Manual transmission leaking from the speedometer drive? Let’s fix it!

Nothing’s more annoying than a leak, and it seems our Datsuns have all experienced leaking from the area where the speedometer cable attaches to the transmission housing. Of course, gear oil gets all over your driveway or garage floor, the undercarriage, and makes a nasty mess. Let’s fix it once and for all.

So, typically, that leakage is coming from the actual speedometer drive pinion. It has two seals, and they wear over time. Mine, for instance, was a close-ratio 280Z 5-speed that I swapped into our 510, and it was leaking like crazy.

This is a task you can easily do using standard hand tools, and it’ll take you about an hour. You’ll want a pair of pliers, a pick (dental pick works good), a very narrow punch (you can use a nail with the point ground off), a 10mm wrench, a hammer, a pan, some gear oil, and some brake cleaner (to clean up your mess).

Get your car up in the air, supported safely on jackstands. Find where the speedometer cable meets the transmission housing (typically, it’s on the passenger side, towards the rear of the trans). Use your pliers to unthread the knurled end of the cable off the transmission. Your speedo cable should now be hanging loose under the car.

Remove the 10mm bolt holding the little tab. This secures the speedo pinion into the transmission case (you can see it in the top corner of the following pic). Grasp the threaded part of the drive and twist / pull it out of the transmission (don’t tear up the threads – if it’s stuck, thread your speedo cable back onto it, and grasp the knurled part with your pliers and pull it out).

Hit the whole thing with some brake cleaner – This will get the gear oil off and make it easier to work with.

You can also see that I’ve already removed one of the two offending seals (and the groove it was in).

Now, take your punch and drive out the tiny roll pin in the side of the housing. It’s located just below the nylon gear. You can see the roll pin (and the hole it came out of) in this pic:

Pull on the nylon speedometer gear, and the whole assembly will come apart.

Now, the toughest part – Get your pick or other suitable tool, and work out the seal inside the drive, being careful not to scratch the aluminum housing. Here you can see where the seal was located (and the tool I used to remove it):

Here are the new seals. I bought mine through Motorsport.

Here’s a pic of the new seal inserted:

Place the drive gear back in place and reinsert the roll pin. You can tap it in lightly with a small hammer.

Here’s a pic of the reassembled unit, ready to be placed back in the transmission. Coat the nylon gear and the outside of the unit with some grease, Vaseline, or gear oil (to avoid damaging the seal), and gently re-insert it in its hole.

Replace the holding tab and 10mm bolt, reattach your speedo cable (make sure the tab in the cable lines up with the slot in the drive), and you’re done. Check it after a drive – if you did a good job cleaning up the general area, you should find that the area remains clean and dry!

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