Monthly Archives: October 2010

Setting a Rear axle gearset

391166953well, this is an attempt to explain how to set up and install a rear axle gearset properly without confusing the shit outta ya. Unfortunately, there are a lot of people that claim to do this and never do it properly as it is a pain in the ass to do it right if you do not have the proper tooling and time. The Golden rule when it comes to rear axle gearset set-up: don’t get pissed off. when you get pissed off setting up a rear axle it will usually cause you to say screw it and throw it together. an improperly set up rear axle gearset leads to improper/premature wear and failure….usually at the worst possible time.

There are 5 critical measurements for rear axle set up, and they are pretty much universal for most all rear diff’s:pinion depth,pinion bearing preload,ring gear runout/carrier bearing preload, backlash, and tooth contact pattern.

-Tooth contact pattern is the most critical of all 5 and is pretty much the only thing you have to go by unless you buy a pinion depth tool. ideal tooth contact of the tooth surfaces is the ultimate goal of rear axle set up. it is checked by smearing grease paint (i use a tube of white from the craft section at the wally-world)on both sides of the teeth of the ring gear and spinning it around to see how the teeth mesh on both sides between the pinion and the ring gear. if you can’t tell the difference between which is the ring and which is the pinion, you probably need someone to help you that does.

there are 4 basic parts to the tooth/teeth of the ring gear. the inside edge is called the toe, outer edge is the heel, the top of the tooth is called the face and the bottom or base of the tooth is considered the flank. the steeper side of the teeth are the drive side, and the other is the coast side: or reverse side.

in a perfect world, we get the perfect pattern in the middle of the tooth AT proper pinion depth. HOWEVER,  no two housings are machined quite the same, so you have to be more concerned about pinion depth of engagement into the ring gear more so than the pattern from toe to heel. if you are too shallow, you will not have enough engagement between the gears and damage will result: too deep and you will have binding issues after you start building heat after some run time and will also have damage.

-Backlash and pinion depth are the two measurements and adjustments that directly effect the tooth contact pattern. use these two measures to adjust your contact pattern. *IF YOU ARE INSTALLING A USED GEARSET YOU WILL HAVE TO GET CREATIVE WITH THESE ADJUSTMENTS TO COMPENSATE FOR AN ODDLY WORN GEARSET*

391166770Backlash is the measure of the play between the teeth of the pinion and the ring gear. i use a dial indicator for this measurement and simply put the tip of the gauge on the edge of the ring gear teeth, hold the pinion stationary, and move the ring gear back and forth. for most gearsets this is ideally between .005-.009″, and this is the guide i use if i have no instructions to go by. circle track cars run substantially more than this due to heat issues. each manufacturer will usually provide their desired specs. you want to accomplish proper tooth contact within the tolerance range of the spec. if you have to be outside of the spec range for backlash to achieve proper tooth contact, either your pinion depth is not right or the gearset is used and has a funky wear pattern cause it was ran before and not set up properly……or was ran in a housing that took on damage and bent. too much backlash will lead to broken teeth and that all too familiar “CLUNK” when you go from forward to reverse. not enough backlash and everything can overheat, bind, warp, and cost you horsepower in the end.

on a gm this is adjusted through the use of carrier selective shims. you will want to have them snug, but not tight. you will overpack them a bit to set carrier preload during final assembly but not for set-up. on the ford 9″-chrysler 8 3/4 centers, this is done simply with an adjusting nut on either side of the carrier. on those, remember to center either adjusting nut in the threads so they spin freely BEFORE you tighten the carrier cap bolts. if you cross the threads or put the wrong cap on the wrong side, you may f*ck yourself, so be careful.

-pinion bearing depth is the distance the pinion sits away from the ring gear. it is adjusted through the use of shims either beneath the bearing pressed onto the pinion or beneath the housing that carries the pinion. from the factory there are many specialty tools available a guy can use to set the shim thickness so you do not have to keep pressing the bearing on and off the pinion to adjust depth. the gearsets are even stamped with a number you can use to set this via using these tools, and makes set up a lot easier as it gives you an excellent starting point at achieving proper pinion depth. i would suggest consulting a factory manual to see how this works. it’s quite elaborate….and the tooling is not exactly cheap, so i have never used it. since most of the rear axles i deal with have been run in demo, been welded on, and used gearsets installed, you pretty much can take the number on the gearset and throw it out the window. basically you need to use trial and error to get the pinion depth right  by reading pattern on the mesh of the ring gear. it is time consumming but cheaper than a depth tool especially if you may only do two or 3 rear diff’s in 10 years……

Now with a Ford 9″ this is a piece of cake, but on a GM or Chrysler pressing the pinion bearing on and off to get the pinion shims right is a long and frustrating thing to do. what i do is i keep a bearing on hand that is machined out a few thousandth’s of an inch so it slides onto the pinion easily yet not loose. you can also use a die grinder on an old bearing to make this. i also have a regular grade 8 fine thread nut on hand so i don’t have to re-use the lock nut that holds the pinion in. also on a 10 bolt gm carrier the ring gear bolts are reverse thread. remember that when you grab your impact gun.

i usually start with a .018″ thickness shim to begin with on the pinion if i have nothing to go by. if it was a complete rear axle and i am putting in a different ratio, i will start with the shim that was on the old gearset and change the shim thickness as the tooth contact pattern dictates it.

with the seal and preload hardware removed, install the pinion in the housing, assemble the ring gear/carrier in the housing. set your backlash to spec. you then mark the teeth with your grease paint and check the pattern on both sides of the teeth. view the pattern against a guide chart. if it is not right, you will have to change the thickness of the pinion shim…..and yes, it all comes back apart, change the shim, and put it all back together. your backlash will change every time you reset the pinion depth, so on a gm you will have to reset the ring gear carrier shims again! set your backlash, then check your pattern again……until you get something centered on the teeth of the ring gear. don’t look at the pinion tooth for contact pattern.

once you achieve proper pattern, now look at how far down toward the bottom of the teeth-or flank-the pinion teeth are engaging. only do this after you have already got a good centered pattern at proper backlash as a first step so you have a solid starting point at “tuning in” your pinion depth. ideally you want a pattern with rounded edges on both sides of the tooth

-if both sides of the pattern are toward the top of the tooth rounding toward the bottom rather than to either the toe or heal…….you are probably too shallow and need to go deeper. increase depth in .005 increments.  yes this will effect pattern, but pinion depth here is more critical than pattern at this point.

-if you have a pie/wedged shape pattern on either the toe or heel of one side of the tooth, that is a good indication of too much pinion depth. you then decrease in .005″ increments.

if you are simply installing the same axle gearset in your car as came from the factory, you probably just need to unbolt and bolt back in everything because all the perameters never changed. the gearset just simply wore out. you would pretty much check a few perameters as a double check and go.

used gearsets may have been installed improperly at one point and ran for some time. getting ideal pinion depth and pattern may be unattainable so you just have to take you best stab at it OR abandon it and buy a new gearset. depends on what you are doing. are we building something for the demo car or a 100,000 mile to go rear out of a 1 ton truck hauling concrete finishing equipment.

However most of the time it is someone swapping in a lower ratio like going from a 3.08 ratio to a 4.56 ratio. now all the adjustments have to change for proper tooth contact pattern. Quite often when you purchase a new gearset and/or a new posi center section or spool, it comes with instructions that give you torque specs and an abbreviated chart to use as a guide for achieving tooth contact pattern. check your pattern at 3 different points on the ring gear. if it is not close to the same, this will be an indication that your gearset is not in there right or the ring gear is warped.

once you have your shim pack settings to where you are fairly confident in the pattern, it’s time for final assembly. The first thing that gets set is pinion bearing preload. This is done through a measure of rotational torque, and is adjusted through the use of a crush sleeve that sits between the pinion bearings. By the book, this torque is measured by use of an inch/lbs torque wrench on the pinion nut. as you spin the pinion, you read the meaure on the wrench. i think it’s usually around 5 in/lbs. i have also used a piece of string wrapped around the driveshaft yoke and hooked to a spring scale. you then pull on it and look at the scale to see the measure of pull.

to be honest, this setting is a real bitch. i have tried doing it by the book and ripped the end off a brand new pinion. i have even used a 4 foot bar and a 3ft pipe wrench under a car with a floor jack to get the job done! the new crush sleeve that comes in the kit is a real pain in the ass. i usually try using the old crush sleeve first and spin the assy together tight using the same nut i had on hand for setting….not the lock nut. if it is loose you need to tighten it a bit. if it is too tight, you will either have to shim it up a bit or start out with a new/different crush sleeve. you want it snug but not tight, a bit difficult to get started to spin but yet spin using your hands without the use of any tools. if you need to use a tool to spin the pinion, or hear the bearings barking….it is way too tight. most people recommend never re-using an old crush sleeve. SO, you can measure your old sleave, use a new sleeve and stick it in a press to get it started. leave yourself about 1/16″ thickness to dial in you preload.

for most rear axles a solid/shim type preload kit can be used in place of the crush sleeve and is considered stronger. in fact most heavy duty rear diff’s are all preload set using solid shims.

on a derby car i have a tendency to set this a bit snug over a street vehicle because of the use of slider shafts. i mean the average lifespan of a rear end in a derby car is 2-5 years….sometimes 2-5 minutes for some people!! once you get the preload set install the locknut and use red locktite….or tach weld the nut(be dam sure you don’t want to take it out if you do that).

-carrier preload/ring gear runout:this is a fairly easy adjustment. on a gm you already have you shim pack. you add about .002-.004″ to either side of your carrier shim pack. you may have to be creative on you shims. i pinch the thin ones between the thick shims and drive em in with a hammer as a pack. the carrier should not be able to pe popped out without using a pry bar….here again if it sounds like the bearings are barking it’s too tight…..but usually you won’t even get the shims in if it is WAY off on preload. on a ford, spin the nuts to zero backlash with your bearings and adjuster nuts at zero lash but no preload, then tighten the nut opposite the ring gear side to set you backlash AND preload at the same time.

now you then double check backlash to make sure you are in range. it may be a bit different than before now that the pinion is fully set. then stick the dial indicator on the back side of the ring gear and spin it. this checks runout to double check the ring gear/differential is not screwed up. i have to admit, i usually don’t even check this measure as it will usually show up earlier in the set up when you check the 3 different points of tooth contact.

finally check your tooth contact pattern one last time. if it is tremendously off from initial set up, you can play with the backlash setting a bit to bring it in.

button it up….don’t forget to put in the gear grease!!!