'97 Mk1 MX-5 V6 - Weights and Measures

Just found this thread!

An excellent resource for my own project doing the same but to a Chrysler Sunbeam.

Glad you have found some of the pitfalls first...... :-)

Not to hijack or anything...

Boring is a relative term, it was supercharged with a eaton m90 from an aston martin DB7, the sound, the throttle response was unreal. 6.3psi and 229bhp.

But turbo is 0.1s quicker to 60 and just works reliably so meh.

Maybe have a look at RX7 T2 diffs, ive heard there the same, but a plate style diff. RX7's tend to do less miles due to the thirst so may be worth a look.

Stay safe everyone!

At least it didn't just snap off like mine did when I went to adjust it after a winter of not being used. I do have some spare mirror innards, I'll see what I've got.

I was thinking of getting some Raydot mirrors from Moss but they won't fit my early doors.

I spent a pretty chilled weekend trying to paint the mirrors nicely... I'm very much still learning as I go with painting, so the mirrors seemed a good little practice job. (and some of you may have seen my 'help!!' posts in the body and paint thread!) I have a load of boring pictures of semi painted mirrors for you...

So mirrors off and the rough areas sanded down / all keyed for primer (pic half way through the job, you can see how poor the old mirror was):



A coat of primer all round, left sanded down with 800 grit, right was the finish out of the can:



Now Jowel had a 3 for 2 offer on the UPOL cans... but that didn't include a proper basecoat. They recommended the satin black as a basecoat to then clearcoat. The satin black finish came out as:



Which looked pretty good to me... Clearcoat over it didn't come out so nicely though!



At this point I asked for some help on the body and paint thread... the advice was that the paint looked too thick and dry, and it needed to be lighter and wetter. I'd always equated a thick coat to a wet coat before so spent a little time puzzling, and realised that I'd been building the 'wet' coat with lots of fast passes as I felt that gave me more of a feeling for when the paint would start running. In reality it was building up a rather pitted coating. I ended up sanding back the satin layer on the mirrors and starting again with much slower passes, giving my 'wet' layer in a single pass, and the result was far smoother paint!

The other comment I got was 'why are you using satin?'... Despite Jawel recommending it, I don't think I'd do it again - the clear finish over it isn't great.

This was the clear on the second attempt:



Improved but not great. I was probably a little too impatient and about 24 hours later hit it with a little 2000 grit sandpaper and some G3 polish and was rather pleased with the results:



Unfortunately that's gone a fair bit duller again, so I'll try the polish again after it's had a week or two to fully harden.

Then gluing the mirror back together... Previously the glass had fallen back into the mirror housing a little bit. From the factory there is a bit of foam tape behind the mirror glass holding it forwards, but it's now a bit too thin to do the job properly. I replaced that with thicker tape that should do nicely:



A friend with a welder tacked the bolts into the metal plates in the mirror to replace the nyloc that was spinning, then I glued it all together. I didn't want to use an epoxy in case I wanted to get inside together so at the moment I've tried with 'Shoe Goo', which is a rubberised plastic glue that works very well on shoes and trainers, and I happened to have a tube of it  handy. It seems to have worked.



The little red bits were painted with a proper basecoat and had a far nicer out the can finish  that I completely failed to get a photo of!

And all done, a car with black mirrors:



Now I need to paint the windscreen surround black too... Not sure I trust myself to do that nicely yet though. Might practice by turning the sills red and have a go at tidying up the rear arches with a little filler. They need re-doing anyway so seem a good area to practice on.

Enough paint... I also spent some time tracking down the various rattles in the car. The main heatshield rattle turned out not to be a heatshield but a panel in the boot that covers the fuel tank filler neck that was a little loose. Two bolts torqued up and the biggest noise in the car has gone! There is still a rattle in the drivetrain under low speed acceleration and higher speed deceleration - this sounds exactly like the PPF misalignment noise which people have limited success in fixing. Definitely needs fixing, although it seems to be slowly fixing itslef - either that or I'm subconciously driving around it.

Finally, I got a lot more awl practice in... The rest of the previously repaired seam in the soft top decided to give up completley as I zipped the window back in one evening, the only bits holding the rear window in were my previous repairs!




Gah. Mostly finished stitching it back up, I'll get a photo once it's complete. So far two of the three main things that I thought made up the cars value when I bought it have failed - torsen and mohair roof! Maybe this car wasn't such a good buy after all!   The brakes were the third thing - they feel really nice but currently squeal like hell - need to get some copper slip in there.

I see what you mean about the car looking ill. It's not the headlights though, it's the offset number plate. It looks like a thermometer in it's mouth.

I like the lights and would keep them if this car was mine. Maybe get a square number plate fitted? (Like in the below photo).

My brother in law is very good friends with BOFI and has pretty much a catalogue build with their 3.3 diff and 6 speed...and many many go faster bits. If you follow them on Facebook or Instagram it's the nardo grey one they're featuring at the moment. I can't remember if the 3.3 was in it when I last drove it but he absolutely loves it and says it's by far one of the best additions. If you have any questions send me a PM and I'll pass on the message or put you in touch.

Following this with interest, always had a V6 swap in the back of my mind. I've now committed another chunk of money to mine so when finished all I'll have left to do is a respray and engine work...seems silly not to

I had a quick chat with BOFI and they haven't found any whine from the 3.3 C+P in any of their builds which is encouraging. The cynic in me says that 'they would say that, they're trying to sell them', but they also said that the blackline diff has had some complaints from customers. The splines on the MX5 driveshafts have a loose circlip around them about a 3rd of the way in which locates in a detent in the splines in the diff, holding the driveshafts lateral position. The blackline diff doesn't have this detent in the diff output splines, and as such the driveshaft isn't held laterally and ends up pushing too far in and the driveshaft scrapes against the diff housing. I imagine a PET or Nylon washer between the driveshaft and the housing would get rid of the noise and as it shouldn't be loaded might even last decently, but it puts me off the blackline diff a little.

I dismantled my dead T2 Torsen over the weekend to see what I could see... (Pictures as promised scimitarse6b )






I wasn't sure what to expect but from the reading up I'd done on the CROWN and pinion contact patch, this was bigger that I'd expected:




The contact patch appears to cover most of the face... As the backlash was in spec I guess the wear wasn't too bad though.

Once the centre was out I had a go at pulling off the bearings. The design of the diff only allows a 2 prong puller, and I only have a very cheap set of 2 prong pullers.




I've got the centre of the puller located on a deep groove ball bearing (I think it's an old spigot bearing from the 944). This gives a good axis for the puller shaft to rotate on and helps stop it wondering off to the side as 2 prong pullers so like to do.

Somehow I don't think they were ever designed for use with a breaker bar though...



That went with quite a bang! 

Much to my surprise I flipped the puller arm around and actually managed to get that bearing off. The opposite one was definitely not budging though and the thread on the puller rounded before it budged.  Now I need to buy a more expensive puller! Bearings only need to come off if I'm replacing them though - and in hindsight I'm not sure I needed to pull it off at all. I can keep pulling the rest of it apart with the  bearing in place.

CROWN gear off:




Fine threaded bolts with lots of locktite - took the breaker bar just to screw them out.

Two more little screws to come out that were JIS heads instead of philips. Luckily I have a set of JIS bits as they were very tight and I wouldn't be surprised if a phillips just rounded them.

So the magical insides of a T2 Torsen diff. Apologies for the blurry pictures - oil on my phone camera lens again :/













So, how does that lot work? When you google torsen diffs, they come up with quite a different internal structure. I may be talking out my [fox picture] here, but my understanding was that Torsen is just a trademarked name (TorqueSensing) and this diffs workings are more inline with every other ATB (Auto Torque Biasing) diff out there.  When both wheels are spinning at the same speed the helical geared shafts aren't spinning relative to the output shafts. When there is a speed differential, these shafts start turning, and due to the helical nature of the gears force the output shafts towards the wheel that's spinning faster, where they push against the square patterned bronze(?) bush, generating friction effectively braking that wheel and sending more torque to the slower wheel as a result.

So, why is mine creating a load of driveline lash and sounding like a bag of spanners? I've no way of measuring the wear on the helical gears, but they all look in decent shape, no missing bits. The only way I can see of generating lash is if the output shafts are moving too much laterally:



And for that to happen, the possibly bronze bushes at each end of the diff would have to be worn.



From the pictures it looks like the loaded centre sections of the bushes look more worn than the locating 'ears'... I'd hazard a guess I've found my problem but as I worked all this out by sleeping on it I've not got back out to the garage to take a second look at the parts.

So my options now... I need to measure the worn section in comparison with the unworn area to give me an idea of how worn the bushes are compared to original spec. I'll try this with my cheapy DTI, but I might also be able to make use of a proper measuring table and calibrated DTI at work.

I don't think I'll be able to find a spec for the exact alloy those bushes are made from, which I imagine is quite important to get the correct coefficient of friction between the surfaces, so having new bushes machined might be difficult, although I imagine I could take a stab at material choice by looking at what other ATB differentials use.

The easier option might be to add a shim behind the bush. There is already one shim there. If I can add a shim to each side equal to the measured wear on each bush my head says 'job done'. Might need to then take down the material on the locating flanges to stop the helical geared shafts catching on them, but that's easy enough.

The issue is that there's no spec I can find for this. On one forum post I found the nugget that the clearance inside the pumpkin with everything assembled should be 10-15 thousandths, so I'll see if I can work out how to measure that...

I'll get some measurements and talk to a friendly machinist and get a second opinion.


Then I need to work out if I want to go through all the faff of setting the backlash and refitting this diff to the car to test a fix that may or may not work! 

When I had a to get a shaft out of my XJS which resisted every puller I could find I got hold of an old steel wheel and the biggest nut and bolt I could find (around M30) welded the nut in the centre of the wheel, bolted the wheel to the hub and tightened the greased M30 bolt up as tight as I could, smacked it with a lump hammer and it moved a tiny bit, kept repeating this until it came free.