Decided to replace a few things this weekend, being the fuel filter, O² narrow-band sensor and the differential axle-shaft seal. The car was sometimes idiling roughly and blowing black smoke when stomped on… so me thinks the O² was original (they only last about 6 or so years). The diff was leaking and you would have noticed this on my photos of the new exhaust system – the smell of burnt diff oil isn’t pleasant!
Fuel Filter: was fairly straight forward and I followed the well documented guide on Miata.net here > http://www.miata.net/garage/FuelFilterChange/index.html
Oxygen sensor: was also simple, but a bugger to get at due to where the new extractors and exhaust system had relocated the sensor – in small space between the gearbox and the body tunnel – not easy but I eventually removed the old sensor, cut the wires and solded a new sensor in-line. After starting the car noticeably idles smoother and was more quickly able to reach a stable idle when first started. I’m expecting my fuel economy to improve also… as it was kinda thirsty (and that isn’t just my right foot!).
Diff Axle-Shaft Seal: completed in about 1 hour and was easier than it seems. As I couldn’t find many guides with pictures, thought I’d add some here.
Step1 – get the rear in the air (the car that is) and take off the wheels. Remember to loosen the wheel nuts before you raise the car. It is way safer to strain on the wheel when the car is on the ground – once loose, you can raise the car and place on stands. I used axle stands, but did so near the jacking points – this let me work on the diff/axles without interference. Now is a good time to drain the diff oil and let it drip out whilst doing everything else.
Once you’ve removed the wheel and undone the four nuts (14mm) attaching the drive shaft to the diff flange you will realise you can’t remove the shaft off the flange unless the wheel hub can move away. Be careful when pulling/pushing on the shaft and flanges – the CV joint ends can pop open and it is possible for the ball-bearings to fall out etc.
Step2 – To make it easier to get the axle shaft off the diff flange, you can just remove the top nut and bolt that attaches the hub to the upper wishbone A-arm. If you can’t get the bolt out, just raise the hub slightly releasing pressure on the top bolt. Once the bolt is removed, you should be able to now pull the hub out slightly enough to release the shaft from the diff flange.
Step3 – The flange is securely held inside the diff, retained by a circular c-clip. It requires a lot of leverage to push the flange out, and I found using a lever between the flange itself and the diff housing to be the only way. After a lot of swearing and cursing, it eventually popped out. Be careful here not to damage anything, but in saying that, I exerted a lot of force and all was OK.
Step 4 – With the flange removed, carefully clean the edges around the diff housing being careful not to let any dirt or grunge enter the diff insides. The flange can be wiped clean where the seal would be seated. You can see the flange with the new seal. You can now also remove the seal attached to the diff. I used a flat blade to slowly edge the seal out, careful again not to scar or mark the diff housing.
Step5 – Now that the old seal is removed, grab the new seal and coat edges with fresh diff oil. Use plenty of oil and even consider coating the flange at this time, at the areas where the seal would rub/seat. Insert the flange into diff housing and use a block of wood and hammer to squarely locate back into the diff. Make sure the seal is fully homed and flush against the diff housing.
Step6 – Final steps are to insert and locate the flange back into the diff. Push as far as it will go, and then use that wood block and hammer to drift back and locate the clip. It should fall flush like before it was removed. Reassemble everything ensuring you remember to tighten securely and replace the A-arm top bolt. Replace with clean fresh diff oil.
Easy 
