YZF600 Front End Swap (Fork Conversion)

Information related to typical modifications performed on the Seca II.
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YZF-600 Front End (Fork)Swap

What You'll Need:
  • 1998-2007 YZF600R (Thundercat) front forks & triple tree (NOT R6)
    1998-2007 YZF600R front wheel
    1998-2007 YZF600R front axle, front wheel spacers and speedometer drive
    1998-2007 YZF600R front calipers
    1998-2007 YZF600R ignition switch
    1986-1990 VFR700 clip-on handlebars
    All-Balls tapered headset bearing kit, P/N 22-1004 (For Seca II)
Thinking it through:
  • The XJ600 fork (left side) measures 30 5/8" end to end. The YZF600 fork measures 32 1/2" end to end. This is a difference of roughly 48mm, not counting the height of the preload adjusters.

    The YZF triple has the same steering stem length as the XJ600 and, the YZF600 lower tapered steering bearing slips right into the XJ600 tapered race (if you've installed tapered headset bearings in your XJ, that is). The steering stops on the YZF600 lower triple are the same as the steering stops on the XJ600 triple. So bolting the YZF600 forks up to the XJ600 is an uneventful ordeal.

    The YZF forks are 41mm and will require the use of a different triple. I ended up using the YZF600 upper triple and gave up on the notion of using handlebars. For clip-ons, I chose to use 41mm clip-ons from a 1990-1992 VRF750. These are quit tall for clip-ons and work well with my back.

    The front wheel on the YZF600 is 17 x 3.5 and uses a 120/60-17 front tire. This has a diameter of 576 mm or a radius of 288mm. The XJ600 uses a 17x2.5 front wheel with a 110/80-17 tire. This tire/wheel has a diameter of 608 mm or a radius of 304mm. That is, the XJ600 front wheel stands 16mm higher (at the axle) than the YZF600 wheel with 120/60 tire. To restore stock geometry to the front end of the XJ600, the YZF forks need to extend 48mm further up in the upper triple. Subtracting the difference for the wheel, leaves 32mm. Both the YZF600 triple and the XJ600 triple offset about 5mm from the steering stem nut, downward; the XJ600 triple is 3/4" (19mm) thick while the YZF600 triple is 1" thick (25mm). All this verbiage basically means, the YZF triple is 6mm thicker than the XJ600 triple. This gives us an additional 6 mm of grab on the forks. Thus, to restore stock geometry to the XJ600, using the YZF front end with a 120/60 tire and YZF triples, the forks need to extend 26mm above the top of the triple. The preload adjusters extend another 12mm above the fork caps. It is not feasible to use handlebars with the YZF600 forks, unless using risers with a minimum height of 40mm.

What to do:
1. Remove your stock front end, including:
  • Wheel & fender
    Brake caliper and brake line (might as well remove the master too)
    Front fender
    Front forks
2. Replace your stock headset bearings with a tapered bearing set. Use the stock YZF600R tapered bearing as fitted on the YZF600 lower triple.

3. Install the YZF600R triple tree

4. Install the YZF600R forks. Set the forks so that the top of the fork tube sits 26mm above the top of the YZF triple.

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5. Trim off the stock alignment tang and install VFR760 clip-ons. Align so that the ends of them are approximately 1" from the tank on full-lock turn.

Here is my bike with the YZF600R triple, forks and VFR750 clip-ons fitted:

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6. Rewire the YZF600R ignition switch to work with the Seca II harness and install the ignition switch.

7. Drill alignment holes in the VFR750 handlebars, for the Seca II switches, and install the switches, clutch lever, master cylinder and grips. I had to add 3/4" spacers and extend the grips 3/4" outward to get all of the switchgear to fit the VFR clip-ons.

8. If using the stock Seca II fairing, install it and verify that the clip-ons do not interfere. Adjust as necessary. Note that I did this on a naked-converted XJ and cannot verify if there will be interference.

9. Fit the YZF600R front wheel. Use the YZF600R front axle, spacers and speedometer drive.

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10.Fit YZF600R, early R6 or R1 calipers and run new lines to your stock master cylinder. I chose to run my brake lines in series but parallel would be just as effective.

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11. Fit the front fender. Here's how the YZF600R fender fits:

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The YZF600R stock front fender is a dreadfully large affair and didn't look good with my N-converted XJ. I ended up using a Ducati Monster front fender and building custom mounts to adapt it to the YZF forks.. Here's the results:
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12. Finish assembly, verify torques and adjust your preload. Take it out for a test-ride.

Here's my finished bike:


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