V6-V8 swap

[JAKES 68GMC]

NO, I'M NOT GOING TO SWITCH THE OLD V6!!!

I was just wondering though, how hard would it be to swap in a SBC in place of the V6? Are the V6 engine stands in the same place as they'd be for a small-block Chevy? I'm just asking in case if the V6 ever decides to go south, which I really don't think it will. I just want to be prepared. Thanks for the info.
Jake

[Fred T]

Can't answer your question. If I were you, I would start looking for a 351, it's a lot better engine than the 305. And the V6 guys won't be gunning for you.

[Bob B.]

It's not too tough if you have all the parts. The motor mounts, and motor mount towers are different. The bell housing is also different, and so is most of the clutch linkage. But, I don't think you will ever really need to. The 305 is still easy to get parts for if you know where to look. And, there are still quite a few rebuildable 305's out there. If you are looking for more power, the 351 is your best bet, as it is a bolt-in. The 401 V-6 is a bit tougher, it used timing gears instead of a chain, and the timing cover is quite a bit different. The 478 has been done, but it is more involved, as they had a different flywheel.

[JAKES 68GMC]

The bellhousing is different? I thought the bellhousing was just a normal SBC one. I have a spare SM465 with a bellhousing that came out of a 292 truck, and the 305 and 292 housing look identical. Maybe I should look again.

I was just thinking, it woud cost quite a bit to rebuild the 305, 750 bucks for a rebuild kit! And for that money, I could have a nice rebuilt 350 for that kind of oney, and maybe even an Edelbrock intake.
But, it's just a thought. My 305 hits on all 6 when she has the gas. Notice the words has and gas are bold. It also has good oil pressure, and it hasn't been fouling the plugs lately. Thanks for all your input.
Jake

[shelby987]

can't help with the swap information, but from what I have heard the GMC V6's are indestructable.......someone on here told me about how he would use it fathers used motor oil for oil changes in his V6 and still got quite a bit of miles out of it.
If you are concerned, I would just keep your eyes open for a possible replacement motor, just the other day someone was giving theirs away, I would be you could pick one up real cheap, maybe even rebuild it and have it waiting in the wings.

[Bob B.]

The '63-up Chevy straight 6's have the familiar Chevy small and big block bell housing bolt pattern. So yes, a 292 bell housing will bolt to a 350. The GMC V-6 however, had it's own unique bell housing bolt pattern. What was interesting was that when a V-6 was used with an automatic transmission in a pickup, an adapter was used to mate the GMC V-6 bell housing bolt pattern to a Chevrolet transmission. GMC V-6 pickups used both Chevy Powerglides and Turbo-400's.

[89 crew]

Originally Posted by Bob B.

The '63-up Chevy straight 6's have the familiar Chevy small and big block bell housing bolt pattern. So yes, a 292 bell housing will bolt to a 350. The GMC V-6 however, had it's own unique bell housing bolt pattern. What was interesting was that when a V-6 was used with an automatic transmission in a pickup, an adapter was used to mate the GMC V-6 bell housing bolt pattern to a Chevrolet transmission. GMC V-6 pickups used both Chevy Powerglides and Turbo-400's.

Are you sure the pattern is unique? I thought it was a standard medium -heavy duty truck transmission pattern.

[Bob B.]

It's unique! V-6's used in heavy duty trucks had an adapter that mated the V-6 bolt pattern to the standard SAE clutch housing. Pickups, light and medium duty trucks usually had an 'apron' type bell housing that bolted directly to the block.

The picture of the engine on the stand shows what the bolt pattern on the back of the block looks like, along with the light truck and medium truck bell housings:
http://www.6066gmcguy.org/Jolly-401v6.htm

The 478 picture shows the bolt on 'standard' heavy truck bell housing adapter:
http://www.6066gmcguy.org/Jolly-478v6.htm

[GMCMAX]

I did the swap on mine, 305 V6 to 350 sbc, yeah, the bell housing, mounts, and towers are different, along with the clutch plate and flywheel, and exhaust manifolds. Had to make a pivot bracket to line the z bar back up, and had to play mix and match to get radiator hoses that fit. If you ever do the swap, find a complete engine with all the accessories and pullies and such so you don't have to try and round up parts to match each other.
Max

[JAKES 68GMC]

Huh, they are different. I didn't think they were, but I guess they are! Next question, is 292 clutch linkage the same as SBC linkage?

[89 crew]

I haven't tried this but when you have a small block engine in a c50 there is an adapter that goes between the engine and medium duty transmission. they don't bolt directly together. will this adapter fit the 478 and a regular pickup transmission, and solve the problem? If not that was a GREAT idea putting a medium duty allison behind a pair of the v6's!

[1969 GMC]

Originally Posted by 89 crew

I haven't tried this but when you have a small block engine in a c50 there is an adapter that goes between the engine and medium duty transmission. they don't bolt directly together. will this adapter fit the 478 and a regular pickup transmission, and solve the problem? If not that was a GREAT idea putting a medium duty allison behind a pair of the v6's!

the reason there is an adaptor there is that the engine has the regular chevy pattern, but the transmissions have generic SAE sized bellhousings. apparently it was cheaper to make an adapter than a new bellhousing.
the adaptor also helps hold the starter on these trucks, as they don't mount to the engine. i have a New Process 5 speed that was behind a tall deck 427 that has this adapter.

[TP from Cntl PA]

If I wanted a V-8 powered truck I'd sell this one and buy one already equiped. I imagine a few of us V-6 nuts(Yes, I am included in on that) would probably pay a decent amount of change for a unique unmolested ride like that, and you could replace it with one already equiped as you want it. Lots of V-8 trucks floating around, less inline powered, and even less V-6 ones.

It would be alot cheaper in the end and definitely easier..........

[JAKES 68GMC]

TTT, not too old of a thread.

So, would I bolt the SBC engine stands in place of the V6 engine stands? Will the V8 distributor be close to or touch the firewall? Will I have to get the driveshafts lengthened/shortened? Whats different about the clutch linkage? Thanks for the replies.
Jake

[Werewolf]

All the frame mounts were the same, to the best of my knowledge, so if you get the right towers and mounts into the right holes on the frame, you shouldn't have clearance issues. Driveshaft issues depend on what transmission you're using, and what you have to use to get it in there. I'd think on the 4 speed, all you'd have to do is change the bell housing, and you're set.

The 250/292 clutch linkage is not the same as the SBC linkage. Get a linkage off a SBC truck.

[BobS]

Jake,

If you ever decide to swap to a SBC (I hope it never happens), I'll trade you everything you'll need for the swap for everything you'll be taking off with the V-6.

Bob

[JAKES 68GMC]

Originally Posted by BobS

Jake,

I'll trade you everything you'll need for the swap for everything you'll be taking off with the V-6.

Bob

Nope. No deal.

Guys, if I pull the V6 out, that doesn't mean it'll never go back in. I wouldn't sell it to anyone. (a truck this odd, would you?) I just think I'd have more fun with the truck if it had a V8 in it. I'd be able to get out of my own way, and be able to cruise at 55 instead of almost being topped out at 55. And it would be easier than putting in higher rear-end gears. (possibly cheaper, too)

My dad offered to buy the truck off me the other night. He said he didn't want to see me change the motor around. I told him "It's my truck, I'll do what I want with it." I worked hard enough to get it, so I therefore I should be able to do what I want with it. Besides, if he had it, it would just be a gumped-up, sh*tty, rusted-out, broken disaster. I'd take care of it, he wouldn't. Now wouldn't you rather see the truck in good shape with a V8, rather than a beater with a V6?

Just my thoughts..........these are the most lengthy thought-over thoughts I've had in years.
Jake

[TP from Cntl PA]

Originally Posted by JAKES 68GMC

Nope. No deal.

Guys, if I pull the V6 out, that doesn't mean it'll never go back in. I wouldn't sell it to anyone. (a truck this odd, would you?) I just think I'd have more fun with the truck if it had a V8 in it. I'd be able to get out of my own way, and be able to cruise at 55 instead of almost being topped out at 55. And it would be easier than putting in higher rear-end gears. (possibly cheaper, too)

My dad offered to buy the truck off me the other night. He said he didn't want to see me change the motor around. I told him "It's my truck, I'll do what I want with it." I worked hard enough to get it, so I therefore I should be able to do what I want with it. Besides, if he had it, it would just be a gumped-up, sh*tty, rusted-out, broken disaster. I'd take care of it, he wouldn't. Now wouldn't you rather see the truck in good shape with a V8, rather than a beater with a V6?

Just my thoughts..........these are the most lengthy thought-over thoughts I've had in years.
Jake

Once the engine comes out............It will never be 100% like it is now. Sell it to a collector and buy a 1/2 ton 4x2 if you want to make a daily driver out of it, then you'll have useability and a V-8 you really are after

Lets face it, two things a 4x4 of this vintage isn't going to do.......

Go Fast
Cruise along cheaply

They have low ratios+Weight, So if you bump the ratios up, you'll kill the go fast your after. Keep them where they are, and it won't matter what engine is in it, it will be turning some rpms.

Regardless of powerplants, its the wrong thing for what you want to do. If you sold it and bought a 1/2 ton you'd be really happy with it. Nice ride with the coils, V-8 with 700R4 for economy, off the line power, and most important, you won't spend all the time and money to change it and its still not what you want.

I seen this same stuff in the tractor pulling world............Guys took pretty rare tractors and made them into pullers.........Of-course they didn't run like they wanted, nor would they ever even with Bill Gate's spending money, so in the end they had a hacked up tractor worth nothing more than what parts were in it, while the guys who kept them as stock, or nothing that couldn't be put back to stock easily had tractors that are worth some serious money.

[71SWBChevy]

Yeah Jake don't screw up that truck by ditching the engine sell it and buy a V-8 truck if that is what you want, someone will pay good change for a GMC V-6, heck if I was closer to you I would take it off your hands.

[Werewolf]

Back when I was 15, nobody could turn me away from taking the I-6 out of my '68 truck and sticking in the 350 I picked up. I wanted the power, yeah, but more than anything else, I wanted .. get this .. that nice V8 rumble out of a glasspack-mufflered dual exhaust. Now, 15 years later, well, if I had it to do over again I may still put a V8 in it, but I can appreciate the old 6. All in all it ran *far* better and gave me *far* fewer problems than that V8 did, let alone the pain it was to convert it.

My dad had a '71 3/4 ton 4x4 .. that's how I got into these trucks. it was a Chev, with the 255 horse 350 in it, backed by the old four speed plugged into 4.11 gears. That poor 350 was taching pretty high at 55. The truck never, never saw 80. I think the best I ever did with it was 70. The power was all there, but the gears were too tall to let it run fast.

Putting a V8 in the truck isn't going to fix all your woes. Especially if it's a stock 350 built anywhere from '73 on to the early 90's. Mid 70's 350's were mostly making under 200 horsepower; they were designed to keep emissions down and get economy up.

It's not the number of cylinders that's hurting the truck. A 305 V6 may as well be a 305 V8. You're dealing with the same displacement. And a 350 is only 45 more cubes.

If you want more power, look into a cam and fuel induction. Bigger carb, better intake, maybe different heads. But right now, the truck is original, and rare. And the other guys are right. If you switch it now and switch it back later, it's not going to be the same. You may live to regret the decision later.

Of course, you're going to do what you want to regardless, so all of this probably amounts to nothing. Any way you go, good luck.

[JAKES 68GMC]

Looks like this one is back to the top again.

I have a one more questions. If I swap in a 350, will I have to change or move the engine crossmember around?? I have a SBC-equipped K20 parts truck, but it just isn't home yet.

[CPNE]

Got news for ya. I have a 350/4 speed 3/4 ton 4x4 with 4:10 gears. It tops out at 55-60 MPH. I also have an 81 1 ton with same drive train and 4:56 gears. same thing.

It's a 3/4 ton 4x4. Unless you do something about your gears and install an overdrive, you'll accomplish nothing by swapping engines other than devaluing the originality of your truck. I think the only upside is parts availability.

No one is telling you this because your truck is sacred. It's your objectives that are out of whack with your methods. Listen to the voice of experience above.

[Longhorn Man]

if it tops out at 55- 60 MPH, something is wrong, or you have little tires off an Escort, you should be able to hit 100 MPH no problem.

[CPNE]

Originally Posted by Longhorn Man

if it tops out at 55- 60 MPH, something is wrong, or you have little tires off an Escort, you should be able to hit 100 MPH no problem.

Of course it'll go faster but the point being the RPM's are over 35K when pushing it beyond that. I'm used to a 2000 silverado with 3:73 and overdrive which is turning 2k at 70.

[ThatOneBlue67]

As much as I am in LOVE with my 350 small block and how fast it goes, these guys have a point. V6 or V8, that transmission and rear end is still going to slow you down. If you're looking to go fast (like me!) consider a transmission with overdrive and a half ton rear axle.

Honestly, if that truck is original and unmolested, I would buy a SECOND truck. And hotrod the SECOND truck, and restore the GMC to original.


I had this crazy idea to try adn convert my 67 to 4x4. In the end, I just took out a personal loan and bought myself a Marty McFly Toyota 4x4. One to keep clean, and one to keep dirty.