351 conversion

[pigslayer]

lookin on craigslist the other day and found what I was told was a 69 model 351e that was bolted to a three speed tranny. so I suspect it was from a half ton or 3/4 ton. I went and looked at the engine and it looked pretty good. fairly clean. removed the oil fill plug and it looked good on the inside. the clutch didn't look to have taken much abuse. at least the current flywheel hasn't. so I bought it reasonable. got it home. looked it over. got it pretty much ready to run. haven't tried yet. but I guess I put the cart ahead of the horse. I'm not sure if this thing will even work in my truck. what I want to put it in is a 48 chevy 3800. it has the 216 in it now and I wanted an upgrade. I know i'll need to fab some motor mounts. but will this work with my transmission or am I now in need of that too. it has a 4 speed with the reverse down and left. I'm assuming it's a 420. so is this possible or do I need to let it go. thanks in advance.

[bigblockv6]

The SM-420 was used from 1947-67, sounds like your 48 has this trans. Don't think your conversion is far fetched, GMC used to promote repowering older trucks and other brands with it's V6.

[Culver Adams]

I don't know much beyond my own experience.

I do know a 216 I6, even if kept full of oil and coolant and not over revved, will show wear. (But how many other engines are friendly enough to let average, motivated mechanics rebuild bearings on a cold night by flashlight?)

My 478 V6, on the other hand, just laughs when I try to find where it is wearing itself out. But it's a brute and your engine is brute's smaller brother. You may want to check the weight of 351 versus 216 and then find what parts it wants to eat for lunch. (Should I mention that today I am welding my 1961 GMC K1500's front upper shock absorber mounting bracket--now in two pieces, thanks to 478 engine weight on top and the winter of winters pot holes on the bottom?) (Photo attached.)

My opinion: If you want the most durable low RPM high torque engine, you can't do much better than what you just bought and SM420, assuming you can use it's low gear and don't need overdrive, is it's perfect mate. They could be going to the prom together.

Regards and Please don't forget how much this group likes photos ;-)

Culver Adams
Minneapolis, MN (Come for the lunker Walleyes: stay for the shock absorber mounting bracket repairs.)
1961 GMC K1500 with (laughing) 478 V6 and (learning how to dance for the prom) SM420

[pigslayer]

well those were my thoughts. the chevy body style I have was from 1947-1953. and that was the start date of the 420 tranny. and the fact that the reverse is located where it is makes me believe it should work. also I have some friends who say that a sbc would bolt up to this transmission so I feel pretty sure I can make it work. I can make a few modifications if need be. I'm a machinist with a shop in my back yard so things within reason can be changed. turned my flywheel last night just waitin for good weather to come so I can bolt her together and give it a crank. this engine is a bit heavier but only around 250 pounds the best I can tell. for a 1 ton I'm sure this wouldn't be noticed.

[pigslayer]

another little question I'm sure someone here could help me with. I have found a gmc transmission from what I am being told came out of a larger truck. dump truck or whatever. if I did decide to use this transmission would the bell housing from the donor 478 mount up to the 351? and flywheel? would I need that as well? what transmission is this? how many speeds? gearing? below is a picture of the transmission in question and the donor engine. any help would be appreciated.

1366

1367

[bigblockv6]

The 478 bellhousing will bolt up to the 351 but the 478 flywheel won't. The 351 has a standard GM 6 bolt pattern while the 478 has an 8 bolt pattern to the crank and also has external balancing. The only way to make this work would be to find a 401 flywheel from a similar application with the 6 bolt pattern.

[pigslayer]

well tried to fire the ol girl up today. I believe she's a runner. didn't hear it run long though. appears the needle and seat are missing out of the carb. so it blows gas back towards the bell housing when it fires. but seemed solid. no smoke. no noises. smooth sound when cranked. hopefully this is an easy fix. found a kit at napa. just wish I could just buy the seat and needle. oh well.

[Culver Adams]

(....tried to fire the ol girl up....believe she's a runner....seemed solid....no smoke....no noises....smooth sound....)

Hi, Pigslayer,

That sweetheart wants to run.

Somewhere I heard, in the days before my time, when a truck was at the stage your's is at, a guy would pull the carb, get one of the neighborhood kids to sit aside the engine to pour gasoline from a tin can into the intake manifold, and the guy would drive the truck around just to get it driving. That was before OSHA and a bunch of other reasons to not do this now. But I sense you are on the edge of discovery and celebration.

Way to go.

Regards,

Culver

[pigslayer]

thanks culver. yeah I think she wants to go. it's hangin from a chain right now so it wouldn't be so hard to pour gas in it. but I had an uncle who was tryin this trick once and the engine backfired through the intake and caught his gas can on fire. not willing to take that risk myself. I'm not very pretty but I doubt this would improve anything.

[Culver Adams]

(....not willing to take that risk....)

Hi Pigslayer,

Very wise.

Regards,

Culver

[BarryGMC]

I think this conversion would be cool. I have thought about putting a v6 in an ad truck for a while. Here are my observations and thoughts. You need a 66 or earlier thermostat housing. The later light truck style angles off to the side whereas the earlier goes up. The 69 bell housing won't really work with the 420 trans. You could use a 465 out of a later truck or find an earlier bell housing. I would stay away from the big truck SAE stuff because the frame is narrow and the cab and floor are really tight with the stock bell and 420 trans. If you really want something cool get a early nv4500 and use it or a later nv4500 and use an auto trans adapter for the bell housing. This would fit OK but you would still need some heavy floor mods. Finally here is what I have collected. 401 v6 with a 420 trans and a watson 3 speed brownie and a ho72 rear with 4.10 gears and a detroit no spin. Have fun.

[pigslayer]

Got her going today. Sounds pretty good. At least with no exhaust. 40 psi idling. Assuming that's good. Doesn't want to idle so well. I'd imagine rebuilding the carb would solve that.