Viper Dana Super 44 Rebuild

[Steve M]

Measuring and Adjusting Backlash

Backlash is one of the many critical measurements that needs to be made when setting up a ring and pinion in a differential.

Backlash is simply the amount of play between the gear teeth of the ring and pinion. During normal operation, the differential heats up due to friction. Heat causes the gears to expand – having the proper amount of backlash between the gears allows for this expansion, and also allows the gear oil to work between the teeth to help prevent wear and keep them within their proper operating temperature range. Too little backlash can cause excessive heat build-up as the teeth get jammed together due to expansion, which in turn squeezes the gear oil out. At a minimum, the gears will be noisy. Too much heat build-up can cause the oil to break down, ultimately leading to metal-on-metal contact that results in excessive wear and permanent damage to the gears and/or bearings. Too much backlash is equally bad - the gears will likely be noisy (whirring, roaring, and/or clunking), and could also be damaged during operation as the gear teeth slam back and forth against each other during acceleration/deceleration. In either case, you’ll be looking at another rebuild. Good for experience; not so good for your wallet.

The amount of backlash required is a function of the differential design and gear manufacturing process. The backlash spec range for the Viper’s Dana Super 44 using OEM 2-cut Dana Spicer gears is 0.004-0.006” (source: 2008-2010 Viper Service manual provided by 13COBRA). For aftermarket gears, I’ve seen specs anywhere from 0.006-0.012”. If in doubt, call the gear manufacturer (or for the Viper, Unitrax). For some gear sets, it may even be scribed somewhere on the ring or pinion gears. If all else fails, here is another resource that will give you a backlash spec starting point for aftermarket 2-cut gears, but I can’t verify the accuracy of the information: https://www.differentials.com/wp-con...structions.png

Measuring backlash requires a dial indicator gauge and a little bit of patience. It’s a little awkward at first, but once you get the hang of it, it’ll go pretty quickly for each setup you try. The most important thing is consistency – making sure the dial indicator is stable is paramount, followed by making sure the pinion gear cannot move during your measurements. That’s why I spent so much time and energy putting together something that allowed me to screw the base of the dial indicator’s adjustable arms directly into the aluminum differential case. If the dial indicator setup is allowed to move at all, you will not get consistent results. The same can be said for pinion rotation – any amount of pinion rotation during backlash measurements will lead to inconsistencies.

In order to get good, repeatable measurements, you need to make sure the plunger of the dial indicator sits square against the drive side heel of the ring gear tooth being measured in the plane of rotation. It’ll look something like this:



Once the adjustable arms and dial indicator are set, hold the pinion gear tight - that’s why you see my makeshift breaker bar jammed against my tool box. Grab the ring gear with your hand, and gently rock it back and forth. Assuming you have any amount of backlash, you’ll hear a nice “click clack” as the drive and coast side of the ring and pinion teeth make contact with each other. Most dial gauges can be zeroed out, but you don’t need to bother doing that here – all you need to do is watch how the needle of the dial indicator sweeps back and forth. As an example, if the needle moves back and forth across 5 tick marks on the dial indicator, you have 0.005” of backlash.

Here's the important part: you won’t be taking just one measurement. Each tooth of the ring and pinion gears is machined slightly differently, so it stands to reason that the backlash measurements will also vary slightly as different teeth are engaged. You’ll want to take at least 10 measurements at different teeth around the ring gear to get an average backlash measurement. If you are hard core, you’ll measure every other ring gear tooth. What you are looking for are any outliers – ideally, you want all backlash measurements to fall within specification, with the highest and lowest readings varying no more than 0.002”. If they vary more than that, it could be indicative of problems with how the gears were machined, or with the differential case itself. More than likely, though, it’s an issue with your gear setup and measurement process. As an example, not having enough pinion or carrier bearing preload would cause the gears to shift around as you are pulling on them, leading to inconsistent backlash measurements.

The carrier shims wedged between the carrier bearing cups and the differential case are used to adjust backlash on the Dana Super 44. As a reminder, the carrier shims also set the carrier bearing preload. The total carrier shim thickness (i.e., the left carrier shim thickness + the right carrier shim thickness) determines carrier bearing preload; the difference in thickness between the left and right shims sets the backlash. This diagram from the service manual will hopefully help:



Again, this will be an iterative process. Acceptable carrier bearing preload covers a range, as does backlash – once you know approximately how much total shim thickness gives the desired bearing preload, you’ll try to keep that value constant as you swap shim thicknesses left-to-right to get the backlash within spec. After a few setups, you’ll have a pretty good feel for what adjustments are needed to find that sweet spot. If you can’t seem to get there with the shims you have, you’ll need to order more until you do. Thousandths of an inch matter, and it’s worth doing right the first time. Anything less than within spec is not “good enough”.