Hopefully you have read through my other posts explaining why I'm doing this so I'll try to get to the point.
The midget 1500 is under-geared. It never had the overdrive it deserved when it inherited the triumph engine and gearbox and so there are lots of aftermarket kits involving Ford gearboxes, angle grinders and bent gearsticks. I prefer to tackle the problem using the special 3.55 diff that was on offer by the special tuning division at Abingdon in the 70s.
Before you even think about changing the diff ratio check that you have the later type of housing. Lots of suppliers will offer recon diffs for the later type but few (if any) will do the earlier one. How do you tell? Well..
How do you tell which ratio you currently have?
There are two ways; the first is to look on the casing to find the number of teeth eg 9/38 which would be 4.2:1. (just divide the second number by the first). This has a problem as the Crown wheel and pinion may have been changed and numbers not altered. The best way is to look at the numbers stamped in the crown wheel itself which obviously means taking the diff out first. Or if you fancy doing the maths use a GPS speedo app, my revs/speed table from previous posts and a clear straight national speed limit road.
Getting your hands dirty
Jack the rear of the car up AND FOR CRYING OUT LOUD use multiple axle stands; you will be pulling and pushing and swinging on the car. The Midget may be light but it will still squash you!
Make sure that you leave enough room either side of the car to pull out the half shafts to change them if you have to. Though they only need to be pulled out about six inches to let the diff out.
Check the half shafts for spline twist and damage. Don't swap them over as you will encourage breakage
To do this;
Pull the handbrake on and remove the wheels.
Let the handbrake off and remove the two screws that holds the drum in place. It should then be possible to pull the drum off (with a little persuasion) You may have to loosen the square brake adjuster on the rear of the drum and obviously this will need to be adjusted back after the job is finished
Its a good time to check the brakes for wear and leaks.
Undo the halfshaft fixing screw and slide out the shaft. Don't get oil on the brake shoes or you will need to replace them.
Use the proper tool to drain off the diff oil (Its a tapered socket and is easy to break. Ebay special)
Now put the car in gear so that the prop shaft won't turn and undo the bolts between the diff and propshaft (good luck! You will need WD40, one ring spanner and one open ended spanner)
Now remove the brakepipe splitter that is bolted to the top of the diff and remove all eight nuts holding the diff in place. (The top one is the hardest to remove as the others probably have had oil on them)
Now carefully slide the diff off the studs.
Fitting the new diff
Obviously refitting is the reverse but I would recommend that you fit new gaskets, bolts and nylocks and rubber O ring seals on the back of the half shafts. Refill with the correct oil. Don't be tempted to over fill the oil as it can get past the halfshaft seals/ bearings. Use a little anti-seize grease on all nuts and bolts. You will be so glad you did.