Motorcycle Rocker Arm Spacers and Shims

Building off my post about what is a rocker box, I thought I would do a post about motorcycle rocker arm spacers and shims.

Rocker arms transfer the circular motion of the flywheels to the opening and closing motion of the valves.

what is a rocker box

Rocker arms sit on the rocker arm shaft and rotate depending on the position of the cam gears.

To keep the rocker arms from banging around inside the rocker box, rocker arms usually have a series of spacers or shims to hold them in place (i.e. keep them from sliding side to side along the shaft).

Remember that the pushrod is connected to one side of the rocker arm and the other side is applying pressure down on the valve springs – they can’t be slopping around in there.

In the diagram above, the item I am talking about is item 12.  Notice in this diagram that the only thing shown are the rocker arm spacers.  No shims are illustrated.  This is your typical stock configuration.

Together the rocker arms and spacer fit perfectly along the rocker arm shaft.  Additional piece of advice – if there is a groove on one side of your rocker arm spacers, the groove is supposed to face toward the rocker arm (i.e. not toward the outside of the rocker box).

When you are running non-stock rocker arm spacers (or have to order a single spacer because you lost one in the garage – yes, me), that is when shims often come into play.

Shims can be bought for about $0.95 a pop and are 0.10″ in thickness.

The aftermarket rocker arm spacers I have encountered are deliberately much thinner than the stock originals.  Honestly, I am not sure why that is.  Maybe it is to make them more versatile with a series of aftermarket rocker arms.  Either way, it is kind of annoying.

My stock spacers fit perfectly and the aftermarket ones require like 5 of these ridiculous shims.  Anyways, the shims go between the spacer and the rocker arm and you should add them one at a time such that there is no appreciable side to side play in the rocker arm when you tighten things down.

That’s it.  A quick post about a motorcycle part that is the size of a quarter.  Good information if you are really working down in the weeds like I do.

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