XJ750 J Maxim fork assembly

Finished fork assembly and figured I'd add some photos of the parts specific to the 750J,which I think is the only model using the adjuster knob on top of the fork for rebound damping control.

This is my first fork rebuild, so I'm not attempting to give a general rebuild procedure. That has been documented much better and by more experienced people than me on many other threads. However, I was previously unable to find details for this model with the adjuster knobs, so maybe it will be helpful to somebody else.

Here are the parts laid out. Everything is fairly familiar to all models, with the addition of an adjustable orifice at the top of the damper rod, the long half-moon shaped rod to change the orifice selection, and the unique top plug that screws into the fork stanchion rod and includes the adjuster knob.



Here are the parts that go in the damper rod. The short barrel is the part that rotates to select the orifice. It is drilled with 3 different orifice sizes, with the 4th selection being a blocked port. The small spring and ball are for position detent in the damper rod. Three of the detent holes can be seen in the damper rod above the groove for the piston seal. The larger hole below the groove is aligned with whichever orifice is in position.



A closeup of the barrel piece with the 3 orifice holes. The larger hole for the detent ball and spring is at the top.



Here is the selector piece installed into the damper rof and retained with a clip. Note the half moon shaped hole that accommodates the long control rod.



Here are the parts for the plug at the top of the stanchion. A smaller o-ring (2.5 x 11mm) to seal the control rod inside the plug, plus the round retaining clip. The wavy clip retains the control rod from the top. Larger o-ring (3.53 x 25.8mm) seals the plug to the stanchion. Adjustment knob indicating 4 positions and its retaining screw.

The knob has 4 numbered positions, with #1 selecting the largest orifice for the least amount of rebound damping. The numbers are not indexed in any manner, unless you get tricky with how you position your control rod when tightening the bolt. Essentially you just turn the knob clockwise through about 180 degrees till the first detent catches, and this is position #1.



A view of the plug from the bottom (threaded) end. The round retaining clip sits in a groove about halfway down. A word of caution! There is a taper on the bottom of the control rod piece which expands this retaining clip gently into its groove during assembly. However, you are fighting a square edge of the groove in the removal direction. One of these disassembled nicely for me. The other was not so nice and I ruined the end of the control rod. Fortunately I had a spare set of forks. You certainly can do a standard fork rebuild without disassembling this piece, but it is necessary to replace the smaller o-ring which will allow your pre-load air to escape if faulty.



And finally, a bit of reassembly, 257 cc's of oil and one fork done. This 100 mL graduate was very handy for measuring.



Done, with new seals, drain plug, retaining bolt and piston seal. In lieu of the special tool to hold the damper rod while torquing the retainer bolt, I used a chunk of 1/2 all-thread, double nutted on each end. The 3/4" nut width fit nicely. I didn't do a professional polish job on the aluminum, but sanded progressively up to 1500 grit then hit it with the buffing wheel. Kinda shiny... Good enough.




Then I got more curious about the specifics of oil flow. As I mentioned, I had another set of forks. The chrome was rusted up enough that I didn't mind some destructive disassembly. In the bottom of the stanchion there is effectively a check valve to allow for different oil flows in compression versus rebound. There are three parts crimped into the bottom of the stanchion, and non removable. The cylindrical barrel is about 1 1/4" long and is the receiver of the tapered cushion, which is that aluminum part that slips over the end of the damper rod. Sandwiched above that are the two smaller pieces of the check valve. This cutaway shows the stack of pieces including the damper rod and rebound spring.



A few more views. The outer half of the check valve ring is cut in half.





During fork compression, oil is being forced from the area above the damper rod, down through the hollow damper rod, out the 2 large holes at the bottom of the damper rod, and finally up past the check valve and into the area between the damper rod and the stanchion, which is increasing in volume as the fork compresses. The check valve disc is lifting up during this time, and as the outer ring of the check valve is cut with reliefs, the oil flow is not restricted.

During rebound, the check valve disc is forced down, sealing it against the top of the cushion. The oil now has 3 paths to return to the inside of the damper rod and back out its top: between the damper rod and the inside of the check valve disc, through a single, fixed control orifice on the side of the damper rod, and through the adjustable orifice at the top of the damper rod. The gap between the rod and the check valve is the largest of these paths (by a factor of about 4), so the orifices are only supplying some additional tuning and not complete control.

Here is the single fixed orifice, about halfway down the damper rod. That hole is about .065". The 3 selectable orifices at the top range from about .030" to .075", which is about .003^2" to 0045^2" in area. The total area between the OD of the damper rod and the ID of the check valve ring is about 016^2" in area.



You can also see the taper in the control rod diameter. It increases at the point where the rebound spring is contacted when the forks become fully extended. When this occurs, the inner ring of the check valve seals much tighter to the control rod, limiting the flow of oil to the orifices during the last bit of extension stroke.

This cushioning is similar to the that provided by the tapered cushion at the end of the control rod, which controls the last 3/4" or so of compression stroke before the fork bottoms out. The cushion traps a small volume of oil and the taper progressively reduces its escape path. This works in the same manner as cushions found in hydraulic and pneumatic cylinders.

Well that went off the rails a little farther than I intended, but maybe not everyone has an extra set of forks for destruction to look closely at all those squiggly little paths that your fork oil is fighting through.

 

Not sure if the photos on this are screwed up or if its just me. If I look at the post on my phone, the photos show up. On the computer it just shows

 

Pictures are visible on my computer.

 

I see a pair of these on FB market place that I can pick up for cheap. I’m thinking of swapping them onto my 650 maxim for the dual break set up.

my question is, do you think I can shorten the spring in order to lower the front end, or will the adjustable damping system interfere with a shortened spring?

 

It won't interfere. The adjuster slides through the rotating piece on top of the damper rod, same as if you bottom the stock height forks out.

If you shortened the fork tube itself rather than adjusting the spring or spacer, you might have to trim the same length from the end of the adjuster rod.

 

Bro thanks for info. I’ll be sure to post when I get it fitted.