85 K10 305 to 350

[Weaver]

I pulled the original 305 out of my truck and swapped in a 350 from a friend. It's been remanufactured within a couple years and runs strong. Starts on the first crank, idles smooth, no misses, no leaks. I did replace the plugs, wires, distributer cap, belt, exhaust manifold gaskets, and all fluids. New power steering pump and the starter, alternater, and water pump are about as new as the reman engine I believe.

Now I would like to swap out the transmission. I think it's a th350 but I would like something that's a little better to drive around on. It's my dd with a little bit of hauling maybe 500 lbs at the most and I'd like to clean it up so no major off roading for me. I'm welcoming anyones opinion on the best combination of transmission, transfer case and rear end. I've been told a 700r4 will do well but I will need to get a different mounting plate for the transfer case.

Thanks for reading and I'll get some pictures up this weekend.

[donut]

What are your ultimate goals?
IMO, for all I know, what you have now may be what you really want. Just freshen them up and go. (and you know they'll fit without major problems.)
DD doesn't mean you NEED OD, sometimes OD and a high rear gear will hurt mileage.

Do you know your current axle ratios? RPM at cruise? And important, budget.
In my mind, easiest and most cost efficient way to upgrade the driveline is sell the truck and get a 3/4 or 1 ton. Ultimately, your goals and budget dictate.

[nekkidhillbilly]

what transfer case do you have and what rear gears. you will want a 3:42 or 3:73 rear. the 700r bolts to a np208.

[crashz]

First verify that you have a TH350 in your truck. Then verify your gears.

If you have 3.73 or deeper (numerically larger) you would likely benefit from the OD transmission. Taller (numerically lower) will be very driveable with the 3 speed.

The TH350 is a simple easy transmission, that in my opinion, is as strong as a TH400 in a lighter truck without the HP losses. Ironically the TH400 is the only transmission that I've had major issues with.

The 700R4 is not as strong, but has a deeper first gear, and obviously OD. Built appropriately, they last behind big block power and mild off-roading. A later model case and high quality aftermarket parts as well as a good locking torque converter are a must. I had a 4L60E (the electronic 700R4) last for 178K miles and still going strong when I traded my 2004 truck. That truck towed in excess of 7000lbs, plowed snow, hauled more than it should, and had larger tires. The only thing I did was regular maintenance (drain, refill with synthetic fluid and replace filter) every 20K miles and occasionally drained and refilled the pan fluid in between. I also had a transmission temp guage installed and always kept temps below 200 degrees F.

If your TH350 replaced a 700R4, putting one back in its place will be easy. Remove the transfer case (likely a NP208), and remove the adapter and old trans. The 700R4 will bolt right it. The only real trick is to properly adjust the kick-down cable to your carb.

[nekkidhillbilly]

he will need a 4wd 700r4 and the oem spacer that goes between the 208.

[Weaver]

Goals are to dd comfortably and have decent gas mileage but keep the 33x12.5 tire or I may step up to a 35x12.5. I will do a little bit of off roading like on the beach and trails but nothing extreme like rock crawls or deep mud runs.

Buying another truck is out of the question but budget wise I believe I could afford a used trans and a set of new gears.

I do have a th350 and an np208 tc. Gears I'm not sure about. I couldn't see a tag anywhere so I tried the lift and rotate trick. 3 1/2 turns of the driveshaft turns the tire once so I'm assuming 3.42. I don't have a tach at the moment so I don't know what the rpm range is at cruise.

I live 30 minutes from work and it's 55 mph majority of the way there. No other roads around are faster than that and I don't plan on going faster than 55. I do plan on traveling with the truck but that's not for another 1.5 years.

[TwoFiftyShifter]

SM465.

3.42s and big tires is tall gearing

[nekkidhillbilly]

if you have the 208 the 700r4 is a drop in you just need a 4wd one and the spacer thing.

[nekkidhillbilly]

Originally Posted by nekkidhillbilly

if you have the 208 the 700r4 is a drop in you just need a 4wd one and the spacer thing.

both should be in a junk stock truck even up in the 90s.

[Weaver]

I appreciate everyone's opinion so thank you all.

I think I'll look into finding a 700r4 trans. I'm sure it will be a little bit of time before I find one and get it installed so I'll keep doing some research until then.

[donut]

DD, comfortable, and gas mileage. No "wheeling"

My .02, put the money toward a good tune and possible dizzy re-curve. Possibly a "rv" cam, for a little more efficiency. I personally don't remember if an '85 would have a rudimentary computer system or not.??

Comfortable, that's up to you. One persons comfort is another mans life sentence.
Mileage, don't expect much. I really would expect 12-16. Higher great. lower, find out why.
DD and mild off road. I think GM didn't do too bad...

Keep in mind, you are dealing with some parts that are 25+ years old.

Gears, until you know what you have and play with tire size and a gear ratio calculator, change what's broke... Used trans is like Russian roulette. Used may work great, or need rebuilding asap.

This post is IMO, your goals don't seem unrealistic for what you already have. I suggested different truck if your goals were more into the heavy wheeling. I'm also not into spending your money for you.
Rebuilding brakes, driveshafts, changing seals etc will eat up a budget quicker than you think.

[TwoFiftyShifter]

You will be turning 1500-1600 rpms at 65mph in OD with those big rubbers. That is lugging and won't help MPG.

[Weaver]

So either a taller gear or smaller tires.

Another question about the 700r4. Is the only difference between the 4wd and 2wd the output shaft?

[nekkidhillbilly]

Originally Posted by TwoFiftyShifter

You will be turning 1500-1600 rpms at 65mph in OD with those big rubbers. That is lugging and won't help MPG.

1600 isnt lugging to bad actually. newer chevies run only 1800-1900 at 70 mph.

[nekkidhillbilly]

yes but you cant take the tail shaft off and change it without rebuilding the trans.

[Weaver]

Originally Posted by nekkidhillbilly

yes but you cant take the tail shaft off and change it without rebuilding the trans.

But if I'm buying a used trans I might as well get it looked at and rebuilt right? Making sure I get a 4wd one makes sense but you can never trust buying used parts from people.