TH350 to 4speed STD....advice

[Petie724x4]

Went to start my truck (72 K10 LWB) the other morning.......not good. Front seal (I'm guessing) puked out on the trans, all tranny fluid out onto the laneway. Now halfway through pulling it out to fix (laying on my back in said trans fluid...) and started thinking.
I've always wanted my 4x4 to be a 4spd....just always thought they were cooler and tougher. I have access to a stock trans (71 4x4 Blazer), I have bell housing/pedals/linkage/floor hump in other parts truck (69 1ton). I'm not looking for a long term laneway project....already got the first snow....the warden doesn't want me stealing her car for much longer.....need my truck.

Send the auto out for a rebuild, or do the swap now? Anyone been here?

[Maxxd07]

You might be able to find a tranny shop that would swap yours out with a rebuilt from their shelf. I am assuming its a th350 which should be pretty cheap. The 4sp swap will take a bit longer than one would think, they always do (any major swap or project never seems to go as planned). That way you could take your time and do the 4sp project once it warms up and you get a chance to gather your parts and wont be in such a rush.

[Ironhorse]

I have a '71 K10 4x4 with 350 and granny 4 spd. Original to the truck, I was specifically looking for a 4 spd in the 67-72 body style. Have had it almost 20 plus years. Have a good heavy duty (aka stiff) clutch in it. Works well in normal driving but in HEAVY traffic as in city driving I'm sure the left foot would get tired really quick. Dont' plan on making any quick street racing style shifts with it, this creature is not made for that. I live in a rather rural area in the flatlands of Kansas so not much problem but also might be an issue in hilly country starts. Takes a bit more co-ordination if not using the park brake when starting on a steep hill. Sometimes when I get really tired or other looser moments I long for an auto box but then that moment passes.

[mrein3]

I have been there only I went the other way. I had a 3-speed manual and installed a th350 out of a Blazer.
Since you have a parts truck sitting there you are in luck. Depending on how you look at it.
Other than being VERY heavy and VERY awkward (the unlucky part) all the parts you need are there.
The driveshafts on your truck are the same length no matter what stock GM configuration you go with. GM made all the transmission length differences up in the transfercase adapter. The longer th350 has a shorter adapter. In other words the transfercase bolts to the same location regardless of what transmission you run.
Now the bad news. The NP205 behind your manual transmission Blazer has a different input shaft than the one behind your th350. You need to change out that heavy, awkward beast when you change out the transmission.
The crossmember is the same and bolts to the same location.
I would use all the Blazer parts you can. The ball studs for the clutch pedal might be unique to the 4x4. Since I was going the other way I didn't have to deal with that stuff, I was just removing it as I went.

I didn't look at the adapter that I took off the manual transmission but if it is like the th350 adapter, the seal to keep the lube in the transmission is in the adapter. It is an oil seal that rides around the drive sleeve. The drive sleeve is what couples the transmission to the t-case. I would seriously consider replacing the seals and possibly the drive sleeve if it has a groove cut it in. Think of it as the front oil seal on your engine. After a while it cuts a groove in the harmonic balancer and allows oil to leak past.

You do not want to do this job twice.

[dan42]

Actually the auto is gonna be better in the snow and mud, in my opinion, unless you are rock crawling - and the four speed is gonna be one suprise after another - I would rebuild your automatic and put it in, and if you really want to change it down the road, then give it some thought and plan it out - remember, steering column is different, pedals have to be changed out, etc.