Rev counter (tachometer) drive components

Got a mysterious leak on the front of the engine?

Oil is landing on the mudguard and the exhaust?

Can’t figure out where the hell it came from?

You too probably have a leaky rev counter (tachometer, tach) drive. On the XS750 triple, the tacho drive is famous for leaking. The guys on the Yamaha-Triples Forum know all about this and call it simply “The Tach Fix”.

When you first look at the rev counter cable as it enters into the engine on the cylinder head’s top right hand side (when sitting on the bike) you’ll notice a small fork shaped clamp and one screw to hold it on. Make sure that the drive unit doesn’t turn in the head when you loosen the rev counter cable off, and then remove it. Then remove the single screw, and take the clamp off, you should be able to gently prise the unit out.

This is what it looks like off the bike:

Drive unit removed from cylinder head

There are two obvious seals on the surface of the unit and I would recommend these be replaced as a matter of course. Any equivalent sized o-rings from the local hardware store should do the job. I recommend that you wear gloves to do this. Overheated rubber parts on any engine have a tendency to release poisonous substances (hydrofluoric acid), more so when overheated, so take care.

Now, the bit that gets most of us. There’s another seal! How many of you have changed the visible seals and it still leaked? Ha. I’ve been there. The hidden seal is in the middle of the drive unit. To get to this little blighter is easier than you think. I assumed that it was a little o-ring that some little guy with small fingers had fitted in the factory. Not so.

It’s a nitrile rubber oil seal like the ones on the engine cases. A very small one. It is 13mm (outside diameter) x 7mm (inside diameter) x 4mm (profile / depth), or as Yamaha list it 7 x 13 x 4.

It’s not always easy to get this seal. Yamaha list it as part number 93104-07036-00, and it’s on the parts drawing for the cylinder head. If you have the parts book, it’s part “45” in the 750E version.

Yamaha do actually still stock it in the UK and in the US.

To get at this seal, you need to hold the drive unit in a vice or other holding device, using soft jaws, or bits of aluminium. This makes sure there is no damage to the drive. The previous owners of my example didn’t do that.

Now, take one large-bladed screwdriver, and insert in the screwed section slot. It must span the width of the thread; turn anti-clockwise to remove it. Be very careful not to let it slip, as any damage at this point will make it difficult to achieve a good seal.

Once you’ve removed the threaded section it’ll look like this inside if the seal is bad:

Take a small screwdriver and very carefully prise out the seal. If you scratch the body of unit at this point you’re stuffed. Don’t damage it. Persist. A scratch will stop it from sealing. The old seal from mine looked a bit done. The seal on mine had all but collapsed, and after a little gentle persuasion to remove it, the seal looked like this:

The inside of the unit looks like this without the seal in it:

When fitting the new seal, gently press it in. It will sneak past the thread and land above the seating area. I used a washer on top of the new seal with a ┬╝ drive deep reach socket (10mm) to press in the seal. Be careful that it goes all the way home, then screw in the threaded section using a high temperature thread lock or gasket sealer to seal the threads. Refit it to the bike, remembering to fit the little worm shaft onto the camshaft.

Take care to fit the key into the fork, this way the oil feed hole into the rev counter drive shaft lines up correctly with the oil feed in the head.You’re done.