Part 10 - Internal Throttle

Running the wires and throttle cables inside the handlebars gives a neat clean look. The internal throttle assembly was ordered off ebay from Get Lowered Cycles as V-TWIN P/N 36-0575 and the Barnett throttle cable was ordered from Jireh Cycles.

Installation requires removal of 4.75” from the right side of the handlebar. Blue painters tape was used to mark the cut line the whole way around the handlebar and the right 4.75” cut off using a hacksaw.

Marking the cut line the whole way around the bar gives you a line to follow all the way through. Typically with a hacksaw, you end up going through the material at an angle which makes a mess of things. With the tape as a guide, it is easier to keep the cut straight.

Handlebar cut to accommodate the internal throttle

The handlebars have an ID of 0.75” and the internal throttle assembly has an OD of 0.80”. The handlebar had to be bored out so that the internal throttle would fit. This was done using a 13/16” (0.8125) Deming drill bit. This job should only be done with a proper drill press with the handlebars rigidly clamped. Not having access to the proper tools, it was done by hand which resulted in a badly swollen finger and a bruise on my wrist inflicted by the drill.This is not recommended practice and can cause serious injury.

Drilling the handlebar ID

After reaming out the bar...

The throttle body needs to be fully disassembled for installation. There is a useful installation manual for this throttle on the web http://www.vtwinmfg.com/Instructions//36/36-0575.pdf, which shows the complete exploded view and guides you through the installation steps.  

Internal Throttle Parts

There are some additional things you should be aware of during the installation:  

1.       Make sure that the throttle body is not inserted too far into the handlebar. The o-ring needs to be completely visible. If you do go too far, you won't be able to get the smaller snap-ring installed. If you go far enough for the snap ring to install, you might still be introducing a small element of friction between the rotating outer tube and the edge of the handlebar. Make sure you have a small gap here. With the o-ring completely visible, things should work out ok.

2.       To eliminate the gap between the hand controls and the edge of the handgrips, a 1.17” spacer is required. Trying to mock up the spacer, I noticed that the spacer would not slide past the cam bearings. The Cam bearings definitely stick out ever so slightly above the surface of the outer tube. If the cam bearings rub on the inner surface of the hand grip, over time, small particles will fall into the mechanism which will eventually result in the outer tube jamming. This may be something that varies from unit to unit. Machine the mounting points for the cam bearings down until you no longer have mechanical interference. We are talking in the region of 5-10 thousands of interference on my throttle so the modification is small but necessary. 

Before and after machining of the spool cam bearing mount surfaces

Installing the internal throttle cam bearings

Handgrip and controls after internal throttle installation

3.       Use blue locktite for final assembly. You don’t want the internal assembly vibrating loose. That will cause a jam which will either leave you unable to open the throttle, or keep the throttle jammed wide open. Either scenario can cause a serious motorcycle accident.

4.       Having a single setscrew clamping down on the motorcycle throttle cable seems insufficient to me. I'm considering drilling and tapping a a secondary.

5.       The cable needs to be lubricated completely. When the cable is fed through the motorcycle handlebars, the cable makes a sharp bend to go down the riser. This bend causes more friction than normal so lubrication is essential.




I have since found an internal throttle on the web that fits a 0.75" ID custom handlebar. This avoids the need to ream out the ID of the handlebar to 0.8" which will save you some time and effort. Here's a link to the website: http://www.motoxcycle.com/internalthrottle.html
The web site mentions that most custom handlebars have a 0.75" ID so this is something you will want to consider. Standard Harley-Davidson handlebars have a 0.8" ID.