Rocker Boxes

made by Thunderbike

Milled from solid metal block

There are several ways to make three-dimensional shapes. As a rule, these are cast using different techniques. However, we prefer to mill even complex shapes, such as rocker boxes, out of the massive block of metal using a CNC process. Only in this way we achieve maximum precision and thus a one hundred percent accuracy of fit.

Tech Rocker Box-Production

Step 1

“CNC” stands for “Computerized Numerical Control” and this means that the entire design is determined of 3D software on the computer and then from this image data the required data are generated, with which the milling machine is fed. At the same time, our machinery is specially designed for the requirements of parts production in motorcycle construction. After we have now determined the data for the milling machine, hardware is used for the first time. An aluminum square block is the base from which the covers will later be milled. This is now fixed in the clamping device in the milling machine. For working out the shape, the various milling heads are available in a reservoir. The particular composition of these tools depends on the later form and is set up specifically for each workpiece.

Step 2

In the first operation, the workpiece is measured using a survey probe. Thereafter, the entire surface of the aluminum block is sized. In this way you get a completely flat surface, which is also oriented to the software of the machine.

Only then is the tool tip fully automatically exchanged for a mold. The automaton always starts with the largest possible tool and works the form only roughly out of its entirety. Subsequently, the respective form cutters are used, with which the subtleties and details are worked out. After repeated tool changes and innumerable passes, the final shape becomes more and more recognizable until there is a part in front of us that already looks like the later cover. In fact, we only have a relief of the later product. In this cover, however, valve springs and rocker arms are housed, for which still the corresponding recesses must be incorporated.

Step 3

The workpiece now has an irregular surface due to the design. If you clamp it in this condition for further processing, you would also damage the surface. As a result, jaws have previously been made which conform to the surface of the workpiece like a negative mold. In this way, the “half lid” without damage safely, clamp the other way around. Once the machine has measured the shape, he knows where to start the cutter, starting again with the largest tool to work out the opposite side. The rocker boxes for the front and rear cylinders are different, hence, for each of the two cylinders as well made a single draft, which will be subsequently implemented. Overall, a set consists of a substructure plus cover, resulting in the sum of four different milled parts. Also, the lower body is as previously described, milled separately.

Step 4

In the end, not only do the new milled parts have to fit together, but above all they have to fit on Harley’s cylinder heads. Thus, the original rocker arms and valve springs not only have to be accommodated, much more they also need clearance and tolerances that need to be considered. Whether these calculated tolerances are actually sufficient, only becomes apparent during assembly on the original cylinder heads. During the prototype production, the just milled parts were repeatedly screwed onto the original heads, to recognize that here and there still a tenth of a millimeter must be removed and elsewhere, possibly a recess should be considered. In this way, the virtual data of the computer were compared with the real forms and thus again exact data for mass production determined. Only when everything fits together and the part corresponds to one hundred percent our creative and qualitative requirements, it becomes an original Thunderbike Part that finds its way into our catalog. Our rocker boxes are available for current Harley-Davidson Dyna, Softail and Touring models with TwinCam engines.

The Rocker Box Set Ribbed for Twin Cam engines can be found in our online shop!

Any Questions?


Nico Schnepel

Customparts, Dealer Support

0049 2852 - 6777-30
Send an email


Roman Reiners

Customparts, Dealer Support

0049 2852 - 6777-30
Send an email


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