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Discussion Starter · #1 ·
I have questions about my 1966 GTO, and I hope someone here can offer some insight.

I own a '66 GTO convertible with a copy of the original factory build sheet from PHS showing it to be a tri-power car with automatic transmission and factory A/C among other options. My GTO was built during the first week of November, 1965, so it's an early '66.

The engine's casting date is J85, which I believe means that the block was cast in September, 1965 -- just about right for a vehicle that was assembled in early November, 1965. The engine appears to be original, except someone removed the tri-power setup and replaced it with a QuadraJet at some point. I have since installed a beautiful, date-correct tri-power setup built for me by Mike Wasson. It runs great!

The mystery is that the code stamped on the front of my engine is YF rather than YR as I think it should be for a '66 tri-power GTO with auto transmission. Some knowledgeable GTO people believe that maybe Pontiac ran out of YR engines at the factory and substituted a YF engine to keep the line running. Others guess that the dealership swapped out the original engine and installed a Bonneville engine in its place.

I've owned this GTO for more than 25 years now and haven't thought much about my YF engine until recently, when I came across a post in another forum where the owner of another '66 tri-power GTO also says he has a YF engine. The post to which I refer is located here:


This makes me believe than more than one '66 tri-power GTO was factory-equipped with a YF engine. I don't think Pontiac's YF engine was very popular in Catalinas and Bonnevilles back in the day, so I have to wonder why at least two early '66 tri-power GTOs are equipped with YF engines.

Do you guys have any insight as to why my GTO has a correct-date YF engine under the hood? Are there more early '66 GTOs with YF engines out there?

Certainly, someone could have replaced the original YR engine with a YF engine before I bought the car, but what are the chances of having correct date codes on the replacement engine?

I appreciate all your opinions!
 

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I believe J on your engine date casting would be October (for 1966 model year). October 8, 1965. I have on file a few documented all original unrestored 66 GTO‘s with various engine component dates that coincide with the cowl tag that I use for references. Every single one of these document GTO‘s the engine build date is typically within 15 days of the cowl tag’s date.

Based on your engine date and your cowl tag that’s at least one month difference. Being that it’s the wrong engine code and that much time difference in dates in my opinion that engine was never born in that car.

The only way to verify the correct born in engine is the protect-o-plate or a build sheet found in the car from the plant with the Engine Unit Number printed on it. Having these documents really increase the value of GTOs because you have evidence of the original engine.
 

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Seeking information on a 1969 Judge my folks owned
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Though it's not a given, engines can often be built weeks or months before the car they are installed in. Though it is certainly possible the original YR block was replaced with a correct-dated YF block, kinda sounds like a production delay thing to me. Let's say the car was a special-order tri-power, but no correct YR tri-power block was available at assembly. My guess is, for 1965 / 1966, the 389s are likely all the same below the intake for that year regardless of the fuel system / induction setup, making the block code purely a formality.

Tri-power dizzy might have a different timing curve to take advantage of the extra air and fuel, other than that, I can't see where it would make a difference, and the dizzy can certainly be recalibrated. If that was the case, rather than delay production / delivery, a YF block meant for a single quad could be fitted with tri-power, installed in the car and shipped to the dealer for delivery. I'm fairly confident the engines WERE otherwise more or less the same FOR that reason, a "just-in-case" if you will.

Just a thought. Truthfully, if the only difference is the engine block code / casting number, and they code out original to the car, it's more of an oddity than a problem. But as I stated, many engines are built some time before the car they go in, so just because the engine build date codes out in the ball park of the car's production date doesn't necessarily mean it is original either. This is one thing I question about concourse restoration and show judging, as I wonder if this is taken into account when "verifying" an original engine.
 

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I know several original owners from the past that ended up getting a GTO different from what they ordered due to various reasons (strike, shortage of parts, or a mistake on Pontiac’s part. In all cases these GTOs showed on the PHS what the car left the factory with.
 

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The engine casting date IS NOT the engine build date. The engine build date can be weeks after the engine block was cast particularly 'special' engines such as Tri-power or RA/SD engines.
Speciality components such as the Tri-Power/RA/SD distributors, intake & exhaust manifolds etc. were built/cast all at one particular time and stored for future use. For example, when the Judge was planned for release in Jan of 69, the RA engine blocks and other RA components were cast and built weeks/months in advance in preparation.
Generally the engine build date will be within 1 - 4 weeks of the vehicle build. All engines were built at the Pontiac MI plant and stored until ready for shipment to the various manufacturing plants across the country.
If your car was built in Pontiac the engine could be cast, built and installed in the car within 2-3 days of the block casting or two to three weeks if the car was built in Fremont on the West Coast.
One other thing to remember, the build date on the Fisher body plate IS NOT the date of the completed car. It is the date when the Fisher body build began. The body would then go into a body bank or storage until the assembly line was ready for production. If a component was not available such as a Tri-Power engine, the body would wait in the body bank until it was.
Not unusual to see an invoice date (date within a day after the car rolled off the line and is found in the PHS documents) 2-3 weeks after the Fisher Body date.
PHS documents will normally list the EUN number especially on the pre 68 cars. Another way to verify a numbers matching engine.
 

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Discussion Starter · #6 ·
I really appreciate all the opinions and additional information I've received here about my '66 GTO "Mystery Engine."

It's likely I'll never know for certain if the 389 Tri-Power in my GTO is the original, but I'll always treasure this car. FWIW, I'm posting a copy of the factory invoice in case it contains any more clues for the experts here:

Music Font Sheet music Parallel Entertainment
 

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I really appreciate all the opinions and additional information I've received here about my '66 GTO "Mystery Engine."

It's likely I'll never know for certain if the 389 Tri-Power in my GTO is the original, but I'll always treasure this car. FWIW, I'm posting a copy of the factory invoice in case it contains any more clues for the experts here:

View attachment 150046
Got a "double post" here as it did not show up the first time and I hit the resend, so second post is the same.

Here is my guess.

With AC, the car should have the return gas line going back to the gas tank and 3.08 rear gearing. I would verify any VIN/Codes on the transmission/rear axle. 3.55 gearing was the standard rear axle gear on the 3 x 2 engine, both manual & auto.

The YF code is for a full size B-body. How would a B-body engine find its way to an A-body assembly plant? So guess #1 is that the engine has been transplanted either from a former owner, maybe a warranty engine (which typically have no letter codes and you will have the GTO "093" heads), or a dealership pulled the engine from a B-body car on his lot to replace a bad engine and then waited on another replacement engine for the B-body - especially if the GTO's were selling "hot" which they were in 1966.

Next guess may be the YF engine was installed due to the AC -which would be an "odd" option on a 3 x 2 car. The compression is lower than the 389CI 3 x 2, and the cam was milder, it has the "066" versus the "068" with the "068" being a rough idling, low vacuum, cam which probably would not have worked comfortably with AC, especially with an automatic & a convertible - keeping in mind that the AC cars had the dashpot to prevent stalling and had to run at a higher RPM when the AC was turned on. The YF engine would have been a wiser pick just because of the AC. BUT, the GTO 335HP engines were available. These still had the higher compression engines and used the "067" cam.

Bottom line? We will never know unless you can come up with the EUN number showing the exact engine used in that car and matching it to your car. And, with Pontiac, factory or dealership, never say never because things are not always as they should be.
 

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I really appreciate all the opinions and additional information I've received here about my '66 GTO "Mystery Engine."

It's likely I'll never know for certain if the 389 Tri-Power in my GTO is the original, but I'll always treasure this car. FWIW, I'm posting a copy of the factory invoice in case it contains any more clues for the experts here:

View attachment 150046
Here is my guess.

With AC, the car should have the return gas line going back to the gas tank and 3.08 rear gearing. I would verify any VIN/Codes on the transmission/rear axle. 3.55 gearing was the standard rear axle gear on the 3 x 2 engine, both manual & auto.

The YF code is for a full size B-body. How would a B-body engine find its way to an A-body assembly plant? So guess #1 is that the engine has been transplanted either from a former owner, maybe a warranty engine (which typically have no letter codes and you will have the GTO "093" heads), or a dealership pulled the engine from a B-body car on his lot to replace a bad engine and then waited on another replacement engine for the B-body - especially if the GTO's were selling "hot" which they were in 1966.

Next guess may be the YF engine was installed due to the AC -which would be an "odd" option on a 3 x 2 car. The compression is lower than the 389CI 3 x 2, and the cam was milder, it has the "066" versus the "068" with the "068" being a rough idling, low vacuum, cam which probably would not have worked comfortably with AC, especially with an automatic/convertible - keeping in mind that the AC cars had the dashpot to prevent stalling and had to run at a higher RPM when the AC was turned on. The YF engine would have been a wiser pick just because of the AC. BUT, the GTO 335HP engines were available. These still had the higher compression engines and used the "067" cam.

Bottom line? We will never know unless you can come up with the EUN number showing the exact engine used in that car and matching it to your car. And, with Pontiac, factory or dealership, never say never because things are not always as they should be.
 
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