Marc Rittner (Part 2) -

YZF750 Fork Swap

 

Many before me have put upside down forks on an FJ1200.  There are lots of nice examples of FZR1000 conversions on Barry Edward’s FJ Mods website.  The first set I ever saw in person was in 2001 on FJ Tom Slocum’s machine (which was later stolen and never recovered.) 

While there is nothing wrong with a properly setup set of conventional damper rod forks, I have developed a preference for the cartridge style forks.  The main reason for this is the adjustability for rebound and compression damping, and I like to fine tune my suspension without having to tear the forks down and change fork oil weight.

The following is a basic parts and task list of what I did to fit a YZF750 front end onto my 1993 Yamaha FJ1200.

Parts list:
(1) set YZF750 forks
(1) set 1.0 kg/mm or 1.05kg/mm fork springs
(1) YZF750 lower triple tree
(1) YZF750 front wheel with Brembo rotors*, axle, speedo drive
(2) R1 monoblock calipers*
(1) YZF750 front fender
(1) YZF1000 Thunderace top yoke
(1) FJ1200 or FZR1000 steering stem (press-in type)
(1) set FJ1200 tapered roller steering head bearings
(1) set ABM 50mm handlebar risers
    (http://www.austreetfighter.com.au/scart/html/81.html)
(1) 7/8” tubular handlebar (superbike or GP touring bend)
* stock YZF750 rotors and 6-pot calipers may used.
 

1. FJ/FZR steering stem needs to be welded and machined to fit YZF750 lower triple tree.

2. YZF1000 Thunderace top yoke needs to be bored to fit YZF750 fork tubes. Drilled and tapped to fit ABM bar risers. Milled to fit FJ1200 ignition switch

3. FJ1200 steering stop needs to be extended to work with YZF750 lower triple tree.
 

YZF750 forks need to have springs upgraded and fork fluid changed


Instructions

 

1)      Steering Stem – Yamaha sportbikes used the same diameter steering stem for many years, from approximately 1987 thru 1995.  However, starting in 1997, Yamaha inexplicably changed the diameter of their steering stems to a slightly (approx. 0.005”) larger stem diameter.  I know this to be true for both YZF750 and R1’s.   This means that the old stems are now a loose fit instead of a press fit into the YZF750 lower triple tree.  This most direct remedy for this problem is to weld and machine and appropriate stem. 

Certain models of FJ1200 have aluminum lower triple tree and a press-in steering stem.  1987-1988 FZR1000 also uses the same stem. The FJ/FZR steering stem was delivered to an aerospace quality welder, who layed down a bead of material that will be machined to the proper diameter.  Don’t take your stem to the muffler shop for this welding.  Only an aerospace quality welder can put the material exactly where it needs to be and not add an excessive amount of heat to the part.

The steering stem was chucked into a lathe, indicated for 0.000” runout, and machined to the diameter needed for a 0.0025” press fit (same as stock).  Once the stem was turned to size, the retaining circlip was reinstalled, and the stem was pressed into the YZF750 lower triple clamp.  FJ1200 tapered roller steering head bearing and dust seal were also pressed onto the stem at this time.

 



 

2)      Thunderace top yoke – the Ace top yoke has the same centers for the steering stem and fork tubes as the YZF750.  The relevant difference between the Ace and YZF750 yokes are that the Ace is flat where the YZF750 is a ‘gull wing’, and the Ace has 48mm fork tube holes, where the YZF750 has 50mm fork tube holes.  The flat top yoke is needed to accommodate the handlebar risers.

Note: The Thunderace top yoke has the ignition switch offset to the left compared to the FJ1200 yoke, which has the ignition switch centered.  The FJ1200 fork locks with the bars turned to the left.  With the Thunderace top yoke, the fork locks with the bars centered.  A small inconvenience.

The machining of the top yoke is straightforward, but needs to be done accurately. 

a)  Bore the top yoke fork tube holes from 48mm to 50 mm.  Have your machinist measure the fork tube diameter for an accurate dimension before boring.

b)  Drill the top yoke for the ABM handlebar risers.  As a base line for positioning the risers, I drew a horizontal line between 6mm clip-on locating holes.  The bottom riser holes are 0.800” inboard from each of these 6mm holes (or 1.875” outboard from the center of the steering stem hole).  The top riser holes are 1.300” up from the bottom riser holes. Spot face/clearance the bottom of clamp to provide a flat surface for the flanged nuts/lock washers

The bolts that come with the risers are 8mm.  The two lower bolts and the upper right bolt are drilled through and secured on the bottom side with flanged nuts Yamaha flanged exhaust nuts) and  lock washers.  The upper left hole was drilled and tapped for the 8mm bolt to screw into directly, as the ignition switch post is directly underneath and provides plenty of metal for threading.

Note:  I drilled and tapped the bottom of the riser blocks for a 6mm cap screw, so that the blocks would stay in place on the top yoke when the 8mm bolts were loosened or removed.  I loosely mounted the risers and bolts, slid in the YZF750 front axle as a handlebar substitute to square things up, and final positioned the risers and tightened the bolts and lock nuts.  Flip the yoke over, and drill and tap for the 6mm cap screw.

c)  Mill each Ace ignition switch posts to shorten by 0.795”.  Also, the ignition switch loop must be shortened by 0.085”.  Some additional clearance between the yoke and the switch body is required, and may be made with a hand grinder. 

 



 

3)      Steering Stop – the YZF750 lower triple tree steering stops just barely engage with the stock FJ frame stop.  It is likely that in this condition the YZF750 lower triple tree will rotate past the stock FJ frame stop.  The remedy was to construct an add-on aluminum block to extend the stop FJ frame stop. 

The add-on block is a piece of aluminum as wide as the stock FJ frame stop, and as tall as the YZF750 steering stop.  A slot is cut in the back side so that it will slip over the stop FJ frame stop.  Bevels are put on the leading edge of the slot to clear the stop FJ frame stop welds. 

The block is attached to the stock FJ frame stock with (3) 6-32 cap screws that are drilled and tapped through the block and the frame stop.  Care must be used when tapping through the block and frame stop, as it is easy to break off the tap.  DAMHIK

The machined aluminum block must be hand fitted for maximum steering stop engagement.  It will be necessary to attach and remove several times until an acceptable fit is achieved.  Once satisfied, apply red loctite to the cap screws and tighten.  This should be considered a permanent modification that requires no maintenance.

 

 

 

4)      Fork springs – standard YZF750 forks springs are too light to support the weight of the FJ1200.  Most people select springs rates from 0.95 kg/mm to 1.05 kg/mm.  You can calculate your spring rate on either the Traxxion Dynamics (http://www.traxxion.com/) or RaceTech (http://www.race-tech.com/) websites.  Install fork springs and service fork fluid per manufacturer’s instructions. 

Note: I was going to revalve the forks, but am glad I did not as they work superbly with stock cartridge pistons, 1.05 kg/mm springs, and 15 wt. suspension fluid.

Assembly

There is nothing tricky about assembling the front end.  It all bolts together as easily as stock FJ parts would.  Here is my order of assembly:

 

1)      Install triple clamps.  Grease lower and upper triple tree bearing and races and install YZF750 lower triple tree as you would install a FJ1200 lower triple tree.  All of the bearings, covers, lock rings, and washers are standard FJ1200 parts (see microfiche)

Install modified Ace top yoke as you would install a FJ1200 top yoke. 

 

2)      Install fork legs.  Slide fork legs into triple trees and loosely snug the lower pinch bolts.  Top of fork leg cap should be flush with top yoke.  Fork legs may be raised a maximum of 5mm without loss of triple clamp engagement. 

 

3)      Install wheel assembly, brake calipers, and front fender, per YZF750 manual.

 

4)      Install handlebar, switches, master cylinders, throttle, and grips.  Adjust to suit.
Note:  I did not require new hoses or cables using the GP Touring bar bend.  In fact, this bend is nearly identical to the stock FJ bars, except they are 1” wider per side.

 

5)      Hold front wheel between your knees, and wiggle handlebar until parts are aligned.  Tighten fasteners in the following order:  lower triple tree pinch bolts, steering head bearing locknuts, top yoke nut, top yoke pinch bolts, front axle, front axle pinch bolts.

 

6)      Attach brake lines.  Bleed brake system.  Double check all fasterners and GO!


Congratulations on a job well done. 

     

 

Stunning job Marc. Well worth the effort !!

Marc's Ace Swingarm Installation

Marc's R1 Rear Caliper Installation

 

BACK TO MAIN PAGE