g741.org

Ahhh, more bits to fix!
I've just pulled the shift tower of our early transmission apart. It needs a new pin (7351048) and plate (7351049) and I'm looking for a source. If necessary I can make the pin, but there was not much left of the plate that goes between the shifter "ball" and the spring that retains it. On the bright side, the slot in the shifter "ball" is in good shape as is the hole in the tower that the pin pivots in. I do realize there are NOS towers available, but it would be nice to fix the one I have.

Andy

Andy, I'd start with John Bizal at Midwest Military.

How are the rest of the parts in the top cover, such as the shift rails? For a truck that has been sitting for a long time, where the internal transmission parts have not been spinning in and throwing the lube around, the exposed internal trans parts such as the shift rails and lugs, upper portions of the gears, etc. will eventually start rusting and pitting. Then, when the truck is driven, the rusted shift rails are not kind to the shift rail bores in the top cover.

Sometimes it's better just to replace the entire top cover assembly. I had to do this on the NP420 I'm rebuilding now.

I've spoken w/ John and he does not stock any parts for the tower, just whole assemblies. Fortunately the trans internals are clean and rust free, unlike the differentials proved to be! Considering the condition of the pin that was indexing the shifter, the transmission shifted well, the shifter was just very sloppy.

Well, fixed the problem . Machined a new pin and spring plate and the shifter works well now. I will say installing the spring that supports the shifter was a massive pain in the rump. I think it took me nearly as long to do that as it did to make the parts... I suspect there's a trick!

ashyers wrote:Well, fixed the problem . Machined a new pin and spring plate and the shifter works well now. I will say installing the spring that supports the shifter was a massive pain in the rump. I think it took me nearly as long to do that as it did to make the parts... I suspect there's a trick!

If you made and installed the new pin like the old, that was your first major mistake. We have fixed many of these; learned long ago the original configuration is worthless if you want your fix to last. The trick is to machine the hole in the tower to get it back perfectly round. Machine the new pin to the diameter that fits a new hole snuggly. Make it longer than the original; cutting enough threads on the end so a lock nut can be installed to keep the pin pulled tight in the bore; otherwise it will soon start flopping again causing the bore in the cover to wear quickly, starting the whole sloppy shifting scenario all over again, and making it much harder to repair next time. The replacement pin needs to be made from a good material such as 4140 alloy; that can be heat treated to prevent excessive wear from happening quickly. Using parts like original to do this repair only assures a return trip to the area sooner rather than later if you keep a nicely functioning shifter.

Charles,
Thanks for you advice. I think the pin fix I did will work fine. I was fortunate that the hole in the tower was still round and the slot of the shift ball was in good shape. The wear was focused on the old pin. Since the old pin was junk I was unable to use it as a template, but with a bit of trial and error was able to make a pin out of piece of NAS hardware that is now comfortably press fit in the orig. hole with a bit of retaining compound for good measure. If this goes south I'm going to tap the hole in the tower and fashion a replaceable pin that can be R&R without pulling the tower off the gearbox. I'm looking forward to getting the truck on the road in the next month or so I can start durability testing ! Of course there's the 1001 other bits we are dealing with...

Andy

I am quickly learning my truck is comprised of over 2000 $20 parts.
I too need to do the pin/ball repair to my shifter. Having done it in the past on tractors makes my solution easier for me. I drill the old one out and tap the hole for whatever thread is reasonable for the project. Make a bolt with a machined end to act as the pin. It usually ends up oversized from the original ball slot as that slot is buggered up. I then mill the slot until it is clean and parallel, make the bolt to fit. Heat treat the bolt and locktite it in.
Has worked for me on Jeeps, Gibson tractors and forklifts.
Just one of those projects on the whenever list.

The design of the early NP trans makes the pin especially vulnerable to excessive wear happening quickly. The reverse throw is the killer, because of the reverse shift fork not being mounted on the rail. On the later 420 trans that has the fork on the rail, the stress level to operate it is considerably less severe and resultant wear in the pin area is way less. I've tried a number of different fixes for the problem over time, and have seen a multitude of different fixes tried for the issue that were done by others to include the installation of a bolt mentioned above. None of these fixes has held up well, and in general have been failed attempts rather quickly. While a lesser fix may work longer in some applications that are not stressed as the early M37 units are; it simply doesn't work well for long in this transmission. If the truck is being driven regularly, in 2-3 months you will be noticing slop returning in the shifter again, especially when shifting into reverse. If you drive only a few miles a couple of weekends a month, maybe yours will appear to be lasting longer. Cutting threads in the cast tower has actually failed faster than most other methods of repair I've seen attempted. Threads in cast begin to wear and fail quickly when stressed hard; with the minimal thickness of the casting in the tower area; there is simply not enough meat for threads that will hold up. I've seen this attempt end up causing cracked towers and a number of other issues in this particular transmission.

Well I'm just sharing past experiences; we've seen various issues that are prevalent in the early trans. They can actually be turned into pretty good transmissions with the right modifications applied; however they are generally not very reliable otherwise.

Thanks for your thoughts and experiences Charles.

What might some of those modifications be that would improve the transmission?

F18hornetm wrote:Thanks for your thoughts and experiences Charles.

What might some of those modifications be that would improve the transmission?

Numerous little things can be done to enhance the reliability and longevity of the unit; however it isn't and never will be a 420. I can discuss some things further when time permits if you want to send me an email.

Reviving the thread since I recalled it and started paying attention to my shift pin (since I have yet to mount the shifter boots). Sure enough, the pin has started floating toward the passenger side when I'm running errands-loose enough that I can reach down and push it back into place by hand.

I'll tackle this fix once I'm back from Topeka.
Gary

I'm interested in any upgrades as well. I still have the transmission I took out and it needs work. I have a good one in it now from Midwest, and wonder how long that pin will last. Although its not driven rough, still crappy design.

Charles, I know your a busy man, but any ideas would be appreciated. You don't have to take the time to go into in-depth details, just some ideas. Give us something to think about....Thanks!!!

F18hornetm wrote:I'm interested in any upgrades as well. I still have the transmission I took out and it needs work. I have a good one in it now from Midwest, and wonder how long that pin will last. Although its not driven rough, still crappy design.

Charles, I know your a busy man, but any ideas would be appreciated. You don't have to take the time to go into in-depth details, just some ideas. Give us something to think about....Thanks!!!

About all I can tell you is that we have designed a fix that has worked well for us. It isn't something that can be done without precision machining capability, and literally every one requires a different technique, depending on how much wear is present and the specific way it may be worn. So far we have not found one that was not repairable; although I'm sure there is some out there that are past the point of no return. If you would like to send your top cover assembly to us, we are happy to check it out and repair it or let you know why it is not repairable if that were to be the case. I will tell you this; in order to repair the pin issue, everything in the top assembly has to be completely disassembled. Most often we see other issues like worn shafts, shift forks, etc., that also need attention. All these multiple issues play a part in loose motion and shifting action. Honesty, if you don't fix it all, you will likely not be happy with the finished product. The ones we have repaired have been better than new and we have had some out there long enough now to show a really good track record. Just let us know if we can help you.

Thanks ..

Not something high on my list to get done as its spare trans was just curious what you found. When I get time Ill take the old one apart and see what I can make of it. I do have machining and welding equipment, so Ill maybe experiment some. If I don't get time in near future, I'll send it your way.

Was fiddling around in the truck today and noticed this... The truck shifts fine but I assume this bit of slop is only going to get worse if I don't take care of it pretty soon.

Charles, any idea on a ballpark cost to repair a top cover assembly?

sloppy shifter pin