big block GMC

[JerryIndy2112]

I have a 71 GMC C3500 longhorn with a 402 in it. The stamp on the block says it is a chevelle engine from 1970. The engine runs fine. But, everything under the truck is leaking and wet. Engine, TH400 transmission and even the rear end.
As of now I am rebuilding the front disk brakes. Had a lot of issues with hoses and the like.
When I got the truck I was able to drive it to get tires on it, the old 16.5. So I have a lot to do to the truck to make it my weekend driver. When I got it up to 50 MPH, it was reving really high in my opinion. I have no tach. So maybe this is normal for the TH400. Maybe the transmission is slipping. I have looked at rebuilding the 402. I am getting prices around $2500 to $3500 to rebuild the 402 to stock. I can buy a 454 rebuilt for around $4000 or less. Most of my searches show the 454 as a long block. Is the 402 I have a long block? To put in a 454 in its place will I need to change motor mounts, radiator and things like that or can I pull out 402 and put the 454 right in? I will also need to rebuild this TH400 or get another in better condition. So I am trying to plan ahead on my restoration. There is body work to do. Cab mounts. New wood bed. Enough to be overwhelming.
The 402 while having good torch is mild on Hp. I looked up the numbers on the block, and it was rated at 330 hp in 1970. I am sure that is good for the truck. It sounds good, but would not cook tires at all. Maybe the old transmission. I would like more from my dream truck.
What does the group have to say? What is on my list of options?

Jerry

[67_C-30]

The term “long block” means the rebuilt engine is complete with heads. It has nothing to do with size. “Short block” means block, rotating assembly with pistons only, but no heads.

Your 402 and a 454 is the same size dimensionally and a 454 will right in place of a 402. Both are considered big blocks.

Your engine is revving higher than you’re probably used to because the TH400 has no overdrive and the truck probably has pretty low gears. An OD transmission would be the best solution, but you could possibly put higher gears depending what differential you have.

[JerryIndy2112]

thanks for the clarification on terms.

[Steeveedee]

Most 400 CI engines with an automatic came standard with a 3.54 axle. To your ear, it's probably revving pretty high because most vehicles made in the last 40 years have some sort of overdrive and a lockup converter. When I don't pull a trailer, I could use an overdrive on the freeway. My truck has the torque for it, as long as the RPMs are in a decent range not to lug it. But I wouldn't use an overdrive gear when towing. I'm actually going to do nothing with my truck in terms of electronic transmissions, fuel injection and so forth, because it's just expense that while giving better mileage, won't give me a return in my lifetime. BTDT with my Buick hotrod and an overdrive trans.

[weim55]

Something to think about: Find someone-shop in your area that specializes in older vehicles and spend the money to have the truck completely assessed end to end. There’s a good chance there’s absolutely nothing seriously wrong with any part of the drivetrain. A thorough check of the engine transmission etc. can can determine if a rebuild is necessary or simply a reseal And replacement of wear components. You could save yourself a ton of money and labor and have a good bottom line starting point before you proceed.

Steve weim55 Colorado

[SavageC20]

Originally Posted by JerryIndy2112

I have a 71 GMC C3500 longhorn with a 402 in it. The stamp on the block says it is a chevelle engine from 1970. The engine runs fine. But, everything under the truck is leaking and wet. Engine, TH400 transmission and even the rear end.
As of now I am rebuilding the front disk brakes. Had a lot of issues with hoses and the like.
When I got the truck I was able to drive it to get tires on it, the old 16.5. So I have a lot to do to the truck to make it my weekend driver. When I got it up to 50 MPH, it was reving really high in my opinion. I have no tach. So maybe this is normal for the TH400. Maybe the transmission is slipping. I have looked at rebuilding the 402. I am getting prices around $2500 to $3500 to rebuild the 402 to stock. I can buy a 454 rebuilt for around $4000 or less. Most of my searches show the 454 as a long block. Is the 402 I have a long block? To put in a 454 in its place will I need to change motor mounts, radiator and things like that or can I pull out 402 and put the 454 right in? I will also need to rebuild this TH400 or get another in better condition. So I am trying to plan ahead on my restoration. There is body work to do. Cab mounts. New wood bed. Enough to be overwhelming.
The 402 while having good torch is mild on Hp. I looked up the numbers on the block, and it was rated at 330 hp in 1970. I am sure that is good for the truck. It sounds good, but would not cook tires at all. Maybe the old transmission. I would like more from my dream truck.
What does the group have to say? What is on my list of options?

Jerry

My truck originally had a 402 in it and now has a 454. Machine shop quoted me $2700 for full rebuild on the 454 so I'd say the quotes are in line. Since the 402 and the 454 are both big blocks you will be able to use the same motor mounts, radiator, and to my knowledge everything else.

However, I agree with having a shop look the whole truck over and see what needs to be done. You may not even have to rebuild anything, just replace gaskets to stop leaks.

I will say, my 454 is great at cooking tires.

[berudd]

Originally Posted by 67_C-30

The term “long block” means the rebuilt engine is complete with heads. It has nothing to do with size. “Short block” means block, rotating assembly with pistons only, but no heads.

Your 402 and a 454 is the same size dimensionally and a 454 will right in place of a 402. Both are considered big blocks.

Your engine is revving higher than you’re probably used to because the TH400 has no overdrive and the truck probably has pretty low gears. An OD transmission would be the best solution, but you could possibly put higher gears depending what differential you have.

Just to clarify, a long block will require more parts to be ready to run. Intake, carb, accessories. Keep that in mind when comparing the price to rebuild over buying. If you bought a 454 I would assume all that stuff from your 402 would swap over though.

[Ol Blue K20]

454 requires a different a counter balanced flexplate and harmonic balancer.

[JerryIndy2112]

Good to know for sure that a big block is a big block. I would not use my intake from my 402. I am told the biggest issue with the 402 is it's airflow. So if I upgrade to a 454, I would make sure to upgrade airflow.

[68Gold/white]

Jerry, nobody here wants you to think less of us for our interest in guiding you in your quest for a complete better pickup build. Sometimes our words may not sound kind, but most here are veterans at working on this stuff. Either making it stock, or hopped up!!!

Whatever intake is on your engine, will likely perform greatly on a 454 also. Your 402 was likely not a huge power maker, even stock. They did run well, even in a 3500!!!

Check online for 454 engines. New ones to be exact. If you have a generous buget, that would be the way to go!!! GM Performance parts, Scoggin Dickey (a dealer for GM Perf parts), Summit, and many others.

Some pictures of your pickup would be cool!!!

[JerryIndy2112]

I am happy to show my truck. She needs so much work. Her name is "large marge" she is a 1971, GMC C3500, 1 ton custom camper. I have original sales receipt.

[JerryIndy2112]

I have her up on jack stands now. Getting front disk brakes done. I was loving the sound of the 402 but was underwhelmed by its power. I priced rebuilding 402. And thought why not get the best powerhouse in my truck. The 402 in it shows it is a 70 chevelle motor. May be original and may not be. It is my truck now so if I want a meaner motor, why not

[Steeveedee]

I don't know what kind of clapped out 402s you guys have, but even with 120k+ miles on my rebuilt engine, it takes off laying rubber and will chirp second, even with that camper shell. I get hole shots from traffic signals on ricers all the time. It's great fun!

[Mike C]

Cool truck. Do you have picture of the SPID you can post? (Service Parts Identification label that is affixed to the inside of the glove box lid). Also a picture of the rear axle cover.

As mentioned before, most big block trucks came with the 3.54 geared Dana 60 rear axle, but as a 1 ton it could well have been optioned with lower (higher numerically) gears for towing performance.

[weq92f]

.
Nice truck Jerry! I would try tuning your motor a bit to make sure it's running proper before giving up on it! These old things leak, but that doesn't mean they're worn out!

If you could get by with some gaskets and tuning that 3k or so could go a long way elsewhere...

-klb

[JerryIndy2112]

This is SPID

[JerryIndy2112]

This is the back axle. Really needs cleaned up. Have not got to that yet.

[Mike C]

That's an HO72 most likely. 7200# GAWR. The C2500 got the HO52 (5200# GAWR) and they are mostly the same.

By having you post pic of diff, we know you do not have the 3.54 that most big block auto trucks came with because they would have had a Dana 60 which is 5500# GAWR. That was my guess since your truck is a C3500 and why I had you take a pic.

The smallest numeric gear HO is 4.10 ratio and more common was 4.57. There was an aftermarket 3.90 gear that was available for a short time in very small quantity so they are un-obtanium. That explains you RPM on the highway. The 3.54 setup in the Dana 60 with a 265 75 16 tires makes for a pretty decent highway cruiser.

[JerryIndy2112]

Thanks Mike,
It is good to get some information on the rear differential. The truck was sold with a camper on it. In 1971 the truck was bought for about $8000.00 dollars. With the camper. I am sure it was heavy, but I have no original picture of it from the dealership, would love to have that picture. I have no intention of putting a new camper on it. Just a nice weekend driver. The gearing makes my experience understandable. I need to find a 3/4 ton with the eight lug rear end and change the axle out? Is that the best solution?

Jerry

[Ironangel]

Originally Posted by JerryIndy2112

Thanks Mike,
It is good to get some information on the rear differential. The truck was sold with a camper on it. In 1971 the truck was bought for about $8000.00 dollars. With the camper. I am sure it was heavy, but I have no original picture of it from the dealership, would love to have that picture. I have no intention of putting a new camper on it. Just a nice weekend driver. The gearing makes my experience understandable. I need to find a 3/4 ton with the eight lug rear end and change the axle out? Is that the best solution?

Jerry

I'd forego the Dana 60 and put a 14-bolt under it. They're easy to set up and work on, tougher than 60's and readily available for cheap. You can drop a Detroit locker in the 14-bolt for like $400 if it don't already have one. Some had factory Gov. Locks (three pin carriers) and gear sets numerically low as 3.42. The Full Floating 10.5" 14-bolt is the cats meow of 3/4 and 1-ton axles..."Jus Sayin"

[custom10nut]

Before you go “Whole Hog” with rebuilding everything change out gaskets and get the leeks stopped. With all the other posted taken it to consideration, I would say your Transmission is the culprit for the lack of power.
Flush it out and put a Shift kit in to see if that fixes it.
If none of this works, then move forward with rebuilding, but it wouldn’t hurt to try

[Mike C]

The 14 bolt can be a viable solution. In either full-float or semi-float varieties. The 3.42 geared setup is uncommon (and you can get 3.23 in the semi-float, but they are hard to find in the 8 lug setup) 3.73 are fairly common and would be a 20% reduction for your cruise rpm which would be noticeable.

Anything up to ‘88 and maybe ‘91 in the square body style I think still has 9/16 lugs to match your front. The newer axles will require re-welding spring perches and relocating shock mounts. The exception being one ton axles have same perch spacing as the Longhorn and 4x4 67-72. But shock mounts require relocating either way.

The Dana is a bolt in if you find a 71-72 donor.