NP 435 replace or rebuild?

[Bsauls]

Hi guys,

I have a 1970 F100 4x4 with Dana 21 transfer case and NP435 trans. 3rd gear has always been hard to downshift to even double clutching. 4th gear is a bit rough as well. So in your experience is it better to have the trans rebuilt or go with a rebuilt unit from a reputable source? IF so what source do you recommend?

[sparky72]

What’s your level of mechanical competence? These old four speeds aren’t very tough to rebuild. I did one myself recently without having rebuilt a transmission of any sort before, and it was pretty easy, honestly. Of course, I had the luxury of time, too.

[Bsauls]

I was a tech for 15years, mostly rebuilt automatics and no 4wd. I just don't have any space to work on a project of this size. No garage and I live in the sticks gravel,dirt and tons of pine needles.

[cep62]

If I'm correct the F100 4X4 used a 2wd trans with an adapter.maybe you could find a good used one.

Also are sure the clutch is completely disengaging?
sagging cab mounts can cause clutch issues.
these old trans don't shift like a new one , especially down shifting.

[JoshT]

F-100s with the NP435 and Dana 21 (I think all F-100s used that config stock) had a "2wd" transmission instead of a "4wd" transmission. Only transfer case they can use is the Dana 21 without possibly sourcing another adapter or going to divorced transfer case. Reverse is same "2wd" NP435 is only trans you can use if keeping the Dana 21, not that there aren't plenty of them out there.

With that in mind this is what I'd do and hope to do if I can find the parts. Find a NP435 "4wd" transmission from a F-series. Find a 2 speed transfer case, preferably a NP205 from same vehicle. Rebuild those, then install. I plan to rebuild and install myself, but even if you can't it might be a worthwhile consideration. Keep the truck running while transmission is being rebuilt, as well as end up with a better transfer case. The 4wd NP435 would also open up a lot of other transfer case possibilities in the future if that might be of interest to you.

Having said all of that... If I was happy with the single speed transfer case and just wanted to get it fixed as quick and easy as possible, and wanted to stay as original as possible, I'd probably drop it off and have the existing transmission rebuilt.

[Bsauls]

Great idea and worth way more than 2 cents LOL. I am new to 4wd and old trucks so even with mechanical experience I feel like I don't know a thing. I think I may look for the 4wd version and transfer case and keep the truck going until I have them ready. I will still probably have to pay someone to install, I"m too old to work on my back in the gravel under the pine trees and my wife wont let me kick the critters out of the barn to work in there

[Bsauls]

the clutch is newish and seems to work fine. 3rd gear is a grinder on upshifts and forget a downshift until you are crawling. I was ok with this for a while but 4th is starting to grind on upshifts now. Not much metal in the fluid but brass is there. I have the original owners manual and it did require double clutch for downshifting, I think the PO didn't do this thus the grind.

[cstoyer]

What type of oil do you use? If you just changed it out did you change it to something that is maybe too slippery for the synchro to work properly? Brass? The synchro is aluminum and the only brass is the washer on the front of the lower shaft next to the needle bearing. A rebuild kit is easy to install with a few tools.

CStoyer

[cep62]

I don't know what your plan for the project is ,
but if you're thinking about an upgraded T case, you might consider
looking for a '76-'79 donor truck , then you could upgrade to power steering and disc brakes too .

(just a thought )

[Bsauls]

I have had this issue since purchase but it's gotten worse over time. Sounds like the bearing was wearing and not the synchro. As much as I would like to upgrade I think the stock parts that have lasted 50 years is the way to go LOL. I went with a front disc brake conversion and love the stopping power now. Thank you everyone for your input, it's great to hear from others that know a lot more than I do!!