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The 23 spline NV4500 is the light duty version that only holds 4 quarts of oil and has a tiny mainshaft. It's just like the cheezy chevy NV4500 only with synchro reverse.

A Ford ZF S5-42 mated to an NP205 will run a guy $500 and will hold up to anything the 4BT can dish out. They're both 100% off the shelf and common parts available everywhere. No funky adapters. Most guys would jump at the opportunity to toss a chain drive aluminum hunk of junk 231 in the dumpster and run a 205.
 

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I checked the numbers and I think I am off on 5 quarts, but the dodge HD version does hold more than the other ones. The rear extension housing is longer for more oil. I do run them a half quart over full or install a PTO filter which adds some extra capacity.
 

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Chrysler Corp. put out Technical Service Bulletin 21-10-98 to cover warranty repair of any NV4500 due to 5th gear nut. This recall includes dealership instruction on how to fix. Most Dodge NV4500 trannies should already be fixed because of this TSB. If one was done by a dealership it was to be stamped 'New Nut' on top of transmission case, but not all were stamped.

One transmission company decided to capitalize on this problem and offered their revolutionary, none other like it in the world, finest, coolest, bestest repair kit ever invented. That was supposed to justify their ridiculous price I guess? Anyway, for some reason their extra-superfine repair kit soon became unavailable with little explanation, but lots of BS. Prior to that this tranny shop posted all sorts of extremely negative comments about all the other repair kits, explaining how they were complete junk, and how none were any good, except for theirs of course. They've now backed off some on all the bum-kicking and negative hype, but the damage was done, and not by the 5th gear nut. Far as I know even several years later now, their wonder kit is still unavailable.
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Where do you get this crap from? Go open up any cummins nv4500. Just pull the extension housing off and wiggle 5th gear and it's nut. If you have an NV4500 and change the fluid in it yourself, ever notice the reddish brown crud inside the tranny? That's not from the synchro or bearing wear buddy, that's evidence of fretting. Fretting is 5th gear, 5th nut, rear mainshaft bearing inner race and reverse thrust loose on the mainshaft and grinding into it. Given time, mainshaft 5th gear will spin on the mainshaft, countershaft 5th gear will wear the countershaft unevenly from being sideloaded by a wobbling 5th mainshaft gear and the rear mainshaft bearing can spin in the transmission case. After this you end up with a boat anchor.

You obviously don't grasp the concept of what the NV4500 5th gear problem is or what it takes to fix it. Fixing it takes a thicker, full spline mainshaft machined from high grade raw material. There is only one company out there offering a full spline NV4500 mainshaft and 5th gear that holds up and it isn't even remotely cheap for the parts or to have it installed. The fact is though Dodge's fixes couldn't even be considered bandaids for the problem. They do nothing to solve the problem. 5th nut is not the issue. The issue is the mainshaft itself.

BTW, that "one company" offering the good full spline mainshafts is called quad4x4. I buy tons of parts from Dan and have never had a single issue with any of his stuff. His torque king mainshafts do hold up where the OE NVG stuff and all other aftermarket parts fail. Ask the hotshotters that make a living running the heck out of thier 5.9 trucks and have gone through half a dozen NV4500's in 300K miles then finally spring for the torque king mainshaft and it never breaks again.
 

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BTW, better than 1/2 the failed NV4500's I repair are equipped with dodges "super nut". Thier fix is a joke. It doesn't take a genius to realize while tearing down your first NV4500 that the nut is still in the same place it was when it was originally installed. The nut didn't back off. The interferrence fit that's supposed to be there between the mainshaft and 5th gear is no longer there, you can wiggle 5th gear all over the place if your lucky, if you're not so lucky you can spin 5th gear freely on the shaft.
 

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Maybe I have come off a little high and mighty on the transmission opinions, for that I apologize. I did not mean to offend anyone or come off as an asshole. I'm not an idiot hiding behind a keyboard making false claims about NV4500's to build up my ego.

I work on these things all the time. Yeah, being in my position I do see the worst of the worst and my opinion could be biased from that. In the NV4500's defense, I do know of one and just one behind a cummins that has made it to almost 400K miles untouched and still going. The tranny is rough, but it's still going. On the flip side, They can fail at 50K miles under heavy use. The average life for one before the driver begins to realize there's a problem is the 150K mile area for one in a daily driven dodge, not some hotshot tow rig, just a driver.

The ZF S5-42 seams to have about the same lifespan behind PSD's (150K miles). Under real hard use, the ZF's lifespan goes down considerably from case stretch to around 80K-100K. The big difference is the ZF just needs bearings and synchros, many times they don't even need synchros. They don't need a new mainshaft and 5th gear like the NV4500.

To me, the ZF is a pretty good option when you look at price and availability. If a guy actually took the time to reshim a ZF every 50K miles to compensate for case stretch the tranny might last indefinitely. The bearings go out when the clearances get sloppy from case stretch. I know that's not practical, but it's an idea anyway.
 

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A girdle for the ZF probably isn't worth the effort. The amount of actual stretch in the case isn't much. .002"-.003" over 30" or so of aluminum. ZF 5 speed guts are impressive. There's a bit more beef there than the NV4500, again, the lack of an iron case is probably the low torque rating reason.
 

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The M5R2 is a great tranny. I build several of them weekly and while that doesn't seam to say much for thier durability, every single M5R2 I've fixed has failed from horrible shifting practice, low ATF or running the wrong fluid, like gear oil. The early 89-91 trannies have weak reverse and 5th synchros which wear early even under easy use, but the later trannies are good. The M5's biggest weakness are the 3 rubber plugs at the rear of the top plate. Put metal ones in, don't shift it like a honda and change the fluid ever 15K and it'll last a good while. The M5R2 is a completely different design from the ZF. You'll never stretch the case of an M5R2. If you rediculously overload one you'll blow the teeth off the input and cluster.

The M5R2 is small, but it has 3 bearings supporting each mainshaft and countershaft. The gear ratios are the best you can get for a diesel. I think if you exceed the 350 lb/ft mark with this tranny under a big truck it won't last long, but otherwise it's a good choice for an economy built DD truck.

The M5R2 behind my 4BT has under 10K on it. I just changed the fluid out of curiosity (atf is cheap) and it looked great.
 
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