1970 C2500 NP 435 transmission problem

[nrapozal79]

Hey everyone and thanks for looking I got this 1970 GMC c2500 4 speed. I believe it's has the np435, I bought the truck and it needed a motor I put a new rebuilt 350 in it and got it running great. But my problem is when the transmission gets hot it doesn't want to shift, I know first and reverse are not synced but to get it into reverse or first it requires a lot of force and makes a horrible loud grinding noise any ideas as to why this may be happening? it has fluid in the transmission, when it's cold the transmission goes into first and reverse smooth very easy and no noise only hot it shows that problem any ideas what the problem may be? thank you for looking

[HO455]

Welcome aboard!
From your post I am assuming you didn't drive the truck before the new engine was installed? It's possible that the transmission was the reason the truck was parked and its engine was harvested for another vehicle.

Have you verified the clutch is completely releasing? Confirm this first. Try starting the engine, on flat ground, brakes off, with the transmission in 1st and the clutch depressed. Does the truck try to move? If so the clutch isn't releasing completely. If this is the case it's time to do a careful inspection of the clutch and the linkages. It is possible to install the clutch backwards.

That being said did you check the alignment of the bellhousing to the crankshaft? If not that would be a good place to start. If you're not familiar with the procedure there are numerous videos on the interweb.

It may be a bad input shaft bearing (or the shaft may be bent) and or the input shaft pilot roller bearings.

Worst case scenario the transmission would need a rebuild. Which isn't a huge deal. Most of the part are readily available and they are pretty simple transmissions to rebuild. Link below to Novak. Great vendor willing to help with technical support.

Good luck and keep us posted.

https://www.novak-adapt.com/catalog/...s/np435-parts/

['68OrangeSunshine]

Something to look for is the condition of the Z-Bar.
[Slang for the shift linkage bellcrank].
After only about 48 years of daily driving in city conditions, the upper arm of the Z-Bar started separating, creating a shorter and shorter throw, and making the gears much harder to shift. Equipment was an SM 465 and 292 L6 on a '68 C/10, but the principles are the same.
Eventually the arm broke clean off, stranding me in rush hour traffic on the way to work.
After welding the Z-Bar back together, the truck shifted fine.

[Accelo]

If it turns out to be the transmission see below;
The NP 435 transmissions are as tough as nails. When the truck is junk not even Saveable the transmission still works. I wouldn't even consider rebuilding one as it would be around $1K. Just pick up a used one can call it good. A recent swap meet I went to must have had 30 of them there. All the way from $25 to $400. Most went home with the sellers. Just one of the most amazing transmissions out there. I would trade the granny gear for an overdrive!
Cheers.

If purchasing used be sure it fits the GM products. They were manufactured for many companies:
he NP435 was made by New Process Gear from 1962 thru 1997. They are found primarily in Ford trucks from 1966 to 1992 and Dodge trucks from 1962 to 1993. They also had limited distribution in GM & Chevrolet trucks from 1968-1972. They are also found in some International Harvester and industrial applications.

[RustyPile]

A good way to tell if the clutch is completely releasing is to check the distance between the pedal and floor when the clutch starts to engage.. The pedal should be 2 - 3 inches from the floor when the clutch just starts to engage. As the clutch disk wears, this distance will close up and periodic adjustment is required.