asked if he would take $150.00 - to my shock he said OK. (the back taxes, and other costs to put it in my name was $850.00- so total $1000 for an unrestored 66 GTO convertable
 
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