The front end swap:
One of the most difficult and time consuming part of this project was making sure what parts are compatible, and then sourcing the parts.
The conclusions of my research is:
Theoretically almost anything would fit, BUT you need to have a steering stem length of a length of at least of the original one, and not much thicker.
If it is a bit longer you could fabric a spacer shim to compensate for the extra length, and if it is a bit narrower/thick you’d have the change of finding some compatible bearings (there are a ton of models available). Slight variances in diameters are easily solved, as there are more than plenty different sizes bearings available.
If the stem is however too short you have the option of punching out the original stem and them pressing it into the new bottom triple tree (I wouldn’t recommend it, as it implies heating and/or material stress), doing some extending with cutting/inserting extra material/welding (absolutely not recommended), or fabricating the part from high strength steel (not recommended unless being done by a reputable shop using good materials- in which case it would likely not be economically feasible.
Unfortunately the head tube on the FZ6 is a bit longer than the ones from common sport bikes, so that doesn’t leave too many viable options.
Another issue is with the ignition lock position and the steering lock- unless you get it in the exact position as it originally was, you won’t have the steering lock functional, and you may also have rubbing between the ignition lock assy and frame.
So, without doing a crazy amount of fabricating, spending too much and/or risking part failure, the only option left is to use the stock FZ6 triple trees.
In my case, that meant sourcing a “new” one, since the original was bent. If only I knew all the above and had just gotten a working FZ6 triple tree from the start…
Next are the forks. Any 43mm fork should work, however keep in mind that when changing the forks you also might have to change the wheel axle (which in turn might mean different wheels/wheel spacers/wheel bearings/brake rotors).
Also, it should be calculated to have the same fork offset and similar total fork length, otherwise it is likely that it will mess up the geometry and possibly make the bike dangerous to ride.
Fortunately, the FZ6 uses the same wheels and rotors as the 03-04 R6 (warning- the 05 R6 axle was found to be too long by some) and the R6S model until 08, and also have the same offset. The only difference is in length, being shorter by about 1cm, but that is not really an issue, especially since the R6 forks are stiffer and might have a smaller sag, so the total length with a rider on the bike would be around the same.
This makes the usage of 03-04 R6 (and later R6S) forks a direct fit. If you are asking yourself (as I did), no the 99-02 forks cannot be used instead due to different offset, and neither forks from next models since those are upside-down.
Does that mean you can not put upside down forks on a FZ6? Sure you could and it has been done, however that implies dealing with the complications related to using a different triple tree, and from what I could find no one got the steering lock fully functional.
Wheels and axle:
An axle from the same generation 03-04R6 (or later R6S) is needed. The FZ6 axle cannot be used due to the different screwing mechanism- it screws in one fork leg, opposed to the R6 axles which are fixed by an outer bolt. You could fabricate a larger (longer an thicker) nut to screw in the FZ6 axle and fill the R6 leg, but it is not worth the trouble, an probably too costly as well.
Do note that the FZ6 triple tree is wider than the R6 one, making the R6 forks sit wider apart, that meaning you need to use some kind of spacers to accommodate for the larger width. Spacers are needed for the wheel and brake calipers.
The extra with also means that the R6 axle won’t fully fill (sit flush at both ends) the forks, but there are numerous long terms reports (5+ years) of there being absolutely no issue with this situation. The most common ways of dealing with this is to 1) make one side sit flush and have it ~9mm short in the other side, 2) try to distribute the axle more evenly, getting to a gap of around 3mm on one side and 6mm at the other (or even 4,5-4mm), keep in mind to adjust the spacers size accordingly, and 3) fabricate a cylindrical part that goes inside the screw and fills the extra space- IMO this is uselss.
Any road you take, the risk of failure is very small to none
Personally, I felt the safest distributing the axle as evenly as possible.
If you’ve got the S1 version FZ6 (04-07) brake replacement is mandatory, as the mounting holes on the R6 fork legs will not fit the original FZ6 calipers. This means getting a set of R6 calipers (same generation as the forks) as well. This alone, along with new fluid (hopefully you weren’t planing on reusing the brake fluid) will give a big braking improvement. For maximum braking improvement, also fit a R6 or R1 master cylinder, and steel braided brake lines. For now, I’ll stick with the stock ones.
If you got the S2 version FZ6 you are in luck, as your calipers are already 4-piston and identical to the R6 ones, so will fit right on.
The shims needed for the calipers are around 2,5mm thick.
Either the stock FZ6 fender will fit using appropriate spacer tabs, or the R6 fender can be used as well. Initially I planned to used the FZ6 fender, but as it was partly cracked all it took was a high speed ride on a Greek highway to make it crack and part ways with the bike. Luckily the bike was perfectly straight when it got pulled under the wheel.
So I ended up buying a new R6 fender. Only issue is that the 03-04 R6 wheels use 120/60 tires, while the FZ6 wheels use 120/70 tires. This means (in theory, at least) 72mm rim to tire outside height for the R6, and a height of 84mm, meaning that R6 forks + R6 fender + FZ6 wheels = 12mm lower fender to tire clearance than how Yamaha designed it. I do not think it’s necessary to “get back” all those 12mm but I want at least 6 more mm. It only measures a clearance of a few mm. At high speed the tire does stretch and increase it’s outside diameter, so you’d want more than that