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Old 12-03-2018, 09:30 PM   #1
JerryIndy2112
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71 GMC Longhorn, compatible engine and transmition

Hey, all.

I bought this 1971 1 ton longhorn GMC pick up. I am meaning to restore it and use it as a truck. It has a 402 BBC in it. I am in the process of getting some tires on it. But for the long term, the engine burns oil and the transmission does not seem to shift up to highest gear. RPM's seem high at 35 mph. Current tires are so bad that I have not tried to open up on highway or open road. There is a lot of work to do on this truck. Body, engine, transmission, door hardware. But at least most of the damn shag carpet it gone now, lol. I talked with a guy about a fresh rebuild of the 402, but is it worth it? Will a 427 or 454 fit in its place without major work? How do I identify the current transmission and even the 4 barrel carburetor? I am new to restoration, but needed a hobby. I worked on cars back in the early '80s and even rebuilt a 350, but it has been years. Can I get a bit of info on these issues to help steer me in the right direction?

Thanks Jerry
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Old 12-03-2018, 09:34 PM   #2
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Re: 71 GMC Longhorn, compatible engine and transmition

If the SPID (service parts identification decal) is on the inside of the glove box that will give you a lot of information.

With a 402 you probably have a THM 400 transmission. The stock carburetor would be a Quadrajet. You could fit any big block in there without too much trouble, but the 402 is a great, torquey engine and you might just want to keep it.
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Old 12-03-2018, 09:43 PM   #3
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Re: 71 GMC Longhorn, compatible engine and transmition

Like truckster said, your truck should've come with a TH400. Pic below will help you confirm based on trans pan shape.
Also, if your carb looks like pic 2, it's a Q-jet - but note that it will also have quadrajet embossed on it somewhere. If you post the carb's part # (you can see this one's the driver's side just inboard of the throttle linkage on the carb body), we can tell you if it's likely original to the motor.
I'd keep that 402 and rebuild it or get it rebuilt (but yes, to answer your question, pretty much a direct swap for any Gen IV big block). Your truck is worth more with it if you ever decide to sell it, and, considering the fact that most had a small block, it's more than capable of any task you're planning to undertake with it.
Also, for what it's worth -a 3.54 Dana 60 likely is under your truck - unless the SPID calls out a 4.10 or other optional gear. You have a great combo. If highway rpms are too high for your liking (which may not be the case if it indeed has the 3.54s), I'd still keep the truck as original as possible and add a Gear Vendors over/under drive. Then you'd be set for anything, to include cross country cruising! Show us a SPID pic when you get a chance!
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Last edited by jocko; 12-03-2018 at 09:55 PM.
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Old 12-03-2018, 09:51 PM   #4
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Re: 71 GMC Longhorn, compatible engine and transmition

Hey Truckster,
I do love the sound of that 402, I think it is a badass motor. But it is also kind of rare. I have some worry about getting parts in the future. The truck came with duel exhaust going through actual mufflers. Not glass packs. Mufflers. the thing is quiet, but the sound is throaty, and sounds like it is waiting for any signal to get busy. I will do some research on the transmission. From my searches the transmission is cheaper to replace than to rebuild. Looking underneath the truck, there are leaks from rear end, tranny and engine. So they all at least need gaskets. The truck sit in Indiana for most of the last ten years. It is originally bought in California. I have all that paperwork. Engine starts right away. No issues there, but it does burn oil, you can see it in tailpipes. So it at least needs a rings. It has 173K miles on it. So it is time.

Jerry
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Old 12-03-2018, 10:17 PM   #5
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Re: 71 GMC Longhorn, compatible engine and transmition

Thanks so much Jocko,
That bolt patern chart will make identification simple. Are you saying I can add overdrive with the rear end instead of transmission? I want truck to be useable, that includes hauling and a bit of highway miles. The son of original purchaser drove it in 80's. Put Shag carpet in it, cut doors to add 6X9 speakers and cut dash for radio. He seemed nice and repentant, but still. He did make my task harder.
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Old 12-03-2018, 10:40 PM   #6
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Re: 71 GMC Longhorn, compatible engine and transmition

Keep that shag carpet too! Ha. Maybe not. The gear vendors (not cheap, around $3K new) installs at the trans tailshaft, requires a shorter driveshaft. Google Gear Vendors on here and you'll find tons of details. But it "splits" the gears so that your current 1st, 2nd and 3rd become 1st, 3rd, and 5th, and the GV provides the overdriven version of each, so 2nd, 4th, and 6th. Makes your trans a 6 speed if you CHOOSE to use the gear vendors unit as a splitter. The other option is to just use it as an overdrive for high gear (i.e. don't split, just think of it as an add on that accomplishes what installation of an overdrive trans would - lowers high way rpm). The splitter function would really just be handy if towing a heavy load or towing in the mountains, etc. Overdrive ratio is .78 for ref = i.e. a 22% overdrive (or reduction of rpm for a given gear of 22%)

Link to their website for technical details. https://www.gearvendors.com/cg2wd3s.html
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Old 12-03-2018, 11:01 PM   #7
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Re: 71 GMC Longhorn, compatible engine and transmition

The 402, while not extremely common, is not so rare that parts are going to be hard to get. As Jocko says, most buyers would be more interested in the truck with the original 402 than with a 454. I'd rebuild it and enjoy it. See if you have the SPID and take a picture and post it. We can give you more suggestions then.
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Old 12-03-2018, 11:04 PM   #8
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Re: 71 GMC Longhorn, compatible engine and transmition

I hope tag is showing
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Old 12-03-2018, 11:24 PM   #9
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Re: 71 GMC Longhorn, compatible engine and transmition

Hey all,
Took a picture of the tag in glove box. It says it is a 400 engine. I was told by seller it was a 402. My research says 402 is common. It says turbo-hydromatic transmission. Wow, I bought the truck and the original owners may have not been fully informed on what they had. They were not enthusiasts, they just passed the truck down till grandson did not want to do restoration. So he sold it for cheap. I have been looking for a classic truck and love the GMC body in these years. The tag does not even specify 400 sbc or bbc. I had assumed bbc. But I may even be wrong about that. Well, I knew this would be a journey. I estimate a 2 year restoration. I know the 1 ton base will haul all I need to work on house and yard. For the price I paid, it is well worth it. But I should have looked in glove box and known right away what I was buying. Live and learn.

Jerry
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Old 12-03-2018, 11:32 PM   #10
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Re: 71 GMC Longhorn, compatible engine and transmition

GM had two engines that they called 400 - both a small block and a big block. The engine you have is the big block, which actually displaces 402 cubic inches. The SBC 400 was not used in the 67-72 Chevy/GMC trucks.

Your truck is also a Camper Special and CST (Custom Sport Truck) which was the highest level of trim available. It's a nicely optioned truck.
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Old 12-03-2018, 11:37 PM   #11
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Re: 71 GMC Longhorn, compatible engine and transmition

A 400 in your year truck is a 402. 400 is just how GM labeled them. That 402 is extremely common; there were probably half a million or more made in the years they were produced. It's really easy to tell of you have a BBC in your truck. The valve covers have 7 bolts instead of the 4 bolts for a SBC (if it is stock).

Looks like truckster ninja'd me!
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Old 12-03-2018, 11:41 PM   #12
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Re: 71 GMC Longhorn, compatible engine and transmition

Truckster,

Ok, that makes sense. Is there any way to tell from tag? The truck is a 1 ton, longhorn. I have sales receipt from 1971, it was sold from GM with a trailer stacked on bed. He put $15 down and paid $150 a month to pay it off. I have a lot of original paperwork. Sorry that I was confused by the 400 vs 402 designation. Sounds right to me now. I really did not know, and that is why I am posting here. I am a noob, and looking to learn. Part of the journey.

thanks
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Old 12-04-2018, 12:56 AM   #13
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Re: 71 GMC Longhorn, compatible engine and transmition

It's a fun (and sometimes frustrating) journey. The board members here are the best. They'll help you out every step of the way.
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Old 12-04-2018, 09:57 AM   #14
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Re: 71 GMC Longhorn, compatible engine and transmition

Looks like a great working-man sort of project. I like it!
This truck will be a good one for you to wrench on if you were doing that sort of thing back in the 80s. They are dead simple and pretty forgiving.
Verify that you don't have a shift linkage problem causing your high RPMs. If the trans is worn out, have that TH400 gone thru and you will never touch it again, except for fluid changes. They are one of the best automatic transmissions ever made.
As said above, the 402 is common enough. Verify that you are using oil before you pull the trigger on any major repairs. How do the plugs look? Do a compression and leakdown test. If it's just running rich, it will blacken up the exhaust.
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Old 12-04-2018, 02:42 PM   #15
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Re: 71 GMC Longhorn, compatible engine and transmition

Quote:
Originally Posted by JerryIndy2112 View Post
Truckster,

Ok, that makes sense. Is there any way to tell from tag? The truck is a 1 ton, longhorn. I have sales receipt from 1971, it was sold from GM with a trailer stacked on bed. He put $15 down and paid $150 a month to pay it off. I have a lot of original paperwork. Sorry that I was confused by the 400 vs 402 designation. Sounds right to me now. I really did not know, and that is why I am posting here. I am a noob, and looking to learn. Part of the journey.

thanks
Jerry
Jerry, Can you elaborate on what you’d like to confirm in your 2nd sentence? The 400 small block didn’t exist in trucks until mid-70’s, so your SPID definitively confirms that your truck was born with a big block 402. Doesn’t guarantee it hasn’t been swapped out for another big block or a small block. Post a pic and we can help. You SPID also confirms it’s a 1 ton Longhorn (by the first “3” in the VIN , and the 133” wheelbase).

Stocker - I thought YE9 Sierra Grandes were introduced in mid-71 along with the Chev Cheyenne Super - but not certain. (As in highest trim level)
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Old 12-04-2018, 07:34 PM   #16
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Re: 71 GMC Longhorn, compatible engine and transmition

Got home after dark, so too late to take engine or transmission pictures. I will get some when I get the opportunity. I am sure it is a 402 bbc. I will also check the linkage, The whole issue may be there. It is hard to get into park and out, so definitely a possibility.
Also there is a chance carb needs adjusted. It does start right up, but idles a bit rough. Plan on spending some time on it this weekend. Also trying to clear a space for it in the garage. Would be nice to work on it in all weather(or at night).

Thanks all for your information.
Jerry
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Old 12-04-2018, 07:55 PM   #17
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Re: 71 GMC Longhorn, compatible engine and transmition

Ok....lets start with the 'real' start point.
What is your budget for improving the drivetrain issue on this truck?
ie: $3000? $5000? $1500? $1000 or?....this is the realistic start point and its good to be honest about this because it has a massive bearing on what you can do, obviously.
....this is job one...question one.

I appreciate the fact that you have the original engine...I would hang onto it.

However, what I would do is pull that motor and put it aside in storage for the moment and drop in a 454.
(you can take the original motor and have it rebuilt over time if you like...or keep it offer as being available for the next buyer)
The 454BB is physically identical to your existing 402BB so all your brackets will bolt right back on.
If you are interested in appearing original...just tell everyone it is a 402.

You will get a nice power/torque bump in that option.

As well, if you find a good used 454 (and they are out there) they often have their TH400's with them.
Its not uncommon these days to find guys parting out low mileage motorhomes that are selling of the low mileage 454/Th400's out of them...and some pretty reasonably priced.
If you go this route...ask to hear the motor, check it out, compression, do your homework.

All good ....and good luck!
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Old 12-04-2018, 09:57 PM   #18
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Re: 71 GMC Longhorn, compatible engine and transmition

Ok Coley,
Good question about budget. I plan to spend 7-10K on the truck over a couple years. I do want to keep as original as practical. Do not want to add electronic ignition or any of that. I expect to rebuild transmission, engine maybe even the carb. Some things I will gladly do myself. I want to do body right, and repaint. May need a electrical harness. Doors are messed up on the outside, mostly due to mirrors being hit. Want to put the stock, period mirrors on it. Not sure what can be done with doors. The back bumper is below the body indents for the standard truck. This is factory, the original package had a camper that hang down over standard bumper. I have no intention of putting any camper back on it. So I need a stock bumper and supports. The wood bed is garbage. Some of the hardware is ok, some is rotted. So bed needs replaced. The previous owner put plywood and mats down to keep stuff from falling out. It has 2 gas tanks, surprise. The thing drinks gas like water. It will need fuel lines. Undercoating. The list is long. Still working on the order of repairs. I am far from rich, but a bit at a time. The big projects need to be planned in advance and saved for. I hope to own this truck for a few years, restore it. Maybe sell, make some money and buy another. I do love this body style. My wife is even excited by the whole project. She also loves the truck style. So all is good. If you can please the wife and make a beautiful truck even better, win, win.

BTW for anyone checking the tread, my transmission is definitely a TH400, the dimples make me sure. Still have not had time to sort the carburetor.
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Old 12-04-2018, 10:26 PM   #19
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Re: 71 GMC Longhorn, compatible engine and transmition

The turbo 400 transmission will not shift without vacuum from the intake manifold to the modulator valve. You should check the vacuum lines before you assume the transmission is bad.

A vacuum leak will also cause a rough idle.
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Old 12-04-2018, 11:13 PM   #20
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Re: 71 GMC Longhorn, compatible engine and transmition

Quote:
Originally Posted by davischevy View Post
The turbo 400 transmission will not shift without vacuum from the intake manifold to the modulator valve. You should check the vacuum lines before you assume the transmission is bad.

A vacuum leak will also cause a rough idle.
I was thinking maybe the modulator, but yeah, this is another check. Though if a 400 never goes into 3rd, there is another failure mode, but I forget what it is.
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Old 12-05-2018, 08:04 PM   #21
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Re: 71 GMC Longhorn, compatible engine and transmition

Took some pictures. The carb is a Rochester Quadrajet.
The transmission is a TH400.
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Old 12-05-2018, 08:11 PM   #22
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Re: 71 GMC Longhorn, compatible engine and transmition

Oh, also I noticed a strong fuel smell around truck. While I was taking pictures I looked in carb and it was wet. So I started it up, it started ok. The issue with burning oil may be a stuck float or something wrong in carb. Carb rebuild is cheap. Will do that first.
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Old 12-06-2018, 05:31 PM   #23
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Re: 71 GMC Longhorn, compatible engine and transmition

That is a big block and a 400TH from your pics, send me the block numbers (on a pad just behind and above where the water pump mounts to the RHS (pass side) of the block and we can look that up to see if its original to the truck and (most people dont know this) but the original numbers I am asking you to send are on the protecto plate if you have that with the original owners manual so you can confirm
Take your time with it and if you can sort through the issues you will be very glad you kept the original motor
The carb though a Qjet does not appear to be the original but the numbers off the LHS (driver side) would tell us what it is specifically
Cool truck!
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Old 12-06-2018, 08:03 PM   #24
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Re: 71 GMC Longhorn, compatible engine and transmition

What part of the carb doesn't look original?
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Old 12-06-2018, 08:18 PM   #25
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Re: 71 GMC Longhorn, compatible engine and transmition

The vacuum advance port is in the wrong place for one
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