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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Hotrodding Basics> My '56 GMC truck has a '66 389ci.... GTO?
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Topic Review (Newest First)
02-13-2013 08:50 AM
Mr. P-Body If it is indeed a '66, that "WW" means a 335 HP 389 (1 4-bbl), manual transmission. The date code is by the distributor and looks like a "tab" that is screwed down, where it's actually "cast in" with four digits. The first digit is the month (A=Jan, B=Feb, etc.). The next two digits are the day of that month. The last digit is the last digit of the year it was cast. An example would be "H205" would be a '66 block (next year's models start in July) cast on August 20th, 1965. That would be an "early build" for the '66 model year.

Also, count the number of expansion plugs in the side of the block. This is how we "narrow it down". A 389 has 2 plugs per side. A 400 (could have a similar casting date, ten years newer) has 3. Logical deduction gets us where we "need to be". And according to the database, that engine WOULD have "93" heads, as it IS a GTO code.

I saw a bumper sticker on Mini in `1970 that said that about Lucas, at Laguna Seca during the Monterey Grand Prix (Can Am cars WOW!!!). On the same "note", a friend, married to a British lady, drank an ice cold Coors (for me) while standing in front of a beefeater at Buckingham Palace on July the 4th, 1986, just for fun.

All joking aside, my father was stationed at Watton, 1942-45. He was a bombsight tech on B-17s and B-24s. In '95, he and Mom made it back for the 50th reunion. The town of Watton (I guess that's the nearby town) welcomed them to the point where they couldn't spend a nickel. Room, food, transportation, events, ALL "free" for American AAF vets! That's CLASS! Thank you to all Brits for the way you treated the greatest generation of Americans since The Revolution, and just maybe, EVER.

02-12-2013 06:24 PM
LATECH X 2 with Bill. The casting number will be inside the valve cover area on the 66 and back heads, look for the last 2 numbers in the 7 or 8 digits that are cast into the head.That will be the number
02-12-2013 05:50 PM
Originally Posted by UKstepside View Post
lol Mr P-Body
The block has WW on it but no numbers can be found on the heads at all
The head number that you need to find will be cast on, not stamped.

02-12-2013 05:00 PM
gearheadslife chevy cavilers and some colbalts have vac pumps, get one and have at it with a wild hair cam..
many 90's caddy's also have the pumps,,

why not a hydra boost instead of vac booster,,
02-12-2013 04:18 PM
Originally Posted by PatM View Post
Incredibly funny . . . Lucas, the prince of darkness.


That is how I understood it too. LOL
02-12-2013 01:38 PM
Originally Posted by Mr. P-Body View Post

Let us know what you find for numbers.

BTW, we "yanks" know why you Brits drink warm beer, ya know... You have Lucas refridgerators... (:-


Incredibly funny . . . Lucas, the prince of darkness.

02-12-2013 11:36 AM
UKstepside lol Mr P-Body
The block has WW on it but no numbers can be found on the heads at all
02-12-2013 07:55 AM
Mr. P-Body Andy,

Let us know what you find for numbers.

BTW, we "yanks" know why you Brits drink warm beer, ya know... You have Lucas refridgerators... (:-


02-11-2013 11:12 AM
UKstepside Awesome info guys - many thanks im such a newbie at yank stuff but i learn fast
If it was a VAG or Subaru i'd be well away. lol.

Im happy to have a slighty rougher idle from a cam, it sounds just awesome. I need to find some headers foe it and make up a stainless exhaust as its stock just now and is shot. I also need to look into the rear end and decide what - if needed - axle to go for. i want it to be a good driver for long distance to shows all over the UK and be able to smoke from the lights

02-11-2013 08:00 AM
Mr. P-Body If it is truly a "GTO" engine, the heads will be "93s". 92s were on all other 389s except GTO.

The Pontiac used the timing cover for the "front mount" in those days. The aluminum versions of later years still have the bolt provosinos, but I would hesitate to use one as a "mount' in a high-torque engine.

Pontiacs are very sensitive to compression versus fuel quality. Make certain your static ratio is reduced to under 9.5:1.

Comp offers the XE series for the Pontiac. The XE256H is probably what you're looking for. Decent idle, good low- and mid-range power. The "next one up" (XE262H) is the most popular grind we have for "stockers". Works very well in 389s with TriPower and stock exhaust.

My "dream truck" is a '59 GMC (short bed, step-side) with a 505 Pontiac in it, pulling houses down... (:-

02-10-2013 12:49 PM
UKstepside It has side engine mounts, middle of block. from the numbers on the block its a '66 engine.
02-10-2013 12:42 PM
vinniekq2 welcome to the forum
looking forward to seeing pics
02-10-2013 12:37 PM
MouseFink 1955-1959 GMC Series 100 and 500 with optional Pontiac V8 engines:
1955...287 CI Pontiac V8 engine
1956...317 CI Pontiac V8 engine
1957...347 CI Pontiac V8 engine
1958...336 CI (small bore version of Pontiac 370 CI V8 engine)
1959...336 CI (small bore version of Pontiac 389 CI V8 engine)

Quick ID:
If the V8 engine in your GMC truck has front motor mounts, it is the original 1955-1959 Pontiac V8.
A 1960 - 1979 Pontiac V8 had side motor mounts and no provisions for front motor mounts. .
02-10-2013 08:48 AM
UKstepside hi huys,
many thanks for the replys.
i'll get the info needed asap but might be a week or 2 as i need to get the truck picked up and brought to me so i can go over it properly to make sure she's sound & safe.

It is believed that its a GTO donk, but the numbers will confirm when i get them.
I do have big plans for her but as with most rodders, it wont happen over night

cheers again. Andy
02-10-2013 08:35 AM
LATECH X 2 with Widetrack here.
Some more info is needed.
I would suspect the rear end gear is pretty low , like 3.56 or 3.73.
Kinda too low for a truly streetable ride with a poncho mill as the poncho mill makes more torque and can handle higher ratios much better.
Way kool that you have a GMC with a pontiac engine in it.
I would love to here more about it.
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