XJ650 to XJ750 front end

I've got an '82 XJ650 maxim and a complete Seca dual disc front end that I picked up with the bike as well as an extra front wheel with a larger discs, which I assume is from a 750.

I've been told that a Seca front on a Maxim would be a bit twitchy and unstable. I was looking at picking up '82 XJ750 forks, but I am not sure what the best way would be to go about swapping parts over. I'm not sure if the triple tree is from the Seca or 750 Maxim. Will the 750 forks swap over to the 650 parts?

Should I just replace the seals and oil on the 750 forks and swap them into the spare front end I have?
Swap them into the installed front end, then move the brakes and stiffener over?

 

I wonder why someone told you the Seca front end would be unstable and twitchy? The Seca doesn't share the reputation of being unstable.
I would not hesitate to make the swap, I would also do the maintenance, new fork oil, & seals before the swap, and probably new headset (tapered roller) bearings.
Cheers, 50gary

 

They (whoever "they" is...) might have been referring to the XJ650 Seca front end, which mounts the axle under the fork tube centerline, rather than IN FRONT OF the fork tube, like the XJ650 Maxim, XJ750 Seca, and XJ750 Maxim's do. The displacement of the "pivot point" due to the axle position, in combination with the rake created by the steering headtube angle (in relation to the frame and the wheels), can create a more sensitive response to steering inputs.

The XJ750 Seca tubes will work fine (in this regard), but they are a somewhat "complicated" fork tube system (some would say "very complicated") due to their anti-dive feature. Both the upper and lower brackets will need to be swapped for your XJ650 pieces (unless you like doing an incredible amount of fabrication work that's not really necessary....).

A more suitable donor fork set would be off of a 1982 XJ750 MAXIM model, or even more preferrably.....because the '82 750 Maxim forks are also a bit "complicated".....are the 1983 XJ750 Maxim or Midnight Maxim forks, which will bolt right on (brackets and all) and are the type of "simple" forking system as what is already on your XJ650 Maxim.

 

Chacal's got it.

The steering geometry will be different with a 650 Seca front end as the axle is under the forks instead of forward of the forks.

I was referring to an 82 Maxim 750 front as it lacks the complicated anti-dive system, which is not present right now anyway (and it does dive a bit). I am hoping the stiffer spring rate and a refresh will compensate for the current dive the front end does have.

It will basically be swapping over the forks, stiffening bracket, braking system, rotors, and the balancing tube?

I've got two spare front wheels, each with different sized rotors, so I'm assuming I will have at least one set with the correct diameter.
Is there a difference in the master cylinder and calipers between the 650 Seca and 750 Maxim?
Is the increased braking that is required for the 750 compensated for by a larger rotor diameter?

Thanks for the input.

 

XJ650 Seca and XJ750 Maxim rotors are 10.5" OD, but the 650 Seca rotors do not have any cooling slots in the rotors, while the 750 Maxim rotors have the air-cooling slots.
XJ750 Seca rotors are 11.75" OD and also have the air-cooling slots.

Both use a 16mm bore, but the m/c's are completely different in orientation, etc. XJ650 Seca m/c's are designed for use on "flat" handlebars, and the mounting bracket is cast so as to give the reservoir a "flat" (horizontal) orientation when mounted on the bars. XJ750 Maxim m/c's are designed to be fitted onto "curved" bars, and this the mounting bracket is designed to "tilt" the reservoir to a level orientation when mounted onto curved handlebars. 1982 and 1983 XJ750 Maxim m/c's differ only in the presence or absence of a fluid level sensor.

Calipers are very different between XJ650 Seca and any of the XJ750 models.

Indirectly, perhaps. The larger rotor size allows for more heat dissipation during high-speed or repeated braking applications.. The XJ750 Seca, being a "sport" bike, might be anticipated to be ridden in a more aggressive manner, requiring more rotor area to throw off the extra heat generated by repeated hard braking actions.