Installing
a 1.8lt MX5 differential into a <94 1.6lt
Why?
The 1.6
differential is a known weak spot in the drivetrain.
Hard driven cars with no mods can grenade these diffs and cars with power mods will
certainly be capable of this! Within 4 weeks of buying my Roadster my first
diff went (due to oil starvation from a cracked case) and 6 years on, my 2nd
one gave up the ghost and this was accelerating in a straight line. So, to
avoid this problem again I decided to upgrade to the 7 inch ring gear which is
in the 1.8lt differential. This is a larger unit and is fairly rugged. In the pic below you can see 1.8lt unit on the right.
It is both
longer and wider so you will also have to change the halfshafts
and driveshaft to 1.8lt units.
Procedure :
First, buy
your parts.
1) 1x 1.8lt
differential (this project sourced it from www.traynors.co.uk
, good service!)
2) 2x 1.8lt
Halfshafts (same source as 1, if you have ABS, make
sure they come from an ABS car as it’ll need the grooved end for the sensor,
see pic)
3) 1x 1.8lt
driveshaft (same source as 1, see pic for differing
lengths, lower one is from the 1.8lt car)
4) 2x hubnuts (Mazda)
5) 2x
differential output shaft oil seals (Mazda)
6) 2x halfshafts C-clips (Mazda)
7)
Differential oil & Gearbox oil (I went for the Redline brand from www.FlyinMiata.com)
8) Exhaust
gasket.
Now, give yourself
a couple of days to do this and try and have someone help as it’s a pig of a
job on your own (believe me… I KNOW!).
1) Night
before, WD-40 every nut you think you’ll have to remove (bar the driveshaft and
halfshaft nuts at the diff as they’re not very
tight).
2) Put the
car in gear and put the handbrake on.
3)Bang
out the staking on the hub nuts (the bit that’s bent inward to stop it coming
off) with a dot punch + hammer. Get a LONG breaker bar to put over your
ratchet’s handle (29mm socket required BTW!) and pull! If the car moves, get
someone to put their foot on the brake. Mine were REALLY tight and needed an
airgun used to remove tractor wheels to loosen them (friendly local mechanics
come in handy here).
4)Loosen
wheel nuts.
5)Put the
car on axle stands under the jacking points on the sills.
6)Remove
wheels.
7)Remove
Brake calipers (a good time to think about changing
the pads and greasing the slider pin!) and ABS sensor (if fitted) and tie out
of the way with cable ties. Remove hubnuts.
8)Remove
the exhaust’s backbox (or from the cat back if you
can to give more room).
9)Remove
brace between lower wishbones.
10)Unbolt halfshafts from the diff’s output hubshaft flanges
(12mm)
11)Unbolt the
driveshaft from the diff (you’ll have to take the car out of gear to move the
shaft round half a turn to get to the other nuts, then put it back in gear so
you can turn against the nuts)
12)Pull the halfshafts out of the
hubs, if they’re stuck, get a lump hammer and give them a couple of whacks,
they WILL come out so be persistant. (ignore the hammer in the pic ;-) )
13)Put a trolley jack under the diff and jack it up so that
it’s just touching the bottom.
14)Unbolt the 2x17mm bolts that hold the Power Plant Frame to
the Diff.
15)One of these bolts had a “collar” on it, in the base of the
collar (bit nearest the PPF) there are 2 grooves in which you can insert the
tip of a screwdriver to lever it out of the PPF.
Be careful
from here on, try not to pull the PPF downwards too much as you don’t want to
damage the firewall nor the gearbox (where the other end of the PPF is bolted)
16)Remove the 2x12mm bolts on each side of the diff carrier’s
arms, then loosen the 2x17mm bolts, be careful not to drop the diff on your
head! This is where having someone with you can be handy ;-)
17)It will be still attached to the PPF by a collar on the
upper part of the PPF so with this in mind, try and lever out the bottom of the
diff from the PPF and unbolt the 12mm bolts holding the spacer between the diff
and the PPF, then carefully remove the diff while lowering the jack, it takes a
bit of fiddling to get it out. Again.. the diff is heavy, so keep the jack under it as much as
possible.
18) Drain
the gearbox oil
19) Drain
the old oil from the 1.8lt diff (you’ll need a 23mm diff for the fill hole and
a 24mm for the drain hole)
20)
Pull/slide the driveshaft out of the gearbox
21) Put in
the new driveshaft (it’ll just slide into the rear of the gearbox)
22) Now you
need to take the stub shafts out of the new diff as they won’t be needed with
the one piece halfshafts you should have received.
This is easily accomplished by using a couple of levers to pry them out of the
diff.
23) Renew
the oil seals in the space from where the stub shafts were removed. (below is a view thru the diff)
24) Put the
new diff on a trolley jack and roll it in under the car. Jack it up and tilt it
back so that it slots back into the PPF (again, this can be fiddly, trick is to
try and get the top in first, then the bottom while lining up the carrier arms
17mm bolts) Be careful not to let it fall off the jack!
25) Screw
on the 17mm bolts to hold the diff in place, then
replace the 2x12mm bolts. And tighten.
26) Replace
the 2 long 17mm bolts not forgetting to replace the collar that was levered out
earlier.
27) Bolt on
the driveshaft to the diff (you can reuse your 1.6lt bolts + nuts here) using
the same method of taking the car out of gear and moving the driveshaft as you
did earlier.
28) Unbolt the upright from the control arm so that you can pull
it back from the car
29) Fit
your new c-clips to the diff ends of your halfshafts
being sure to line the “open” bit of the clip so that it’s facing upward before
inserting into the diff. Tap it fully in using a rubber mallet on the end of
the halfshaft. Try pulling it back out to ensure it
is seated properly.
30) Pull
back the upright and insert the (now copper greased) splined
end into the hub.
31) Bolt
the upright back onto the control arm (you may need to jack it up a bit to get
the bolt holes lined up again)
32) Replace
the ABS sensor and brake calipers (don’t forget the
copper grease on that sliding pin!).
33) Bolt
the rear suspension brace back on.
34) Refill the
gearbox oil and differential oil (upper most bolt is the fill hole!)
35) Admire
your work ;-)
36) Fit new
exhaust gasket if required (mine was a slip-fit) and replace backbox.
Job done!
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