Different Differentials & The Pitfalls of the Easy Swap

I dig cars, and I do car stuff. I started fairly late in life, though, and I’m only just starting to get into the whole modification thing. Now, as far as automobiles go, you can pretty much do anything you set your mind to – engine swaps, drivetrain conversions, you name it – it’s been done. But such jobs require a high level of fabrication skill, automotive knowledge, and often a fully stocked machine shop to match. Those of us new to the scene tend to start a little bit smaller.

So where does one begin? Well, there’s a huge realm of mods that can be done that are generally referred to as “bolt-ons”. This centers around the idea that the install process of the modification is as simple as following a basic set of instructions to unbolt the old hardware and bolt in the upgraded parts. Those that have tread this ground before me will be chuckling at this point – so rarely is a bolt-on ever just a bolt-on. As follows, the journey of my Mazda’s differential upgrade will bear this out.

The car in question, currently known as the “Junkbox MX-5” until it starts running well enough to earn a real name. It somehow looks passable here, but in person I promise you, it looks awful.

It all started when I bought the car, back in December 2016. I’d just started writing for Hackaday and my humble Daihatsu had, unbeknownst to me, just breathed its last. I’d recently come to the realisation that I wasn’t getting any younger, and despite being obsessed with cars, I’d never actually owned a sports car or driven one in anger. It was time to change.

After realising all my favourite JDM metal was outside a budget I was comfortable spending, I settled on an automotive classic – I’d have a Mazda MX-5, known to the Americans as the Miata. It’s a Japanese take on the old British sportscar – a convertible roadster with an engine in the front, driving the rear wheels. I wanted the cheapest one I could find, and well – safe to say, I got it.

When I rolled up to buy the thing, it looked okay – some shabby paintwork, sure, but it drove great! It was a 1990 with the smaller 1.6 litre engine, and no performance parts to speak of, but I wanted a project anyway. I happily parted with $3000 and all was well, for about fifteen minutes. You see, on the way home, a terrible, awful noise began to emanate from the car’s drivetrain. My heart sank at rather a high rate of knots.

After limping home praying the thing wasn’t about to completely explode, I sprinted to the Internet for help.

“Weird deceleration noise”
“Scraping mx-5 deceleration”
“Miata drivetrain shuffling noise”

To my relief, thousands of people had exactly the same problem I did – a strange shuffling or scraping noise on deceleration in gear. To my exasperation, it wasn’t clear what the problem was – was it the differential, the gearbox – an exhaust heat shield? Eventually, I took a trip to a local MX-5 expert who indicated it was likely the gearbox or differential, but despite the noise it would probably be okay for another 50,000 km or so until I could get it fixed.

Now slightly more comfortable, I resolved to put up with the noise while I got on with real life. In the mean time, the car rewarded me with more trouble, like overheating – eventually solved by a radiator replacement you can watch in the video below.

I was very much getting my money’s worth out of this car, but I wasn’t worried. I’d bought this car expecting to upgrade virtually everything on it anyway, so I wasn’t too cut up about the worn out parts. After much research, I had decided that I wanted to upgrade to a limited-slip differential (LSD), which allows better torque distribution between the rear wheels for better grip and handling. Plus it helps you do mad skids. I settled on a 4.1 ratio unit from the more recent 1.8 litre cars, sourced from a wrecker a state over.

The Torsen LSD from a 1.8 litre MX-5, along with tailshaft & halfshafts with hubs attached. Apparently removing the halfshafts from the hubs is insanely difficult so I’m thankful the wrecker left them attached.

Now, I initially wasn’t too worried about the swap. The 1.8 litre LSD in itself isn’t a direct swap into a 1.6 litre car. However, if sourced with the tailshaft and halfshafts from a 1.8 litre car as well, everything should just bolt up. Sounds easy, right?

Well, that’s where the trouble starts. I’ve only been doing bigger mechanical jobs like this for a few months, but I was confident after reading a few tutorials that I could complete the job in just a few hours. But the tutorials I read assume a certain level of experience that I quickly learned I didn’t have.

First problem – tools! I’d happened across a great trolley jack and sourced some jackstands so I could get under the car. After getting everything set up, I slid under the car and prepared to start unbolting the differential mounts. Armed with a 3/8 inch ratchet I got in a cheap $50 toolkit, I was quickly disappointed. Drivetrain and suspension components are generally held on with very large fasteners, done up to a very high torque. We rounded off the 3/8 socket extension in the first five minutes, and called it a day while we waited for the hardware store to open.

This socket set was integral to getting the job done. A good set of tools will make your wrenching activities much more pleasurable.

The next day, I was back, armed with a tough new socket set. To say this transformed my experience is a total understatement. Suddenly, with a 300 mm extension and the 1/2 inch drive breaker bar, the nuts holding on the differential were no match for my raw strength. Instantly what felt like an impossible task seemed again to be a quick, one-day job. Until the next pitfall.

Things on cars get stuck a lot. Put a big bolt through a few parts, wang it up to 150 foot-pounds, and blast it with dirt and water for 27 years. Remove the bolt, and you might find it’s all still stuck together. As you’re trying to free things up, the initial instinct might be to reach for a hammer. But it’s not so easy –  sometimes it’s not obvious if you’re actually hitting the right thing. Other times, you’re on the ball, smack the part free – only to realise you’ve just put your screwdriver through a brake line. It often takes finesse, a cup of tea, and a quick question to the relevant forum or Facebook group to identify where one should place one’s hammer, and how hard one should swing it. On this job, there was confusion about a bushing that was holding the old differential on to the power plant frame – thankfully, the fine people of the Mazda MX-5 of Australia group confirmed I could bash away with impunity, and I did so.

A stubborn bushing. Hammer away, they said, and hammer I did.

Next problem was bolts. Despite ordering “everything” I needed to fit the diff to a 1.6 litre car, the wrecker had only supplied two nuts to go with the four bolts that attach the tailshaft to the differential. I suspect they might have been lost in transit, as everything was shipped loose. I wasn’t able to follow my initial plan of reusing the old nuts either – they were a smaller size. In the end, a last minute dash to the parts store netted me some slightly larger nuts that fit. It might sound strange to say that bolts are a problem, but you might be surprised. Automotive manufacturers generally use a higher grade of bolts than is typically available at the local hardware store, and in a much wider variety of thread pitches. Replacements are accessible at a specialty fastener distributor, but these all close at 5:00 PM and don’t open on weekends – cold comfort to the shadetree mechanic with a full-time job. I was thankfully saved by the auto parts store which did have the correct pitch nuts I needed on a Saturday, albeit at $7 for a pack of three. It was a better option than driving out to the wrecking yard to yank a single bolt off the nearest Mazda, though. With that solved, I felt confident I could finish the car that night.

The halfshaft, impudently refusing to seat fully in the differential. The green section should be fully flush up against the differential housing & seal.

I moved on to the next job – fitting the new halfshafts to the new differential. To my surprise, they didn’t just slide in. A quick search found that they required a heavy pounding with a stout hammer, so, after much wrestling with the shafts, which still had the hubs attached, I got the left one in. Buoyed by my new success, I was excited, and started immediately on the right, but to no avail. Repeated blows did nothing to force it in to its full depth. I once again begged the forums for insight, who all reported that it was difficult, but more hammering should do the job. At this point though, fatigue was setting in and I decided to finish up with a fresh mind and body in a few days.

Now armed with everything I needed, on the Monday, the work went quickly. A quick strike from the hammer seated the previously immovable half shaft. I didn’t stop to ask why. The differential was bolted up and filled with fluid, the powerplant frame aligned, and the transmission topped up with oil. I was excited – the car was close! All we had to do was refit the rear brakes and take it for a spin. The right side went together in a snap – I was well acquainted with the brakes by this point, having done an upgrade to the later model brakes when I found my car’s rear calipers were both non-functional.

It was dark by this point as we were working on the last wheel. For some reason the brakes just weren’t going together, and we kept fumbling around as we tried to take shortcuts, not wanting to disassemble the caliper any more than necessary to get it back together. Eventually, we realised the problem.

Yes, that brakepad is severely bent. I can only presume that it was due to incorrect fitment during the upgrade, but nonetheless. Not wanting to stop, we found a spare pad with the parts left over from the brake upgrade, and got halfway through putting everything back together until we realised the spare pad was the wrong type and it was all over. My three-hour differential upgrade was now going to bleed into a fourth day.

A good brake pad (top) versus the bent one (bottom). Note the highly irregular wear.
The bent pad up against a rule. Colorized for clarity. I’m still not 100% sure how this happens on a single piston caliper.

By this point it was Wednesday, and armed with a new set of rear pads, I once again disassembled both rear brake assemblies and fitted everything up. I lazily adjusted the handbrake as, in several hours of trying, I’ve never been able to get it right despite following the proper instructions. I wasn’t surprised or that bothered when the handbrake largely failed to work.  With the wheels now back on, the car was gently lowered down off its stands, ready to drive.

To say I was nervous was an understatement. While it may be a “bolt-on” job, drivetrain components are more than capable of doing a lot of damage in the event something goes wrong. Reversing out of the driveway went well though, and there were no immediate catastrophic sounds as I drove the car to a friend’s extended driveway for testing. With the engine dialled up to 5,000 rpm, I dropped the clutch and to my delight, the car spun both rear wheels as expected – without flinging rotating metal components all over the place. The drive home further confirmed my success, with the awful scraping sounds now absent from my drivetrain.

I’m pretty darn pleased with the job, and can’t wait to test the car further at an upcoming skidpan day at the nearest racetrack. There’s still a long way to go, and I’m sure this won’t be the last part of the MX-5 to suddenly and unceremoniously fail on me. At the end of the day though, I managed to suffer through a “simple 3-hour bolt-on job”, even if it took me four days – and I’m all the more experienced for it.

 

69 thoughts on “Different Differentials & The Pitfalls of the Easy Swap

  1. You bend a brake pad by having one end not move because it is hung up on the caliper, the slides, or just not quite in the right place.

    Now that you know to look and make sure the pads are properly seated, you’ll manage to find something else to screw up on re-assembly. Trust Me.

    Welcome to the wrenching fraternity. It’s addictive. I got a Miata to have a reliable convertible. The other one is a 1976 Triumph TR6. One of the very last ones built. After 32 years we are old and dear friends with nary a bolt untouched.

    1. At one point just after high school I bought a cheap truck to replace my dying cheap truck. For a couple of weeks I would alternate which one I would work on and just drive whichever one ran that day.

  2. Typically the half shafts have a ball that holds the retaining ring outwards. Typically pushing the shaft into the cup releases that ball and allows the clip to compress and the cup to slide into the diff.I always push the shaft in then whack the end with a rubber hammer and they always pop right in. Removal is typically the same – push the shaft into the cup while pulling the cup out of the diff.

  3. HEAD GASKET? DID HE SAY HEAD GASKET?! *pant* *pant*… Sorry, but I do own (and love) a MGF… These cars are ridiculously easy to work on with ordinary tools and just the minimum of electronics to get in your way, but, boy, did Rover know why they made it so…

      1. Yes but engine comes out a subaru fast. Leaving gearbox in situ and removing the radiator for clerance i have removed the engine and pulled the sump off an ej20 in a gc8 in 56 minutes. That includes driving it over the 2 post lift. Engine out there lovely to work on and would rather do that than a gead gasket in an mgf in situ (id also drop the subframe and engine on an mgf) k series requires a lot of tlc to do gasket well and properly.

        1. Subaru flat 4’s are pretty easy to remove and replace. Made easier because they didn’t change the fitment to the transmission or the intake manifold to head flanges for a long time.

          Why that’s a good thing is if you don’t get a replacement engine from the same year you can simply swap the original intake on with all the crap attached to it.

          So a Subaru swap goes like this. Disconnect throttle linkage, fuel line and any electrical and vacuum connections. Unbolt and set aside intake manifold. Remove the two engine mount bolts and unbolt exhaust pipe flanges from bottoms of heads. Remove four bolts attaching engine to to transmission. Lift engine out. Some engines may have a torque rod between the rear plate and the firewall, so remove that if it’s there.

          If you need to replace the clutch it’s far easier and faster to pull the engine than the transmission. The last swap I did, a really big friend of the friend I was helping just reached in and picked up the disconnected engine. That one had the torque rod – the used replacement he bought didn’t. So we got a rear plate gasket and swapped rear plates, taking the opportunity to install a new rear oil seal.

          Any time you have an engine or transmission out where there’s a clutch, if it has a few thousand miles on it or more, it’s a good idea to install a new or rebuilt clutch disk, just to push off having to pull things apart to change the clutch disk later.

        2. I had various ‘flat 4’s (started with 1970 Beetle, doesn’t everyone?)
          You need to do an Alfa-Romeo ‘Alfasud’ have to lift body off engine/transmission (easy if you have a4 pot lift, very difficult for DIY’er) Worste clutch change I ever did (but, it drove over 200 miles with no clutch at all- locked solid)

      2. Weelll,…. If you have a lift, the rear frame falls out of the chassis pretty easily with rear axle, engine and gear in one piece and then you have all parts very nicely accessible. So removing the engine is not that much of work except that you have to open the lines containing the hydragas liquid. And it took me quite some time to find some means to refill and re-pressurize them again. Trust the Brits to add some fancy yet unnecessary quirk to every car they build. But then again, that’s why we love their cars, don’t we? So, yes, I can, but being a mid-engine, it is no fun.

        1. yeah full disclosure I own a garage (in the UK) so sadly do this day in day out so my perspective is a little skewed, but the engine comes out a subaru on the floor easy peasy, on the floor id do an mgf gasket in situ but as a job in the work i’d drop the engine/subfrme using the lift to save time and my back.. the hydrogas setup is annoying but in the UK the hydragas pumps are easy to come by most garages that have been around more than 10 years will have one as rover/brittish leyland used the system for a long time.

          the OP is going about this the right way, you learn by getting stuck in

  4. The old adages that say things like “double the time and change the units” (2 hours = 4 days and so on) are roughly accurate and (from experience) are worth allocating for. That way, if you get it done more quickly, then you have time for a cold beverage and some gloating about how you brought it in faster than you thought you would.

    The cost? Just double it. Then add a bit for specialty tools, sealants, lubricants and the parts you messed up the first time. All you have to do is look up one of those studies that shows what an assembled car is worth in separate parts to understand that you’re on the wrong side of the efficiency of mass production, and it’s absolutely more satisfying than sitting in meetings or worrying about the color of the cover on the TPS reports.

    1. “The cost? Just double it. Then add a bit for specialty tools, […]”
      The Handyman’s Motto: “No project is worth doing unless it justifies the purchase of a new tool.”
      B^)

      1. Or making a tool or using a torch and hammer to alter a tool – which you will only use once.

        I do have one 5/8″ open end wrench that’s twisted into an interesting shape, specifically so to be able to tighten the injector pipe nuts on a front drive Buick V6 diesel. That one has come in handy a few times in the past near-30 years since it got bent. Haven’t had to further bend it.

        1. I have a ‘lot’ of modified tools from when I worked on construction equipment. Ingersol-Rand with Duetz engines and various hydrualic’s (DYK, Maserati made hydrualic parts)

  5. One point about rounding off fasteners; those look like 12 point sockets. If so, you REALLY want 6 point sockets for heavy wrenching. 12 points for finesse situations where the nuts are hard to reach and your socket wrench has limited throw.

    1. I only made a (hopefully constructive) criticism but I want to congratulate you for your bravery. You’re doing and learning exactly the way I did. Dive in, screw up, learn. What’s the worst thing that can happen; you’ve wasted some money on a junkyard differential. It does help to have an experienced mentor around for things that are difficult to convey through Youtube videos. Knowing when to stop turning due to the feel of a rusted bolt before it snaps. Stuff like that. It’s the same way everyone learns a new skill, electronics, etc; jump in.

      1. Yeah, thanks for the tip re: sockets. These claim to be a special kind of “edge grip” thing that strips less but I imagine that’s marketing wank.

        I’m just glad to no longer be using my Chinesium set. Ugh. So much pain.

          1. Probably. The Chinese are perfectly capable of making excellent tools, and crap tools. Happily, Stanley did not pull a Sears and is still selling quality stuff though it’s from China.

            Would you look down your nose at an iPhone? No? So don’t assume all Chinese stuff is crap just because the cheap no-name Bunnings/Harbour-Freight stuff is.

  6. You’re not alone. All of us who have delved into the workings of our transportation have experienced the same; and often much, much worse. Salve your feelings with the knowledge that professional mechanics are usually just as bad, and invariably much more expensive. In nearly 50 years of car ownership, I don’t believe I’ve had more than a couple ‘professional mechanic’ experiences that ended satisfactorily. Either it cost way too much, the problem wasn’t fixed to my standards, or it took an inordinately long time…often all three. Most frustrating is the problem which they simply cannot properly diagnose, even after throwing hundreds of dollars of new parts at it (but which usually ends up being something disgustingly simple in the end).

    1. Sounds like Frank Zappa and his experiences with mechanics. The song “Flakes” was about that.
      “Flakes, flakes
      They can’t fix your brakes
      You ask ’em, “Where’s my motor?”
      “Well, it was eaten by snakes”
      You can stab ‘n’ shoot ‘n’ spit but they won’t be fixin’ it
      They’re lyin’ an lazy
      They can be drivin’ you crazy
      Swear to god they got the most
      at every business on the coast”.

      Quick oil change places are a lot like that too.

  7. Tell me about it. I recently had to rebuild the transfer case in my jeep. It was supposed to take a good day. It was sitting on jack stands in our shop for a month.

    Running the rebuilt transfer case for the first time was EXTREMELY nerve-wracking. Also you could feel the relief in the air.

    1. Yeah, there’s a lot of stress and second guessing when you finally get the car back on the road when you’ve just done a job that pushed the limits of your abilities…

      1. Indeed. I have never done major major work to a car, but I’m assuming it feels similar to when you first hit the power button on a 1kW audio amplifier you’ve just spent a few days assembling.

    2. LOL, I did transmission on Jeep Liberty (including taking transfer case apart -just for a look) Took a fair bit of time and was no where near as ‘easy’ as it was made to look on You-Tube. Got everything assembled correctly but it wouldn’t change gear (automatic) There are two 9 wire electrical connectors that ‘look’ identical and fit the plugs but get them wrong and it’s dealer time ($118.00 to swap connectors) Live and learn :)

  8. i did engine swapping and turbocharging and all that nonsense for many of my younger years. but these days I prefer electronics, because cars are entirely too much time, money, and space to be much fun anymore. now I only do the necessities or things that will save me money. I got quoted $800 to do a timing belt that I did in 10 hours.

    1. Because you provide no details :-( $800 for a professional to change a timing belt is quite reasonable if we believe it took you just 10 hours as a baseline. $800/10=$80 per hour “Burdened Labor Rate” (look up what that means please). Remember, your time with worth Real Money! And I’m not even considering the cost of parts.

      Of-course, the burdened labor rate plumets if the shop doing the work is using Illegal Aliens or perhaps H1-B Visa holders from a foreign country (both of which displace jobs for “Regular” U.S. Citizens). So maybe you didn’t choose your shop carefully – assuming lowest cost to you is your primary objective without Any other consideration.

      1. Unless they were doing it on a weekend when you don’t normally work and just sit browsing Reddit/gaming all day. Plus the fact you might not have gone out the night before and spent $200 to earn a hangover. Basically they earnt $800.
        Not saying $800 is unreasonable. If you have the skills to save $800 in 10 hours when you’d be doing nothing otherwise is time well spent.

        1. Just to add, sometimes it’s beneficial too. I was once quoted $500 to replace a radiator, I asked them the brand they would be swapping it out with (2004 Nissan with no OEM parts available anymore) and it was some known junk brand they can get cheap from suppliers locally. I decided to go with a “decent” aftermarket brand for about $300 which had to be ordered from interstate instead and install myself, which took no more than an hour (including disposal of auto trans fluid and coolant at the local drop off center).
          So while it might have cost about the same (including new fluid and time), had I not asked, I might be up for another radiator in a year or so.

      2. I have a rule for major DIY work and that is I have to save enough to pay for my time at $100 per hour unless I just really want to do it for the fun of it. The rule always works on a BMW or Mercedes.

        1. The fun factor is one reason I do all of my own work on my Bronco II, including an engine swap last year. Another factor is that it was my dad’s truck and my mom gave it to me when he passed away, so I feel like it’s honoring him to maintain it myself.

          For our newer cars, we’re better off getting work done at a reputable shop.

    2. $800 sounds about right. Assuming that includes about $100 in parts and likely a special tool or two. In my younger days things were different but now I’d gladly pay $800 to not have to do it.

    3. Yes, Pad-obi-wan,
      You just nailed why hot rodders are mostly fat balding men like me…
      The mind can experience the same thrill of adventure and accomplishment of hot rodding with programming skills and a soldering iron. With $10 dev boards, it is a LOT cheaper for youth to get started in a hobby.

      (okay, I’m being a bit tongue in cheek here)

  9. Reminds me of the three days it took me to do a waterpump in an old Volvo Brick. They just don’t do things–even simple things like this–the same way Americans do. Often it’s a better idea, but different enough to throw an old Missouri farm boy for a loop.

  10. I love this article, and the way you write, “…cold comfort to the shadetree mechanic with a full-time job”. Priceless.

    I have rebuilt/modified many cars and I guess I’d be considered a kind of rodder in your country (though all were street cars on the outside; we call them “street sleepers” here). Anyhow, part of the learning a few people have mentioned, and I wanted to comment on, is the whole ‘feel’ thing: no amount of net research will tell you when a thread is about to strip, or how to solve that problem, but experience will, in time. And the wielding of a hammer by an experienced mechanic (or fitter and turner) is just a beautiful thing to see—and that can’t be taught, only learned.

    Great article.

      1. Once or twice! I used to run what looked like a stock standard Volvo 2 series sedan, but it had a destroked 400c.i. Chevvy small block, long rods, Brodix track heads, Motec fuel injection, dual 3″ extraction system (which dumped over the rear axles) and ran the stock steel wheels (but widened, kept factory hubcaps) and the little drinking straw exhaust out the back—the car looked stock. It ran high twelves at the track, though. A very fun project, but definitely was not my first.

          1. If you can find one, get the turbo: they can be chipped for quite incredible performance and the station wagon versions were raced successfully (the tailgate on the wagon body gave close to 50/50 weight distribution, I was told). A very fun project!

  11. You know what, this article is brilliant and i’ll tell you why. People when doing tutorials on what they make or fixed,usually leave out their blunders, especially simple ones that are easily remedied. Its not the fault of the content creator, they just assume it would be boring to say “my socket set wasn’t up too the job” or “my shed didn’t give me enough room” but these little hurdles are usually faced by newbies in their own situations and they just blunder their way through and lose confidence that they know what they are doing. This article shows the nuuance that people who know what they are doing don’t show.
    Kudos man, big fan of your writing style.

    1. I agree completely. It’s great to see that other people face the same hurdles that I do… I’m not in a hurry with my hobbies, but every project takes 10x what I estimate.

    2. Thanks mate – this is exactly what I was trying to convey with this article! All the little things that don’t make it into the online tutorials that make it sound oh so easy. Glad you liked it :D

  12. This may not be the case with your handbrake, but with mine, on my old Triumph, I found that I had to lift the rear end up (so I could get to the adjustment mechanism) and then lift both sides of the suspension to roughly where they’d be when I was driving, and THEN adjust the cable length. If you do it with the suspension dropped, it’ll be too loose with the suspension loaded.
    (I also changed the cable pivot fulcrum point so it simply pulled more cable: means I have to pull harder on the ebrake, but also means it doesn’t drag at all no matter what the suspension is doing when disengaged, but does go to full tight when pulled.)

    1. That’s interesting – I may have to do something similar. I also suspect my cable is badly stretched, as the odo is reading 380,000km and when I got the pro mechanics to adjust it last time (and it worked) I noticed they wound the in-cabin adjuster to the full length of its travel.

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