How I Rebuilt My Final Drive

Last Updated January 7, 2000

General Comments and answers to a couple of questions:

Hello! This document is a description of how and why I rebuilt my Triumph Spitfire final drive. I didn't touch the differential assembly, so if you're looking for help with that you're out of luck! My purpose here is to cover the basic procedure, particularly the steps aren't really obvious until you've done them once or twice. Hopefully this will help get you up to speed quicker or at least demystify the process a little. It's the kind of thing I would have liked to have seen before I started my rebuild.

Rebuilding your final drive can be fun if you relax and take your time - patience is a virtue on this job! Getting the correct settings is important and you need to make careful measurements. You'll probably wind up putting it together and taking it apart several times as you home in on the correct shim thickness, etc, but be patient and the correct results will follow. If you get frustrated, take a break and try again later! Do it once and do it right. A clean work space is also highly desirable. My workbench always has grime from the last job on it, so to "clean" it I cover it with a layer of clean paper shop towels. Then I can see where everything is and I know that once I clean something it won't pick up a lot of crap from the workbench.

Lastly, remember, when you start taking something apart and you've never done it before and you're not sure what you're doing and perspiration starts to bead up on your forehead and the room starts to feel really hot and your palms are getting sweaty - Don't Panic! The fear reflex is pretty normal and it's okay to be a little nervous. Just double check your work as you go and ask questions if you're really not sure how to proceed. Everything usually works out in the end. And if it doesn't... well, whatever you're fixing probably wasn't working so great in the first place and you can always find another one somewhere... :)

Please please please do yourself a favor and get the real shop manual for your car. (See previous discussion) You can't get by with some aftermarket manual for this job.

What about those special tools I see them using in the shop manual?

Don't panic, the only "special" tools you'll need will be a good dial gauge, a pair of calipers or micrometer, a torque wrench, a small spring scale, and some way of grabbing the input flange to tighten the nut. I originally thought that in order to set the correct pinion depth (more on this later) you would need the special rebuild pinion and the special jig to hold a dial gauge that goes with it. I realize now (after rebuilding a different final drive at work) that with patience and some gear marking grease you can get by without them. Although if you're lucky enough to have them definitely use them! You won't need a "hypoid spreader" either. I tried to buy one and couldn't find one anywhere, anyway!

A few of these tools are pictured here. The crazy looking vice grip is the flange grabber! It also doubles as a motorcycle clutch holding tool. I don't know where to get one of these - I got mine from a friend in exchange for a tune-up...

I can't find anyone who supplies pinion preload shims - what do I do?

Don't panic - you can make your own! More on custom shims below...

Why did I rebuild my final drive?

There were two reasons, really. First was leaky oil seals, the second was a funny noise I >thought< was the final drive. To make a long story short, it turned out the be the universal joints. They're so close to the final drive and I couldn't tell where the sound was coming from and they >seemed< alright... Yeah, it's a little embarrassing, but I learned alot and I'm sure I made things better rather than worse.

Oil Seals

The output shaft oil seals can be replaced with having to disassemble the final drive, and in fact you could even replace them with the final drive still on the car, although I would say it's easier to just remove the final drive rather than fight with the driveshafts which would be constantly in the way. The output shafts and outer bearings come out as a unit, after unbolting four mounting plate bolts. The final drive also has two differential carrier bearings, but these stay inside the unit. There's no real trick to getting the output shafts off although you may that you can only loosen/remove one bolt at a time - the output flange interferes and has to be rotated out of the way for each bolt. Once the bolts are removed you can tap the flanges with a rubber or dead-blow hammer to remove the shafts. Next you'll need a hydraulic press (or >maybe< a gear puller) to separate the bearing and mounting plate from the shaft. Once the plate is removed, the oil seal is accessible and can be removed and replaced. Assembly is the reverse of disassembly - piece of cake.

Replacing the input shaft oil seal requires removing the input flange from the input shaft. To achieve this, you will need a flange grabber of some sort, the right socket, and a big socket driver - the nut is on pretty tight. Also, if your final drive uses the "collapsible spacer" means to adjust pinion preload, you >may< need to open the final drive and replace that bad boy - but you >might< be able to get by with re-using the old one. I say this conditionally because I've haven't tried it - my final drive used shims to adjust preload - but this worked for me on a Toyota final drive I rebuilt at work, but Triumphs may not be as accomodating. Use your own judgement on this one. How will you know if you can reuse the spacer? Basically, if you can get the correct preload without surpassing the torque limit of the flange nut then you're probably okay... If you're wondering what all this preload business is about and what a flange grabber looks like, you'll see more in the next section on the total rebuild.

The Rebuild

The first step is remove the final drive from the vehicle! That's not too hard so I'll assume you can figure out that part yourself. Drain the oil! Next, unbolt and remove the output shafts, as mentioned above. At this point you should be ready to undo the main housing bolts. What you see should look like this ->

Now it's time to unbolt the bearing caps and the pry the ring gear (aka crown wheel) and differential assembly out. This may look scary, but it's not - the screwdriver won't damage anything:

Keep track of the bearing races! Unless you're replacing the bearings, you'll want to keep the bearings and races as matched sets. Also it doesn't hurt to keep track of the shims you find. Measure them and put them in a plastic bag or something. This will give you some baseline information to compare against later if you need to. It's also probably a good idea to keep track of which bearing cap went with which bearing, although in my case it turned out there was only one side each could go on, due to the way the locating sleeves came out. Now you should have something that looks like this:

Time to remove that pesky flange. Below is a closeup of the business end of my vise grip flange grabber and a shot of me loosening up the pinion shaft nut. Can you tell what's wrong with this picture? Where's the final drive mounting plate?! I already removed it because I already had the flange off earlier - your mounting plate will still be on at this point because you haven't removed the flange and it won't come off otherwise! Also, don't make the same mistake I did - when you're setting the pinion preload, don't forget to put the mounting plate on first! Otherwise you'll wind up with this nice rebuilt final drive and then you'll look over at the bench and say "Oh sh*t." Oh yeah, I've been there, baby!

Once the flange is off, tap out the pinion shaft with a rubber hammer or dead-blow hammer. The only thing holding it in is the first bearing (trapped behind the oil seal), which is a slight friction fit on the shaft. You should wind up with a pile of parts like the one below. Once again this is a good time to measure the shims you find. In my case I found that one of the shims was partially missing. There were some strange shim fragments left from the last guy. This was when I started looking to order some shims and couldn't find any. Eventually I learned to make my own at the dining room table!

Now at this point if you're renewing the input bearings (which I didn't) you'll have to press the races out of the housing. Or possibly you can beat them out with a drift of some kind. Refer to your shop manual!

Congratulations - you've succeeded in tearing down the final drive! Now it's time to start putting it back together. It's time to start making measurements!

The first measurements you'll make are ring gear runout and side float. To measure runout, place the ring gear and bearings back in their original places, minus the shims. Tighten down the bearing caps. Make sure the bearings are firmly together and the ring gear isn't bottoming out against the casing. Position a dial gauge perpindicular to the ring gear along the edge where it meets the differential carrier. Spin the gear and measure the total deflection on the dial gauge. It should be less than 0.003" (0.076 mm). In practice I found this hard to measure - the rear face of the ring gear had various machining or casting grooves on it which caused a lot of scatter in the measurement not necessarily attributable to runout. With true runout you'll see that one side of the ring is always closer and the opposite side is always farther. Mine turned out to be okay, as far as I could tell! Oddly enough the shop manual doesn't say what to do if the runout is not within tolerance! I think there's only two things you can do: make sure all the differential carrier bolts are tight, or replace the bearings. This is why you want to make sure the bearings are not sloppy when you make this measurement or you might buy a new set for nothing.

Measuring side float will take some patience and maybe even an assistant. To measure side float you place the ring gear and bearings in the housing (minus the bearing caps this time, no shims, either) and, making sure the bearings are firmly together, you slide the whole thing side to side and measure how far it goes. This is pretty easy but there are two ways to go wrong. The first is that it's possible for the ring gear to bottom out against the housing, which will give a low reading because the assembly won't get full travel to one side. The other problem is keeping the bearings firmly together - this is where an assistant would come in handy. I measured off of the differential carrier stub shaft rather than off the bearing races which can wobble a little. Make sure the ring gear moves freely (at least a little bit if you can't get a full turn out of it because of bottoming out) and settle the bearings. In my case this turned out to be about 0.073" as close as I could measure. Take several readings and see how repeatable they are. If you get the same answer a few times it's probably the right one. This number is important because it will determine the total shim thickness required to get the correct bearing preload later on. More on that later.

Now comes the part I wasn't able to do! Now that I'm older and wiser I think I have a solution. It's time to set the dreaded pinion depth by determining the correct pinion head spacer. This is the shim that goes between the pinion head and the main pinion bearing. Now the official shop manual starts throwing out all these special tools you're supposed to be using that you'll never see in your lifetime. The first is the "dummy pinion". This is just like the real pinion except it has no teeth and has a "built in" pinion head spacer of exactly 0.077" (1.95mm), and the "pinion" is smooth on top. As if that wasn't bad enough now you're supposed to have this crazy swiveling dial gauge mount that allows you to measure the distance from the bottom of the carrier bearing seats to the top of the dummy pinion head which it mounts on! Needless to say I didn't have any of those things so I said "ignorance is bliss" and continued on my merry way. I rationalized this by saying that since I wasn't replacing the bearings that this would be okay and it was probably set right before I started, anyway... although nothing else was correct...

So here's what I would do now. First off use the pinion you've got. Unless you're a talented machinist and feel like reverse engineering a dummy pinion. Measure the pinion head shim you've got or get yourself a 0.077" shim and use it. If you use the shim you've got you'll have to adjust the measurements you're about to make to compensate. Oil the bearings, put everything back in the housing, but leave out the pinion preload bearings, for now. Leave the flange on the bench. Tighten the nut gradually until you get the correct preload (15 to 18 in-lbs starting torque). More on measuring that later.

I think it would be difficult to measure from the bottom of a curved surface, so instead of measuring from the bottom of the bearing cups I would measure from the flat part where the bearing caps seat. This is exactly one half diameter of the bearing outer races, so you'll have to measure them with calipers and adjust the measurements by subtracting half the diameter. Now measure from the flats to top of the pinion. This is probably easiest on the side closest to the pinion. If you can't get a measurement from the other side, don't panic - the manual just averages the two measurements, anyway. Subtract one half the bearing diameter. Now you have the measurement from the bottom of the bearing cup to the top of the pinion. Keep in mind that the pinion may be higher than the bottom of the cups, in which case you will need to mark this as a NEGATIVE reading. If the pinion head is lower than the cups, this is a POSITIVE reading. The textbook ideal measurement is zero, if you have the 0.077" pinion. If your shim is less than 0.077", add the difference between it and the 0.077" shim to the measurement you just made. If your shim is thicker, subtract the difference from the reading you made. Your total shim should be set to [measurement you calculated from pinion head to bottom of cup] + 0.077" + (0.077 - the size shim you used). As you can see if you used the magic shim the third term drops to zero and you get the textbook case. Shims are available from 0.075" to 0.096" in increments of 0.001" as necessary. At least they used the available in those sizes. Check your supplier to see what you can really get. If you can't get the right one, get the closest that's a little smaller and make up the difference with a homemade shim. More on homemade shims below.

You might want to make that measurement before you renew the bearings (if that's on your list of things to do) just to baseline the unit and see what you've got. Also it's good practice! In any case, if you got the wrong answer you can fix it later by the trial and error method. So just do the best you can and we'll work it out later! The closer the better, of course, but don't lose sleep over this one. In fact if you're really lazy, just reuse the old shim and move on to the trial and error method as outlined below.

If you made the correct pinion head measurement, you could follow along with the shop manual and set the pinion preload. If you have a collapsible spacer to set preload, you might only get one shot at this, so you better hope your pinion head spacer is dead on. However, I think it's safe to say you can do this later after you've sorted everything else out. I'll cover this below.

Okay, let's assume you've got the correct pinion head shim, or at least one that's close. Now it's time to make another differential carrier side float measurement. This time we measure the float from the far side of the case to full mesh with the pinion. This gives you your "mesh clearance". Same procedure you used before. Make sure the pinion is tight enough not to slop around, but don't panic if you haven't set the preload yet. Just make sure it's firmly in there.

Time to set the ring gear backlash and bearing preload! In my case I got 0.054" for mesh clearance. Get out your calculator, it's time for simple math. Take the meash clearance you got and subtract 0.005" as an initial setting for backlash. This gives you your crown wheel side shim pack thickness. In my case this would be 0.049". Next take the total side float (0.073" in my case) and subtract the shim pack from the other side. In my case 0.073 - 0.049 = 0.024". This is the other shim pack thickness. Finally, add 0.002" for bearing preload to each pack (0.004" overall above what you measured as total side float (0.077" for me)). So this would give 0.051" ring gear side, and 0.026" pinion side in my case.

Now it's time to check your settings and shift things around if everything's not right. This is the possibly time consuming and patience testing trial and error part I was going on about in the preamble.

The first thing you have to do is put the bearings and shims back in the housing. Assuming you don't have a hypoid spreader, you can do the following:

Angle the bearing races and carefully place the shims as shown above. Do this for each side. It may be helpful to have an assistant holding the ring gear or minding the shims since this is almost a three handed job. Lower the assembly until it looks like the picture above. Now all you do is tap the bearing races with a soft hammer and the whole mess drops right in slicker than snot. It doesn't look like it would, at first, but it'll drop right in there unless you severely overshimmed. But you were careful, so I have confidence you didn't. Refit the bearing caps and tighten them down. Give the wheel a spin to settle the bearings!

Time to measure the backlash! The pictures above show the setup I used. I was able to mount the dial gauge post through one of the housing bolt holes. However you choose to mount the dial gauge, make sure there's no relative movement between it and the ring gear or you'll never get an accurate measurement. That's why mounting to the housing itself is a great idea. Place the business end of the dial gauge on a tooth as shown above. Measure the slop between the ring gear and pinion by rocking the ring gear back and forth. It should be between 0.004" and 0.0065" if everything's perfect. I was only able to get within 0.004" and 0.0065", but life's not perfect. If the fit is too tight you'll need to re-shim. Move a few thousandths from the ring gear side to the pinion side. 0.002" moved from one side to the other will give about a 0.002" change in backlash. If it's too loose, go the other way. Just make sure your total shimmage stays the same. Make the backlash measurement at different rotations of the ring gear. You'll probably get different readings at each spot, but they should all be within tolerance wherever you measure when everything's shimmed correctly. Once again you may have to make your own shims to get the correct distribution of shims.

Now at this point if you got the correct backlash tolerance, the book says you're all set! If you find it impossible to get the correct backlash tolerance, the pinion head spacer will need to be adjusted. That's okay because it's easy to check if your pinion is spaced correctly...

To really see if you're set up correctly, you'll need some gear marking paste. Basically this is a brightly colored grease which you paint onto a few of the ring gear teeth at a few places around the ring gear. If you can't get any gear marking grease you >might< be able to get by with white lithium grease instead. Don't quote me on that since I haven't tried it. Gear marking grease is available mail order or from differential rebuild houses. You spin the gear around several times and see what kind of contact pattern you get with the pinion. You will be able to tell if the pinion is too close to the ring gear, or too far away.

Time to read the pattern!

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Time to set that pesky pinion bearing preload!

Making homemade shims