The RC51 uses a geared valve train instead of a traditional timing chain system. The gears that make up this valve train are straight cut, meaning that the teeth are cut into the gear perpendicular to the face of the gear. Using this type of arrangement requires "play" in the meshing of the teeth. This "play" allows the cams to "bump" against the cam drive gears causing a deep ticking sound. In order to alleviate this, Honda engineers put a secondary gear on the cam shafts along side the primary gear. This secondary gear is slightly offset from the primary gear. The secondary gear is spring loaded so that when the cam is torqued down, the two gears can align with one another. By adding this spring loaded gear, the "bumping" between the cams and the cam drive gears is dampened. By removing these secondary gears we gain in two ways:

1) We reduce the overall weight of the bike. 2) We reduce the weight of the cam assemblies by 15.5oz - resulting in improved throttle response. Cam Silencing Gears (Figure 1) - Removed items WARNING: The noises from the engine will be louder after this mod is performed. The new sound is similar to that of a Dry Clutch. Note: If you have the proper tools, now would be a good time to inspect/adjust your valve clearances.

First, remove your lower inner fairing, lower fairings and gas tank. The gas tank will need to be removed for both cylinders. This differs from the shop manual, but it will make life much easier when doing the front cylinder. Rear Cylinder: Disconnect the air supply hose from the PAIR check valve. Remove the Pair Check valve cover (if using Dan Kyle's block off plate, this can remain attached). Removed the spark plug cap. Remove the three cylinder head cover bolts (Figure 2). Remove the cylinder head cover (Figure 3).

Next, remove the timing hole cap and crankshaft hole cap (Figure 4).

Rotate the crankshaft counterclockwise and align the "R T" mark on the flywheel with the index mark on the left crankcase cover (Figure 5). Note: The engine will be much easier to turn over if both spark plugs are removed prior to this step.

The index lines on the rear cylinder camshafts must be flush with the cylinder head surface and facing outward as shown (Figure 6). If the index lines are facing inward, rotate the crankshaft counterclockwise 360o (1 full turn) and align the "R T" mark with the index mark.

Next, remove the camshaft holder bolts (Figure 7), camshaft holders, dowel pins and camshafts (Figure 8). Watch the dowel pins, these can remain but do not lose them. Also, note the positions of the camshaft holders. These should be replaced in the same orientation from which they were removed. Note: It is recommended that only one cam be removed from the engine at a time.

If you look closely at the camshaft (Figure 9), you can see that the teeth on the Silencing Gear are slightly offset. This is what allows it to dampen the lash that is created between the Cams gears and the Cam Train Assembly.

Remove the C-Clip and the Bellville washer. Then use the washer to pry the Silencing Gear free (Figure 10). Be careful because the Silencing Gear is spring loaded. Once the secondary gear is removed, the spring (large C-shaped object) may remain attached to the cam. If this is the case, use the washer to pry the spring from the cam in the same manner the secondary gear was removed. Note: The spring and/or the C-clip can shoot across the garage, so eye protection may be a good idea.

Figure 11 shows the disassembled cam and secondary silencing gear. From the top: Spring, Silencing Gear, Bellville washer, C-Clip.

Apply molybdenum oil to the camshaft journals and cam lobes. Install the cams in their proper locations making sure the index lines on the camshafts are flush with the cylinder head surface and facing outward as shown before in Fig. 6. Figure 12 shows the cams before and after the modification. Apply oil to the threads and seating surfaces of the camshaft holder bolts. Install the camshaft holders in their proper locations with the "IN" (arrow) mark facing to the intake side. Apply oil to the threads and seating surfaces of the camshaft holder bolts and tighten the bolts in a crisscross pattern in 2 or 3 steps. Cam Holder Bolt Torque: 23 N.m (2.3 kgf.m, 17 lbf.ft)

Clean the old sealant off of the Cylinder Head cover and the top of the Cylinder Head where they seal together. Apply sealant to the cylinder head cover gasket semi-circular areas (Figure 13). Install the cylinder head cover onto the cylinder head (Figure 14). Install and tighten the cylinder head cover bolts. Making sure that the two silver and one black bolt are in the right spots. The black bolt DOES NOT have a triangle near the hole as seen in figure 2. Cyl. Head Cover Bolt Torque: 10 N.m (1.0 kgf.m, 7 lbf.ft) Connect the air supply hose from the PAIR check valve (if you don't already have the PAIR System removed, now would be a good time to see the PAIR Removal Guide).

Front Cylinder: Remove the two mounting nuts and oil cooler from the stay and move it forward (Figure 15). Remove the two socket bolts and oil cooler stay. Disconnect the air supply hose from the PAIR Check valve, remove the PAIR check valve cover (if using Dan Kyle's block off plate, this can remain attached). Disconnect the crankcase breather hose from the cylinder head cover. Remove the spark plug cap (if not already removed). Remove the three cylinder head cover bolts and the cylinder head cover. The front cylinder head cover requires some finagling to get out of there. BE VARY CAREFUL NOT TO DAMAGE THE CYLINDER HEAD COVER GASKET! The coolant hose will need to be pushed and shoved around a bit in order for the cover to come out easily. The oil cooler will need to be moved as far to the bottom as possible. Note: We found that the oil cooler could be dropped down an extra bit by removing the front fender. Every little bit helps. Note: For easier access to the top bolt, remove the airbox and approach it from above (Figure 16). Note: If you remove the airbox, this would be a good time to put the tall rear velocity stack back into the airbox (if you've previously removed it). See www.rc51.org for more details on the effects of the "short velocity stack mod". You may also want to see the Stock Air Filter Modification.

As you can see in Figure 17, there isn't much room for moving around when working on the front cylinder.

Rotate the crankshaft counterclockwise 450o and align the "F T" mark with the index mark (Figure 18). Again, make sure the index lines on the front cylinder camshafts are flush with the cylinder head surface and facing outward. The index lines on the intake camshaft can be seen more easily from the top of the bike. Repeat the same steps you did for the rear cylinder. Install your spark plugs (if removed), your tank and bodywork. Note: The RC51 employs this type of noise dampening in a few other places on the bike. Those will be addressed in later guides.

***Jason Hinton and Troy Gwin assume no liability for any damage done by this modification. Perform this at your own risk. A portion (if not all) of your warranty will most likely be voided by performing this mod.***

Guide by Troy Gwin & Jason Hinton (JaSoN)

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