4.3 to 305

[barebones 1500]

alright, the other day i checked my oil and noticed alot of coolant in the oil. i figure that it is the cylinder head gasket because it still runs great and i think a cracked block or something would make it spit and sputter. i am worried that this might be something i am going to fight along with the oil leak that i have been fighting since i got it, so i think i am going to swap in a 305 v8. but i have some questions.

my truck: 1990 c1500 SCLB 2wd w/ 5 spd tanny and stock 4.3

1. will my stock tranny bolt up to a 305 along with the flywheel?
2. can i reuse the stock harness?
3. will the stock ECM give me problems? i don't plan on building the motor as of yet so it will be stock.
4. i know i will have to change the the motor mounts and at least modify the fan shroud, but will there be anything else i will have to change to get the motor in? also i plan on running new fuel lines when i swap in the motor so i can run those where ever needed.
5. what year motor should i look for and how much do they normally go for?
6. and last, is there anything i am leaving out?

sorry for the long post and all the questions but this is my DD so i will have to do this swap in a weekend and i want to make sure i have all my ducks in a row. TIA for all the help, i think it should be a straight forward swap but i haven't done one like this so it is kinda new to me.

 

[jcro61]

1. I beleive so
2. maybe im not sure
3. yes it will. you need the ecm for a 305.

 

[Swims350]

I would say yes the motor will bolt to your trans. but not sure on flywheel. maybe find a 305 that's in front of a 5 speed and get the flywheel if not just try yours or buy one northern has them fairly cheap. As for wiring harness probably might have to change the plugins on it though maybe this or that? Maybe find one that has wiring harness? I'd say 88 and up 305 since it's be tbi like yours. 87 would be too but some things are different. Any would work but if you're gonna use tbi you need a tbi v8 intake. I think you need the tbi for a v8 and injectors for it as well.

If I was gonna do it I'd try to get as complete of motor as possible maybe even down to wiring harness and sensors. Then for fan shroud and such just try to find one in a junkyard, but why not go 350? aint much difference maybe more power or torque? maybe a tad bit difference in gas mileage not enough to notice really.

 

[barebones 1500]

i have thought about going to a 350 but i am not sure if i can afford a v8 in general so i figured i would start small, i will have to look up some specs on the two to see what the ratings are and weigh the pros and cons. i definitely am going to get a complete motor from tbi down to headers so getting parts together won't be a problem. getting the motor harness won't be a problem either but getting one that covers the motor and tranny will, i talked to my mechanic and he told me that 5% of these trucks came with any kind of manual tranny. so finding one with the motor i want and the same tranny and have it be close enough to get it will be next to impossible. which brings me to another question, now since my ECM won't work, will the new one work fine with the 5 spd even if it came out of a truck with an auto? i know that all the old trannys were mostly mechanical so i don't there will be a problem other than the fact that my vss is not working, as far as the **** light in the dash goes i don't need it cause i hooked up a 8 inch sunpro tach with shift light and i go of that. as far as the fan shroud goes i think i will do an e-fan swap while i am in there just to minimize the search for these trucks, not that common in my local yards. once again, sorry for the long posts. i just want to be thorough. and thanks for all the info.

P.S. if anyone knows a good place to look up the specs on the tbi 305 and 350 please let me know, i hate crawling through the underbelly of the internet.

 

[Swims350]

I don't see much difference in them, they are the same outside and alot of the times same top end, maybe computers injectors and bore size being the difference. Alot of times ppl say it's a 350 when its actually a 305.

I don't see why your trans is a problem I mean what all does the computer even work on a stick shift? I know it can't or shouldn't control it, I would guess on only speedo and shift light for it the safety switch is on the clutch pedal I think. The shift light is for engine rpms so it might not even be part of it either, as for the speedo not sure, my 88 auto is in the t-case as for 2wd not sure. I would think a 90 model isn't tons and tons of computer control crap, like say a 2k or 98 might be. I know my 88 has a bit, but not a ton of it. Plus you aint changing the trans so yours would work with it, maybe you can swap chips in your computer? if not I would think a 5 speed truck with v8 should work as long as it's 2wd and 5 speed? I wished I was there man to tear into it and check things out. I'd find out exactly what trans it is, nv3500 4500 t5 whatever. That would help a bit I bet for the net search on swapping it out, or maybe check out 67-72chevytrucks.com they have a 88-98 section and chevytech knows alot on almost anything. There's some gm guys who work for or used to work for gm and know alot of the factory crap of what ones had and year ranges.

 

[jcro61]

Listen, when you buy a motor, buy one with a wiring harness and ecu. Get one that is pre-93 as even if it is automatic it will be a non electric trans. So the only plug will go to the converter lock up. You can just cut that off. It will make no difference to how the engine runs, wont throw codes or anything like that. This is honestly not a hard swap. Other than that you just need random bits. Motor mounts, maybe fuel lines and fan shroud, maybe not. Probably new y pipe. New radiator hoses.

You will need a new radiator too.

IMO if you want a v8, just get a v8 donor truck to swap everything over.


but if you have a basic socket set and a torque wrench you could change your head gaskets yourself and see how that goes. its not too hard to do if you have a manual to tell you the tightening sequence and torque specs on the heads/intake

 

[Swims350]

good point on a radiator can't hurt. Hoses you could probably trim to fit or change, aint too expensive, but either way it's gonna be more then head gaskets on the v6. those are cheap and easy. if you can do a motor swap you can go head gaskets no problem. You could also go carb'd lol.

Manifolds is also a good point not sure if they'd be different or not.

How does the speedo work on yours cable or what?

 

[jcro61]

the radiator for the 4.3 is different than a v8. it will probably overheat if you dont change it. the y pipe will be too small diameter for the v8 and might also not line up. not sure on the lining up but will def be too small. I would definitely change the radiator hoses if I did this swap. Unless they are fairly new they probably need to be replaced anyways.

 

[FastOrange]

1. will my stock tranny bolt up to a 305 along with the flywheel?
2. can i reuse the stock harness?
3. will the stock ECM give me problems? i don't plan on building the motor as of yet so it will be stock.
4. i know i will have to change the the motor mounts and at least modify the fan shroud, but will there be anything else i will have to change to get the motor in? also i plan on running new fuel lines when i swap in the motor so i can run those where ever needed.
5. what year motor should i look for and how much do they normally go for?
6. and last, is there anything i am leaving out?

1. Yes, they are identical
2. Yes, but you might have to extend some wires
3. You MIGHT get away with a 4.3L computer, but i would try to find a 305 one
4.Motor Mounts on the frame and engine have to be V8 Ones, Fan shroud has to be cut, Need new rad hoses, and new rad, 4.3L is tiny
5.any 305 from 1987-1995 Will work
6. dont think so.

 

[barebones 1500]

i know that head gaskets are not hard to do, i could crank that out in a couple of hours. but like i said before it could be one of those things i will fight till it dies like the oil leak, i go through about 3 quarts a month and my pressure if off so i think the pump might be on its way out. i didn't think that the computer controled the tranny because the only plugs i have on mine are the vss and reverse light switch and my speedo is electric. i did think of radiator hoses but not the radiator or exhaust but those won't be too hard to track down.

my basic plan is to either find a complete motor out a truck and then go to one of the local yards for the odds and ends or find a donor truck that was rear ended or has a blown tranny or something. i hope i get lucky and find a truck with a good front clip so i can replace all my bad pieces as well. i don't plan on doing this swap till next spring so i have time to track down parts and finance the job, also i can wait for one of those weekend holidays because one yard does a half off weekend so i can score lots of goodies for cheap. lots of good info coming in and please keep it coming, every little bit helps. i will start doing some research and price checking to see what it will cost me.