[Whit]

By Dan Guarino

Owners of diesel trucks equipped with NV4500 5 Speed Transmissions have been looking for a cure for 5th Gear failures from as early as 1993. The problem has been experienced with both General Motors trucks using the 6.5l V8 Diesel and Dodge trucks with the 5.9l inline 6 cylinder Cummins Diesel engine. So, exactly what is the problem and what is being done about it?

“I Lost 5th Gear!”

Almost all cases of Mainshaft 5ifth Gear failure can be directly attributed to insufficient support for 5th Gear. In the classic case, the nut retaining the Mainshaft Fifth Gear backs off allowing the Gear to slide back in the transmission extension housing and out of contact with countershaft 5th gear. This failure usually occurs without warning or noise. You are driving happily along in 5th gear, either you (or your cruise control) lets off on the accelerator for a moment. You step on the accelerator to resume speed and the engine races, but your transmission is no longer transmitting power! White faced and shocked, you mutter “what the #%!!” as you check to make sure the shifter is still in fifth, then downshift to fourth and let the clutch out. Your truck responds instantly as power is returned to the rear axle. Your blood pressure lowers a bit and you try to upshift to 5th. Again, no power and no noise! You mutter “Oh no, I lost fifth gear!” Back into fourth and away you go wondering if you are going to make it home and HOW MUCH IS THIS GOING TO COST ME TO FIX!

Typical 5th Gear failure on a stock NV4500HD Transmission. Note the reddish/orange sludge discoloring the shaft. This is a sign of wear or “fretting”. What you see is actually oxidized metal particles worn from the gear and shaft. Vibration and cyclic loads are the main causes of fretting




Every time someone tells me “I lost 5th gear”, I am reminded of an occurrence in my youth. I was riding to school on a Brockway school bus. We had a lady bus driver who was a chronic gear grinder. One morning, when she was unable to force the transmission into fifth gear, she radioed the school that “I lost 5th Gear”. The reply was something to the effect “Wh’ad ya do drop it on the ground?”. I presume she had little solace with that reply, but she bravely continued on in 4th gear and we all eventually arrived at school. Like my bus driver, you too will probably also make it to your destination after your 4500 turns itself into a 4 speed because this type of failure normally does not cause incapacitating damage in the short term. Chances are, what occurred inside your transmission was simply the 5th gear wore into the mainshaft. Once the gear is loose, it will work the nut off until the gear slides back and your day is ruined.

 

[Whit]

Nut Woes: Band-Aid Approach to A Serious Problem

The initial attempts to prevent 5th gear failures focused on the retaining nut which was a flat nut without a lockwasher in the original configuration. New Venture first added a lip on the nut that could be crimped into a keyslot, then a concave spring lockwasher, and lastly a split nut with a crossbolt to increase the thread contact. The aftermarket weighed in with at least two styles of nuts with set screws. Several chemical thread locking compounds have also been tried. None of these “fixes” were successful in keeping the gear on the shaft. They failed because they address a consequence of the failure instead of the cause. (To find out the reasons why, read on.)

Clockwise from top left: Flat Nut, Concave Washer, Crimp Locknut, Crossbolt Locknut, Aftermarket Nut with 2 Set Screws, Aftermarket Nut with 3 Set Screws and Brass Pads



Overloading Aggravates Problem

Before discussing the problems associated with factory Partial Spline and Aftermarket Full Spline Mainshaft designs, I want to point out that they are good quality parts made by reputable manufacturers. The 5th Gear failings they have are not metallurgy or workmanship related. Shaft wear and gear failure is primarily due to design, but diesel engine harmonics, and overloading also are significant factors. Owners of diesel powered NV4500 equipped trucks often greatly exceed the recommended horsepower and load ratings for their vehicle. We have seen these trucks with engines putting out over 500 horsepower and 1,000 ft-lbs. of torque. We have also seen combined truck and trailer weights exceeding 40,000 lbs.. Now, when you consider the NV4500 is rated for up to 460 ft-lbs. of torque and 19,000 lbs. combined truck and trailer weight, you can see why part of the responsibility for Transmission problems must rest on the shoulders of truck owners who grossly exceed the vehicle design limits and insist on making their ¾ or 1 ton work like a 2 or 3 ton truck

 

[Whit]

Mainshaft Types and Weak Points

To support the 5th Gear, the factory Mainshaft has both a splined area to transmit torque and a smooth machined surface for the fully internal splined gear to press onto. The end of the Mainshaft is drilled and tapped for a pull rod so the gear may be pressed on the shaft without damaging other components inside the Transmission. With this type of shaft, only the tips of the gear splines are in contact with machined support surface of the shaft. This area is also directly under the gear teeth. Over time, the gear splines will actually wear into the shaft. As this wear continues, the fit between the gear and shaft becomes progressively looser. At this point, every time you accelerate or decelerate in fifth gear, the gear moves back and forth between drive and coast. This back and forth motion against the nut is what eventfully causes the nut to back off. Since the mainshaft bearing is inboard of the gear, the Transmission will usually operate in all other speeds.

Stock New Venture Mainshafts from Dodge NV4500HD Transmissions.

The top shaft had the typical fifth gear failure. The splines are worn and you can see the wear marks in the support surface to the right of the splines.

The lower shaft is a new takeout in perfect condition. Note, the support surface is smooth

The aftermarket introduced two types of imported Mainshafts with near full length splines in an attempt to remedy the problems experienced with the factory shafts. The splines on these shafts are considered “flexible” in the sense the gear is not supported by pilot rings at either end so it can “flex”, or rock under load. Both styles are used with the same fifth gear design as original equipment. These shaft are most often sold with an Aftermarket Mainshaft Nut that has two setscrews.

First on the market was a shaft with splines that were made to provide a fairly light interference or press fit with the gear splines. This shaft design was introduced by Blumenthal Manufacturing of Oklahoma City. Next on the market was a shaft made for gear wholesaler Midwest Transmission of Zumbrota, MN. With this design, the shaft splines are near full length, but machined to be a slip fit with the gear. This shaft is grooved to allow a 3 piece thrust washer to be inserted between the gear and retaining nut. Both of these shaft designs received tremendous positive press coverage as permanent cures for 5th gear failures and were enthusiastically sold by almost everyone in the transmission repair business, including us. Unfortunately, great confidence in these designs has not proven to be fully justified. While the percentage of 5th gear and related failures is probably lower with either full spline shaft design in comparison to the factory shafts, the cold truth is full spline shaft failures are occurring.

With the press fit type full spline shaft, gear failures are identical to the original equipment shaft. In other words, the gear loosens up on the shaft and eventfully works the nut off the threads. To have any chance of keeping fifth gear in place, the press fit between the shaft splines and gear splines with this type of shaft must be quite tight. We rejected nearly 50% of the shafts we bought of this design because they did not provide a tight enough fit with the gear. Long before we ever even heard of any failures with this type of shaft, I discussed the importance of accurate machining with the importer and the need to increase the spline contact to prevent the same types of problems we had with the factory shafts.

Full Spline Shaft with Light Press Fit Splines





The other full spline shaft utilizes a thrust washer between the gear and nut. With this design, I know of no instances were the nut backed off. On the downside, this shaft was designed so the 5th gear will easily slip onto the shaft. This slip fit means the gear and shaft splines may eventfully wear themselves away, at which point you could “lose” fifth even though the nut is still in place. This is a very noisy failure as the spline nubs jump over each other, plus all the worn away spline material contaminates your oil. There is one other major issue with this shaft. A secondary function of the fifth gear is to keep the mainshaft rear bearing tight against a thrust washer. With this shaft design, the loose fitting gear cannot hold the bearing tight. If this bearing is not held tight, it tends to wear into the shaft and thrust washer which increases end-play. Excessive end-play can lead to shifting problems, shaft misalignment, and catastrophic failure. This style of shaft is somewhat weaker than other designs due to the reduced shaft diameter at the thrust washer groove. Breakage is not common, but it does occasionally snap in high load situations. Despite the potential shortcomings, this shaft design is very popular with both professional rebuilders and shade tree mechanics because assembly is fast and easy without any press tools.

Full Spline Shaft grooved for thrust washer.

 

[Whit]

The Surprise Culprit Is…..

The 5th gear splines used on all NV4500 Transmissions are of side fit, involute configuration. Involute splines are self-centering, have curved sides, and offer greater torque-transmitting capacity than any other type. Side fit means only a portion of the flanks of each spline are in contact with the mating spline and it is this characteristic that is culprit #1 in the Case of the Lost 5th Gear.

Side fit, involute splines are widely used and well proven for transmitting inline torque. Common examples include input shafts, output shafts, synchronizer hubs, transfer case chain sprockets, propeller shaft slip yokes, pinion yokes, and axle shafts. This spline style is not often used to transmit torque in applications were the torque input is from the side as is the case of the NV4500 mainshaft fifth gear, unless there is some type of additional support (such as pilot rings) to prevent the gear from rocking on the shaft. None of the previously discussed mainshafts use any type of ring or pilot to prevent axial movements occurring under load although the factory gear does derive some weak support from the unsplined portion of the shaft.

If you take a magnifying glass and view a cross section of a 5th gear pressed on the mainshaft splines, you will clearly see the no-contact areas between the tips of each spline and the root of the matching spline. You can also see how short the contact surface is between mating splines. The 5th gear to mainshaft contact area of the mating splines is less than .040” high. The unsupported, no-contact areas or voids, are greater than 1/2 the distance from the minor diameter of the shaft to the major diameter of the gear. With both the factory and full spline shafts these unsupported areas are where wear and eventual failure begin. Such movement will be minute at first, but over time it will increase as the matching parts wear. To eliminate movement, we must have effective and rigid support for the 5th gear. To do this we need to change the design of the shaft, gear, and retaining nut.




Now, lets turn our attention back to the much maligned factory Mainshaft with it’s partial 5th gear spline. These shafts have a spline length of just under ¾”. In addition, for additional gear support there is a smooth machined surface just over 1” long. For manufacturing reasons, there are also reduced diameter areas between the splines and support surface and between these two features and adjacent portions of the shaft. The matching gear (which is also used on both full spline shafts) is 2.25” long with internal splines cut the length of the gear. The gear splines are beveled at each end.

Many people assume New Venture choose partial splines over full splines because it was cheaper. The fact is it is more expensive to make a partial spline shaft. Why? The splines are rolled before the shaft is hardened. After the shaft is heat treated to harden it, the support surface is ground to a very, very precise dimension. New Venture could have saved money by rolling full length splines in the first place. So why didn’t they? Well, I can’t be sure, but a comment made to me by a New Venture engineer about the importance of keeping 5th gear concentric on the shaft had to be a consideration. New Venture tried to use the gear splines pressed onto a smooth support surface instead of pilot rings. This approach is reasonably effective with gas engines and moderate loads. However, with diesels, the engine harmonics and horsepower in heavy duty applications creates loads beyond what this design can handle without wear given the shaft size. With a factory Mainshaft and Gear, you typically “lose” 5th gear when the gear splines have worn into the shaft to the point where repeated axial gear movement works the nut free. New Venture tried to fix the problem by addressing the consequence (by changing the nut design) of the gear working loose, instead of the cause (insufficient gear support). End result: A big problem and a bad reputation for an otherwise excellent Transmission.

Cross-section view of 5th gear pressed over unsplined portion of factory shaft.

Look through magnifying glass and note how splines have already slightly indented shaft when pressed on.

Now, let’s take a look at the press fit, full spline shaft. The theory here is longer splines are better, but the side contact, “flexible” spline with it’s limited spline to spline contact and large unsupported areas makes this design subject to the same types of failure as experienced with the factory Mainshaft. Some support can be derived by having a very tight fit between the gear and shaft. However, with the “average” fit, premature failure is a distinct possibility.

The concept of the thrust washer style full spline shaft is simple. The design assumes the gear is going to come loose anyway, so they grooved the shaft to accept a split thrust washer between the gear and nut. With this design the thrust washer will probably keep the nut on. The problem here is the designers concentrated on preventing the nut from backing off and ease of assembly. In this they were successful, however, by making the shaft so fifth gear and the rear bearing would be a slip fit, wear and fretting is almost guaranteed. Result: Potential for reduced service life.

 

[Whit]

What are the NV4500 Design Specifications?
Current Production NV4500 Specifications*:

Synchronization.........................................All Gears
Main Case..................................................Cast iron
Shift Cover................................................Aluminum alloy
Gear Bearing.............................................Caged roller
Main Shaft Pilot........................................Straight roller
Lubricant....................................................SyntorqLT Synthetic 75w85w gear lubricant
Shift Pattern...............................................Same as Getrag
Tailhousing................................................Aluminum alloy
Support Bearing........................................Timken tapered roller
Input Thrust...............................................Flat thrust bearing assembly
Dry Weight...............................................195 pounds
Rated Gross Vehicle Weight...................HD: 16,000 pounds, LD: 15,000 pounds
Rated Gross Combined Weight...............HD: 21,000 pounds, LD: 19,000 pounds
Torque Rating............................................HD: 460 lb.-ft. LD: 410 lb.-ft
Oil Capacity Transmission Only..............4 quarts, but we recommend 4.5 quarts
Oil Capacity including TC adapter ...........5 quarts (when replacing Getrag)
PTO.............................................................Standard 6 bolt on each side
Dodge HD Input Shaft................................1 1/4" 10 spline with .750" pilot tip diameter
Dodge LD Input Shaft................................1 1/8" 10 spline with .750" pilot tip diameter
GM Input Shaft...........................................1 1/8" 10 spline with .590" pilot tip diameter
Dodge 4x4 Output Shaft............................HD: 29 spline, LD: 23 spline
Dodge 4x2 Output Shaft............................HD: 31 spline, LD: 30 spline
GM 4x4 Output Shaft................................32 spline
Bellhousing Bolt Pattern ..........................top 10.078", bottom 9.738" Height 6.043", 5.600" index
Bellhousing Bolt Pattern 1993-95 GM...top 10.078", bottom 10.394" Height 4.685" 5.125" index

*NV4500HD models are used with Cummins Diesel & V10. NV4500LD models are used for all other applications.


What are the major design changes?

The NV4500 series transmissions have undergone many minor and major design changes since 1992. The minor changes, while important for durability and shifting ease, are not easily identifiable to the layman trying to determine the features of a particular transmission. The following list shows the major changes we have identified. To precisely identify a particular transmission by part number, get the numbers off the build tag on the left PTO cover and the decals on top of the top cover.


The light duty 1" 19 spline input shaft used in 1992-93 was changed to 1 1/8" 10 spline in 1994.

1992-93 models had a coarse thread shift stub. This was replaced by a square stub that accepted a press on shift lever for 1994-1997. A metric threaded shift stub was introduced for 1998.

1992-1996 had an interlock mechanism built into the 1-2 syncro to prevent high speed downshifts into 1st or second gear. This feature was dropped during the 1996 model year

In 1997, New Venture changed the design of reverse gear from a dual plane to a single plane. The reverse gear ratio was changed from 5.61:1 to 5.04:1.

For 1998, the shift tower and shift handle were redesigned. 2WD models no longer had a speedometer drive (the speedo works off the rear axle). 4x4 tailshaft housings were changed to eliminate mountings for the transfer case shift lever. Also, with HD models, the mainshaft nut was redesigned late in 1998. This change first showed up on transmissions with an AD suffix on the Dodge part number (ex: 52108131AD)

 

[95Z28A4]

Now I Understand

I've been reading about the NV4500 5th gear failures, but did not fully understand the cause. I would say a picture is worth a thousand words, but considering the reading I've done, I'd say these pictures are worth several thousand words. Thanks for taking the time to post this.

Mike

 

[Whit]

Dan is well known to be a guru on this and we all stand a chance to learn something from him if we pay attention

I have a crap load of other great articles too....will post up

 

[kfowler]

Whitmore,

What are the issues to deal with if I am thinking about replacing my nv4500 with a later used 6 spd in a '97 2500 4x4 ?

Thanks,

Keith Fowler

 

[Whit]

Whitmore,

What are the issues to deal with if I am thinking about replacing my nv4500 with a later used 6 spd in a '97 2500 4x4 ?

Thanks,

Keith Fowler

I will do a seperate 6 speed replacment article on that ...hang on I think I have one

 

[69hemigtx]

Whitmore,

What are the issues to deal with if I am thinking about replacing my nv4500 with a later used 6 spd in a '97 2500 4x4 ?

Thanks,

Keith Fowler

Besides locating the correct parts and setting aside the time to do it, money will be the biggest issue. NV5600s are nowhere near cheap, so plan on paying a fairly large amount unless you happen across a smoking deal.

 

[Pantherman]

Is there any year that the NV4500 didn't suffer from the 5th gear nut coming loose?

 

[Whit]

Is there any year that the NV4500 didn't suffer from the 5th gear nut coming loose?

they all did.........trick is not to lug in 5th gear

 

[Paul(in NY)]

Is there any year that the NV4500 didn't suffer from the 5th gear nut coming loose?

The following is a cut/paste of a email I received from NVG.

"

_____________________________________________

From: "John Malone" <[email protected]>



Sent: January 2, 2003

Subject: NV 4500 Manditory Repair



Paul;



The NV 4500 Transmission as used in General Motors, Chevy and Dodge Trucks form its introduction to 1995 suffer a design problem in the 5th gear lock nut retainer. It is probable that during the life of the NV 4500 Transmission, the 5th gear retainer nut will loosen and allow the final 5th Overdrive Gear to fall off the shaft. When this happens SERIOUS damage will happen to the Overdrive. The more miles the transmission has, the more apt the fail to occur.



The update to eliminate this problem is a new 5th gear Nut with a newly designed retainer. The retro fit requires special tools and can not be done properly without the special tools. All Authorized New Venture Transmission stations can make the retro fit. The cost which includes a modification to the output shaft is $250.00, Two Hundred Fifty Dollars.



All transmissions modified by New Venture Transmissions will have the words "NEW NUT" stamped into the case by the top left Overdrive mounting bolt. If these words are not there, consider the transmission as unmodified and a serious threat of failure. Transmissions built after January 1st, 1999 feature a completely new 5th gear retention system eliminating the 5th gear nut problem.



I hope this answers your question on the 5th gear nut failures.



Sincerely

John Malone

NV 4500 Customer Support

New Venture Transmissions

Syracuse, NY"

 

[nhdiesel]

I would like to take a minute to defend the NV4500, which has gotten a bad rap when it comes to the 5th gear issue. The "problem" is actually fairly rare, and is usually found on vehicles that have been used hard, lugged in 5th, and have well over 250k miles. Most transmissions (and especially the vehicles they are in!) are worn out by that point.

I wouldn't consider the 5th gear nut to be a defect or a problem, but rather the first signs of wear in the transmission. If it is caught and a proper fix performed, the transmission can have many more years of useful service. Thats more than can be said for most transmissions with 250+k miles.

I'm not saying to ignore it completely, but I sure wouldn't lose sleep over it. If you have the transmission out of the vehicle anyway and want it in tip top shape, go ahead and do an upgrade to lower the chances of 5th gear failure. But I wouldn't bother pulling a perfectly good NV4500 out of a running vehicle with fairly low mileage just to deal with 5th gear.

Jim

 

[Paul(in NY)]

I would like to take a minute to defend the NV4500, which has gotten a bad rap when it comes to the 5th gear issue. The "problem" is actually fairly rare, and is usually found on vehicles that have been used hard, lugged in 5th, and have well over 250k miles. Most transmissions (and especially the vehicles they are in!) are worn out by that point.

I wouldn't consider the 5th gear nut to be a defect or a problem, but rather the first signs of wear in the transmission. If it is caught and a proper fix performed, the transmission can have many more years of useful service. Thats more than can be said for most transmissions with 250+k miles.

I'm not saying to ignore it completely, but I sure wouldn't lose sleep over it. If you have the transmission out of the vehicle anyway and want it in tip top shape, go ahead and do an upgrade to lower the chances of 5th gear failure. But I wouldn't bother pulling a perfectly good NV4500 out of a running vehicle with fairly low mileage just to deal with 5th gear.

Jim

Absolutely Agree. The NV-4500 is my transmission of choice for my trucks.

Paul

 

[Grigg]

That's some great info so far, is there more to come?

Thus far we have covered the "problem", the quick fix, the real problem, and two less than perfect solutions.

What's the real solution? Is there a main shaft and 5th gear design that won't destroy each other? Or are all of the available "repairs" prone to the same failure, just at different times?

Grigg

 

[briney]

just to recap, is the main drama someone putting 25k on the towbar then wanting to drive in 5th at 45mph ?

 

[kaptar]

The Torque King mainshaft kit thats made in Japan is $600. the kit thats made here is over $1000. Thats a lot of cash for a shaft,gear,3nuts,1 bearing and some loctite. Oh I forgot the video. With prices like that NV5600s don't look to bad. Some people have even welded the gear and nut on the shaft, cheap temporary fix but I guess it will work.

 

[Grigg]

If good parts cost $600-$1,000 and you still have to buy the transmission and do the work lots of other transmissions look better. I think even if I already had an NV4500 I would think twice about the upgrade at that price.

I considered the NV4500 for my project, but decided on a Roadranger RTO-6610, I feel better about that decision now, less money up front, and as bonus there is no obvious weakness in the Roadranger.

Grigg

 

[nhdiesel]

just to recap, is the main drama someone putting 25k on the towbar then wanting to drive in 5th at 45mph ?

Thats exactly my point. I don't have any actual numbers (I don't think anyone does), but I would estimate that the 5th gear issue probably happens to 5% or less of high mileage trucks. Now, what were these trucks used for? Towing 15,000+ lb. trailers their whole lives? Lugged in 5th gear while doing it? I have heard of plenty of people with over 500K miles on their NV4500 without a rebuild. Go to DTR and do some reading.

If you are swapping an NV4500 into another 3/4 or 1 ton vehicle (or smaller) and won't be towing huge trailers, just install it and forget it. Remember that web sites like the one shown in the first post make their money selling the upgrade parts, so of course they will push the worse case scenario. I'm not saying that their info is wrong, or their parts are not good...just that they use scare tactics to sell their products.

Jim