How to renew the fork oil on the Yamaha Thundercat

You will need: good socket set with hex bits, a torque wrench, a rear paddock stand, a small jack.

First, go out and buy a litre of good quality fork oil. 5W is specified, but some people prefer 10W for a stiffer feel, or mix your own 7.5W.

With the bike on its side stand, remove both halves of the front mudguard. Slightly loosen the fork caps, the front axle pinch bolt and the front axle. Unbolt both brake calipers and tie them out of the way. Slightly loosen the bottom yolk clamp bolts – these can just about be accessed from below with the fairing in place by turning the steering to the opposite lock. Finally loosen the top yolk clamp bolts slightly. You may need to displace the bar clip-ons to access these. Set the preload adjusters on minimum (fully out).

** Mark the front each fork tube with a bit of tape so you can get it back in exactly the same orientation **

Now raise the front wheel just off the deck. The easiest way is to use a rear paddock stand then lift the bike using a small jack under the down-pipes (no need to remove fairings). If your down-pipes are badly rotten you will need a front stand — one that engages with the steering head, not the forks.

Unscrew the speedo cable, unscrew and pull out the axle and remove the front wheel. Note how the spacer and speedo drive fit.

Fully loosen off the top and bottom yolk clamp bolts, and the two smaller bolts that clamp the clip-on bars to the forks, and you should be able to drop each fork down out of the yolks and free of the bike. If they seem stuck, try twisting them first. You can use a flat-blade screwdriver to wedge open the yolk clamps, but take care not to scratch the fork tubes. Do NOT push them upwards as you will scratch the sliding area on the bottom yolk.

This is a good time to check the straightness of your fork tubes, using a straight edge such as a steel rule, or even better a dial gauge. More than 0.2mm runout is officially “bent” and you could have them straightened, but in practice even 1mm or so is fine provided you fit the forks back the right way (hence the tape).

Now the forks can be taken apart. First unscrew the cap assembly (beware there is some spring pressure on it). Release it from the damper rod by loosening the locknut and unscrewing it. Now you can withdraw the spring top washer and spring. Smells horrible, doesn’t it?

Hold the fork upside-down over an old container and gently pump it to drain out the old oil. It’s a good idea to leave it upside-down for a few hours to drain completely.

Meanwhile, measure the free length of the spring. It is 424.5mm as standard, if it’s dropped to less than 419.5mm it’s too short and you should consider replacing them. Slightly stiffer springs, such as Hagon’s progressive rate springs, can really improve the Cat’s front end.

Carefully measure 434ml of fresh fork oil and pour it in (421ml for 1996 bikes). Slowly pump the damper rod up and down until you can feel resistance all the way up the stroke. Now slowly pump the fork tube up and down, but by only three or four inches to distribute the oil.

Use a ruler to measure the air gap from the top of the fork tube to the oil surface with the fork fully compressed. It should be 124mm (137mm for 1996 bikes), if not add/remove a little oil.

Replace the spring, top washer and fork cap. Try to get exactly 12mm of threads showing on the damper rod below the locknut, and lock it. Screw on the fork cap but don’t try to tighten it yet.

Slide the forks back into the yolks, taking care to get them in the same orientation.

Pinch up but don’t fully tighten the top and bottom yolk clamp bolts, ensuring the forks are at the correct height. They are level with the top of the clip-ons as standard, but you can let them protrude up to 12mm to make the bike faster-steering.

Put the front wheel back on and pinch up the axle. The bike can now be lowered back to the ground.

Tighten everything up to the following torques settings:

Bottom yolk clamp bolts: 38 Nm / 28 ft-lbs
Top yolk clamp bolts: 30 Nm / 22 ft-lbs
Fork caps: 23 Nm / 17 ft-lbs
Clip-on clamp bolts: 13 Nm / 9.6 ft-lbs
Axle: 65 Nm / 48 ft-lbs
Axle pinch bolt: 20 Nm / 14 ft-lbs
Caliper mounting bolts: 40 Nm / 29 ft-lbs

Replace the calipers. If you have trouble getting the calipers back onto the disks, use a screwdriver to force the pads apart a bit first.

Reassemble the mudguard. Reattach the speedo cable. Reset your preload adjusters.

*** Pump the front brake lever until firm ***

That’s it!

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