Marc Rittner (Part 1) - A Decent Engineer At Last !


Okay, I'll admit it. I'm a 'fly by the seat of my pants' type of guy. I see it and think, yeah that'll fit and off I go.

Well mercifully for all FJer's out there Marc Rittner takes a different tack. He actually carefully plans his modifications and records them with meticulous detail.

Marc has very kindly sent me the details of the modifications he has carried out on his glorious FJ. I'm sure you will find them very interesting. Perhaps even reassuring :>)


For those that do not know firsthand, the compulsion to modify your motorcycle is a disease that can only be treated with more modifications.  Although I have been a carrier for this disease for a long time, I can state with certainty that it was Barry Edwards FJ Mods website that has caused a recent inflammation.

I started out with some relatively simple modifications to my 1993 FJ1200.  These mods included the standard YZF600 17” rear wheel swap, and a grafting on a complete YZF600 cartridge fork front end.  Although a significant set of improvements that would have satisfied most normal people, true modifiers are not normal, and I had to do more.

I read Barry's writeup on how he put a complete Thunderace rear end (swingarm, wheel, brakes) on his FJ, and thought that would be a pretty spiffy upgrade for my ride.  The only concern I had was that Barry’s instructions were a bit vague (vague ! I spent a good 2 minutes working that lot out ! ) on precise dimensions where certain parts would have to be machined.  Not being the type to leave these dimensions to chance, I set out to gather definitive dimensions required for best results

The following is a basic task list of what I did to properly fit the Ace swingarm and wheel assembly to the 1991-1993 FJ1200:  Other years may vary.

  1. Build a swingarm/wheel jig to determine stock and modified dimensions
    (This was for prototyping only.  You do not need to do this).
  2. Ace cush drive hub needs to be machined to provide proper chain alignment.
  3. Ace swingarm pivot area needs to be narrowed to fit FJ frame
  4. Ace swingarm crossbrace needs to be relieved for shock spring clearance.
  5. Honda shouldered countershaft sprocket needs to be used to provide chain to tire clearance.
  6. New, shorter dogbones need to be made to lower wheel/raise bike rear.
  7. New spacers need to be made to fill the gap on Ace swingarm between dogbones.


1)  JIG – I built a jig out of plywood and 2x4’s that would allow me to mount the stock swingarm and wheel, and compare dimensions with an Ace swingarm and wheel.  A piece of posterboard was put under the wheel, and a right angle measuring square was used to transfer the dimensions directly from the wheel to the posterboard.

The results from this jig exercise confirmed that the FJ and Ace wheels were both on the exact same centerline.  It also confirmed Barry’s finding that the cush drive needed to be machined to provide chain alignment. 


2) CUSH DRIVE – Material needs to be removed from the face of the cush drive in order to offset the rear sprocket toward the bike centerline.  Before the cush drive can be machined, it is first necessary to remove the (6) 10mm studs.  The way I do this is to securely clamp the cush drive in a vise between soft metal jaws.  I then double-nut a stud so I have a means of unscrewing them from the cush drive using a wrench.  Don’t use a pliers or vise grip unless you plan on replacing the studs.

Note:  the studs are held with Red Loctite thread locking compound.  The only way to get the studs loose is to heat them. I use a small propane torch, and heat the studs from the back side.  Don’t be stingy with the heat.  Once you have properly heated a stud it will unscrew quite easily.  Remove double nuts from the removed stud, install them on next stud, heat, unscrew, and repeat until all 6 studs are removed.

The proper amount of material to remove from the cush drive face is 0.195”  This was determined as follows:

Amount of sprocket offset needed =  0.320”
Flipped rear sprocket offset              -0.055”  (Will explain later)
Honda countershaft sprocket offset  -0.070”  (Will explain later)
Material to be removed from cush =  0.195”


3) SWING ARM PIVOT AREA – The Ace swingarm pivot area needs to be narrowed in order to fit into the FJ frame.  ‘FJ Tom’ Slocum told me that the Yamaha YZF750 used a 3-piece needle thrust bearing on its swingarm to handle side thrust loads.  I thought that would be great on the FJ, as I could machine the pivot for minimum clearance but still have zero friction with no binding.  These thrust bearings are composed of (2) 1mm hardened races, with a 2mm radial needle bearing sandwiched in between.

As Barry described, it is necessary to press the bearings deeper into the swingarm pivot area.  I made a bearing press pilot tool that allowed me to precisely locate the bearings.  The tool was actually machined for 0.305” depth, but there is a few thousandths of ‘spring back’ when the bearing is pressed in.  .  The final depth that the bearings are pressed in is 0.302” per side.  

The proper amount of material to remove each side of the swingarm pivot is 0.302”  This was determined as follows:

FJ Pivot Inner Sleeve                          =   9.697”
Thrust Bearing clearance (.005”/side)    -0.010”
Thrust Bearing width (2) x 4mm            -0.315”
New Ace Pivot area width                  =  9.372”

Ace Pivot Inner Sleeve                        =  9.976”
New Ace Pivot area width                    - 9.372”
Total material to be removed               = 0.604”  (0.302” per side)

Now that the bearing is pressed in, we must remove the excess aluminum material from each side.  My machinist made a special piloted cutting tool that we simply insert into the swingarm pivot tube bearings, and drive with a large ½” electric drill.  The cutting tool easily removes the aluminum, but stops cutting as soon as it contacts the chromed steel cage of the swingarm bearing.  Net result is a foolproof way to locate the bearings and narrow the swingarm.  Total elapsed time is about 20 minutes (10 minutes per side)



4) RELIEVE CROSS BRACE – If you plan on using a stock shock, it is not necessary to remove any material from the swingarm crossbrace.  But if you intend to use an aftermarket shock (Ohlins, Penske, etc) where the spring may be a larger diameter and extends further down the shock, you will need to machine a relief in the swingarm crossbrace.  If a relief is not provided, the swingarm will hit the shock spring and will not lower down enough to have proper geometry or ride height.

Pictures do a better job of showing the problem and explaining the solution than I can.  I will provide a simplified explanation of what we did. 
First, we flipped the swingarm upside down and clamped it to the mill table, with the axle end clamped up on blocks.  Total parts angle is 20 degrees.  Cutting head is vertical.
Second, we used an adjustable flycutter set to 3.30” diameter to cut a relief in the crossbrace, extending down 1.125” from the ‘top’ edge of the crossbrace.  We removed as much material as we dared without breaking through the tubing wall.

Note:  1991-1993 shock links have an ‘offset’ mount for the shock clevis.  Cut the relief slightly off centerline (1.550” from left dogbone attachment point).  It is best to remove most of the swingarm gusseting material to provide clearance around the shock spring.

It may also be necessary to remove some swingarm gusseting material to provide clearance around the shock spring.



5) COUNTERSHAFT SPROCKET – Use Honda part number 23801-438-000.  This is a 17 tooth sprocket that fits a 1983-1986VF750 or VF-1000F or R.  It is also Sprocket Specialists part# 541.

The reason to use this sprocket is that it has a wider shoulder than the stock FJ sprocket, which moves the sprockets out from the bike centerline.  The stock sprocket shoulder is 0.040”.  The Honda sprocket shoulder is 0.110”.  Net results is moving your sprocket out 0.070” from the bike centerline.

In order to maintain the same countershaft nut engagement, one shoulder should be machined flush to the sprocket, or to a slight recess.   The recess I machined is 1.675” in diameter, and extends 0.030” below the surface of the sprocket.


6) DOGBONES – New dogbones must be made.  Use 1” wide x ¼” thick steel bar.  Drill holes for bolts on 3 7/8” centers.  Round the corners of the barstock for appearance.  Paint to prevent rust.  This length dogbone will allow approximately ¼” clearance between the rear tire and ground with the bike on its centerstand. Make shorter if you wish for the rear to be higher. 

7) SPACERS - Needed to fill gap on Ace swingarm using FJ linkage sleeves.
Dimensions are 1.160” ,  0.673” I.D., and 0.100 thick.  Can be made from aluminum or steel.  These are used to keep the dogbone linkage centered, and to keep dirt away from the swingarm linkage bearings.

Note:  Photo shows special adjustable dogbones.



 1) Grease all swingarm bearings with a good EP chassis lube.  Grease and insert FJ1200 swingarm pivot tube.

2) Assemble swingarm pivot area end caps.  Use FJ1200 end caps, not Thunderace.  Be sure to include stock washer underneath 3-piece thrust bearing.  Grease thrust bearing with a good EP chassis lube.  Install end cap assemblies onto swingarm.

3) Position swingarm in frame.  Insert swingarm pivot bolt and torque to specification.  Check swingarm for side clearance.  There should be none!
Note – if you use an endless chain, please install at the same time as the swingarm.

4) Install dogbones.  Grease pivot sleeves with a good EP chassis lube and insert into swingarm and shock link.  Attach dogbones using stock FJ bolts.  Use machined spacers where dogbones attach to swingarm.

5) Double-nut cush drive studs and install in machined cush drive.  Use Red Loctite locking compound.  Torque studs by hand until pretty tight.  It feels like about 250 in/lbs to me.

6) Install rear sprocket on cush drive hub.  Make sure the  rear sprocket is ‘flipped’ with the recess TOWARDS the bike centerline.  This small amount of offset (0.055”) is considered in the machining of parts.  Torque the nuts to specification.

7) Install machined Honda countershaft sprocket, with shoulder toward the bike centerline.   Install locktab washer and nut.  Torque the nut to specification, and bend locktab to secure the nut.

8) Install rear wheel and stock Ace rear brake assembly per manual
Note: read Barry’s cautions regarding tire selection.  Some 180 series tires will be too wide and will foul the chain.  You can use a large rasp to take some rubber off the edge until the chain clears.
Tires known to fit include Pirelli Diablo Strada, and Bridgestone BT014 and BT020. Tire known NOT to fit is Dunlop D208.

9) Congratulations on a job well done !

Ace Conversion Options

1)  Replace stock rear caliper assembly with R1 Rear Caliper assembly

     See separate writeup

2)  Replace stock Ace chainguard with Pyramid Plastics Ace carbon fiber hugger

     Minor trimming is required at front left corner of hugger to clear the FJ1200
     subframe.  With the hugger installed and the rear shock absorber disconnected,
     manually raise hugger/swingarm/rear wheel assembly.  Note where hugger hits
     subframe, and mark area for trimming.  Remove hugger, trim, reinstall, check,
     and repeat as required until you achieve the needed clearance.

    I'm sure you will agree that Marc has done a very thorough job in recording his work and has been most generous in sharing his labours. Well done Marc !


Marc's YFZ750 Fork Installation

Marc's R1 Rear Caliper Installation