NA diff swap

  1. My model of MX-5 is: __1991 NA
  2. I’m based near: __
  3. I’m looking for technical help or recommendations on: __
    While recovering after changing the differential on my 91 V-spec, I’d like to pass on a couple of observations to anyone contemplating such a move…
    The used diff arrived looking as though it had been unearthed while excavating a Viking tomb.
    The shafts rotated however, without any excessive backlash or play, so I cleaned it up and set to.
    I fitted new side oil seals, knocked out the old rubber bushes, and fitted some polyprop road spec bushes from Bofi racing.
    The manuals say “Prise out the old oil seal with a screwdriver…” This seemed a good way to fatally score the aluminium casing and drop a load of crud into the diff itself, so I popped out the halfshafts and used a screwdriver with a sharpened end as a chisel, gently tapping around the seal until I had lifted it 3 or 4 millimetres. I then se-sawed a bit of string around to clean underneath it. (Pics).
    Then with the front wheels on ramps and the back on axle stands, the ordeal began…(I bought this car “off the boat” 19 years and 110K milesago , and AFAIK, the diff had never been disturbed).
    Despite a 24 hour soak in plusgas, undoing the two main bolts that secure the aluminium PPF “girder” to the diff required applying a 3 foot breaker bar and socket while lying on my back underneath with 6 inches clearance between me and it. The bolts were both seized, but cracked loose after some obscenities.
    Then the manual states “Screw a length of M14 x 1.5 studding into the forward bolt hole and pull down the sleeve…” well, good luck with that…mine was seized enough to resist a slide hammer.
    Unless you can free this sleeve, which should slide down far enough to clear the top spacer, there’s only one way to separate the diff from the PPF, you have to replace the bolts a couple of turns and tap gently with a hammer to knock the captive nuts out of the top of the PPF. You may need a mole wrench to hold them while you remove the bolts
    The manual says that this will render the PPF unserviceable, but whoever wrote that hadn’t tried getting the diff out of a 30 year old car…
    Before finally separating the diff and PPF, make sure you have supported the PPF. If you allow it to drop more than 4 or 5 inches, the engine cam angle sensor can hit the bulkhead because the diff acts as the rear engine mount. Once everything was loose, I was able to lift the nose of the diff slightly with my right hand and pull the PPF sideways just far enough to clear the diff, which was then freed from the subframe (6 nuts) and lowered on a trolley jack.
    There are any number of videos on Youtube showing this being done, it all looks so easy, and it is if everything comes apart as it should…but it may not.
    If you can’t separate the diff from the PPF in situ, then just undo the PPF from the gearbox, having first supported the gearbox, removed the wiring clips, earth wire, and speedo cable.
    Re-assembly was a doddle…(everything had been cleaned and greased)
    The cuts are healing, the bruises and contusions are fading, there’s no oil on the drive, and I now have some slip and decent gearchanging.

T/pete

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My dad constantly used to remind me that, "Nothing worthwhile is ever easy. "

Seems to me that the corollary is mostly true, in that the hardest things are often worth doing.

Sounds like you’ve achieved something worthwhile there and I’ll tip my hat to you for sticking at it and bringing the job home.

Thank you for your comment…I agree with your Dad! Sounds like we’re of a similar generation…



Here are the pics that I forgot to add.

And I forgot to mention that it now goes round corners as it should.

And if anyone knows of a competent re-sprayer in the Southampton area, I’d be pleased to hear.

T/pete