Moss Miata

Changing the Rear Differential vent

by John Collings


Q - What the heck is it?

A - The differential vent relieves pressure that builds up in the differential during normal driving.

Q - What happens if I ignore it?

A - It can build up with gunk over time and become blocked. This will lead to seal failure in the pinion seal or axle seals due to excessive pressure buildup. If the seal failure is not caught in time the loss of oil in the rear differential will lead to differential bearing and gear failure. (Remember the rear differential only holds 1 quart of oil!)

Q - How often should I change it?

A - It may be a little overkill but I change mine once a year when I change my differential oil. For the sake of about $15.00 for the vent and $20.00 for a quality synthetic 75w-90, $35.00 a year is a small price to pay to prevent a potential $1500.00 repair.

Q - How difficult is it to replace?

A - If you have a spare 15 minutes and can remove a bolt and re-install it, you too can do this!

Tools Required


1 - 14mm box end wrench, yup that's it!

The Procedure


The vehicle does not need to be put up on jack stands to perform this repair, although it will give you a little more room. Please make sure your vehicle is safely secured if you do.


1 - Locate the differential vent as shown in figure 1

Figure 1

2 - Slip the box end portion of your 14mm wrench over the cap and down to the base of the vent. The 'nut' section is located on the base. (See figure 2 for 'nut')

3 - loosen the vent, usually you just need to break it free and you can remove it with your fingers. The cap should spin freely so make sure you grab the 'nut' section.

Figure 2

4 - Grab your new vent and install it in the reverse order, and that's it! Now that was easy!

Back to the Garage

17 April, 2010

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