Shimano Dura-Ace 7900
This article will discuss the Shimano® Dura-Ace 7900 components. The group is lighter then previous editions through changes in design features and the use of aluminum and titanium fasteners. These materials do not pull up and feel the same as traditional steel fasteners. When possible, use a torque wrench. The basic service features and adjustments are similar to previous Dura-Ace editions.
The rear derailleur is reading 170 grams on the DS-2 table top scale.
The chainring bolt heads face inside, toward the frame (Figure 1 and Figure 2). The bolts use a 12-point star-shaped fitting known as Torx®, in the T30 size. Use a wrench such as the Park Tool PH-T30 (part of the PH-T1 set). (Torx® is a registered trademark of Acument Intellectual Property, LLC.)
Figure 1. Bolt heads are on the inside of the spider arm
There are no separate chainring nuts. The outer ring is hollow and is manufactured with internal threads for the chainring bolts. When chainrings are replaced, secure chain ring bolts to 12Nm.
Figure 2. Internal threads are part of the outer chainring
The Dura-Ace uses the Hollowtech II system. The threaded bottom bracket bearings are installed into the shell. It is important the shell faces be parallel. If necessary, the faces should be faced (Figure 3).
Figure 3. The aluminum shell of this carbon frame allows for facing with the BFS-1
The crank is a Hollow Tech II system. When installing the left crank, secure each crank bolt to 12-14 Nm (106-122 in lbs). Alternate between each bolt, pulling up each bolt back and forth. Each bolt should be confirmed to be at full torque twice with a torque wrench such as the Park Tool TW-5. A "click-type" torque wrench will click immediately on one bolt, and then click on the other bolt (Figure 4).
Figure 4. Secure the pinch slot by working bolts back and forth
Derailleur service and overhaul features are the same as previous models. However, there are titanium fasteners to lighten the derailleur (Figure 5-7). Again, when unfamiliar with a material, use a torque wrench whenever possible.
Figure 5. Lower cage pivot and pulleys. Pulley bolts are aluminum and cage is carbon fiber.
Figure 6. A-carbon fiber cage. B-hole drilled for weight saving in cage pivot. C-titanium pivot bolt.
Figure 7. Even the cable pinch bolt is drilled to save weight. Pinch plate has two tabs to ensure proper cable orientation.
Figure 8. Front derailleur mounting bolt is titanium, use a torque wrench to achieve correct load on fastener
The brake levers secure to the handlebar with a strap system similar to other levers. Use strap hex fitting on top of the lever body. Pull rubber cover back to expose fitting for 5mm hex wrench (Figure 9).
Figure 9. Secure brake lever to handlebar
The brake cable installs through the front of the levers. Remove the nameplate screw and nameplate (Figure 10 and 11). The plate pulls away forward. Use care not to scar the exterior of the plate.
Figure 10. Remove nameplate screw
Figure 11. Pull nameplate forward from top
There is a reach adjustment screw behind the nameplate, adjacent to the cable hole (Figure 12). This screw turns a stud that will rotate the lever down and closer to the bars (Figure 13).
Figure 12. Location of reach adjustment screw
Figure 13. The lever position range from fully open to fully tightened position screw
The cable access hole is the inner hole at the lever. As seen from the front, it is the left most hole for front shift lever, and the right most hole of the rear shift lever (Figure 14).
Figure 14. Rear shift lever with brake cable fed through lever and body
Shift cable feeds from underneath the lever (Figure 15). Pull rubber cover forward to expose cable access hole on the outer edge of each lever. Shift small lever several times to move the cable anchor socket to the correct position.
Figure 15. Shift cable installs from under the lever body
Feed cable end through cable anchor and out the top of the lever. Shift cable exit has two options. The option to the outer groove routes the shift housing to the backside of the handlebars (Option "A" in Figure 16). The other option routes shift housing to the front side of the handlebar (Option "B" in Figure 16). The brake cable exits from the inner hole and brake housing must be routed to the front side of the handlebar. The routing option to the backside of the handlebar ("A") has less of a bend at the lever.
Figure 16. The two shift cable routing options. The thicker brake wire is routed from the brake hole.
It is recommended by Shimano that an aluminum end cap be used rather then a plastic end cap for the shift housing (Figure 17). The end cap will be subjected hand pressure while the cyclist is riding "on the hoods." The aluminum end cap must fit easily into the lever body, do not force and end cap into place. The single strand brake housing does not use an end cap.
Figure 17. Brake housing and shift housing in place
Use strong tape to hold the housing before the handlebars are wrapped. Route housing to appropriate stops. There are in some situations two routing options. Traditionally, the rear shifter is routed to the right housing stop (Figure 18). This may lead to more severe bends in housing. In some situations it may help to "cross over" the shift housing (Figure 19). Do not cross if cable wire results in rubbing the frame tube between bottom bracket guide and housing stops.
Figure 18. Rear shift housing is routed to right housing stop. Note bend of rear shift housing at headtube. The shift housing is also rubbing on headtube.
Figure 19. Front shift housing is routed to right cable housing stop, and rear shift housing is routed to left stop. Housing makes single, smooth arc as it approaches stop. There is no rubbing of housing on the headtube. Cables must then cross back below downtube to reach appropriate routing at bottom bracket cable guide.
The chain length is determined the same as other derailleur chains. The chain is not symmetrical, and must be installed only one direction. Route the chain so the connection rivet is in the left side of the outer plate when viewed on the lower section of chain, between lower pulley and lower section of the rings. This will insure the connection rivet will contact the cogs first as it wraps to the cassette. There is a correct inside and outside of the chain (Figure 20).
Figure 20. Press connection rivet into left side of outer plate as seen on lower section of chain
The chain is clearly stamped with the model number on one side. This should face outward, away from the spokes and toward the mechanic (Figure 21). The plates facing inside have no lettering or text marking.
Figure 21. Of the two rivets in an outer plate, the connection rivet should be the first rivet to contact the cassette cog. Note stamping on chain facing outward.
The brakes are dual pivot brakes and are adjusted and serviced as other dual pivot brakes. However, there are Torx® fittings used to secure pivot adjustments on caliper arms (Figure 22). These are titanium bolts, and use of a torque wrench is recommended. (Torx® is a registered trademark of Acument Intellectual Property, LLC.)
Figure 22. T-25 titanium bolt as pivot of caliper arm