The internal throttle installation process is pretty straight forward, but can become a little confusing, especially for a first timer. The instructions most of them include are a little vague, and miss some of the finer points. We will go through the steps needed to do a proper throttle install in two segments. Part 1 will cover the throttle itself and part 2 will cover making and installing a throttle cable. When done properly, an internal throttle can really clean up a set of handlebars, and in turn, the bike it is mounted on.
Your throttle kit will come with a bunch of parts; all in some stage of semi-assembly. It’s a good time to take everything apart just to make sure you have everything, and also to check out how it all works. It’s a pretty simple mechanism, but it’s also pretty neat. The spiral cut groove in the grip-sleeve transforms rotational movement from your hand into lateral movement in the the cable sled, which is attached to the braided-steel inner cable. The inner cable is thus pulled outwards and since the the cable-housing is pinned by the housing stop, which is locked into the body of the internal-throttle , the braided-steel inner throttle cable moves independently of its housing. This allows it to exert a pulling force on its opposite end, which is attached to the cable cam on the carburetor (or throttle body) . When it pulls on the cam, the lateral force is transferred back into the rotational movement required to twist the butterfly in the carb.
In the above photo, you can see the cable housing stop (upper left), cable sled (upper right), and the body of the throttle that inserts into the end of the handlebar, as well as the set screws that accompany each item. The cable housing stop uses a smaller set screw than the rest of the items. The three loose set screws are for locking the throttle housing into the handlebar itself. All the larger set screws are 10-32 thread, and the smaller one is 8-32.
To start, you want to position your handlebar controls where you would like them to be. In this case, we are using some custom controls requested by the customer. You want to be sure to mock up both the throttle and the clutch sides together to make sure you get them to look exactly how you want them. Once you have the controls located where you’d like them, make some witness marks and then measure out how much grip area you will want / need.
Here I am calculating how much grip area I want to leave, based on the overall length of the Biltwell grips I am using and based on how much length I need to look and feel right.
Mark out both sides of the bars and trim to the desired overall length.
Once you have trimmed both sides to that point, it’s worth mocking the bars and controls up on the bike again to confirm your decisions before moving onto the next step, which is trimming an additional 4” off of the throttle side. This way, you can make further trimming adjustments without having to account for the 4” of missing handlebar. Once you are SURE you like everything, cut the last 4” off and square up the end as much as you can. Since the bars are usually in a shape that will not easily fit in a lathe or mill at this point, I use a known square edge to mark my edge and get things close.
Once you’ve got the end of the bar as square as you can get it, clean and de-burr the edges inside and out. Also, if the tubing in use has a welded seam inside it, take a moment to knock that down as much as you can.
Now that your bar is nice and clean, you’ll want to check the fit of the throttle tube inside the bar. The internal throttle kits often come with a shim stock sleeve in case the ID of your 1” tubing is a little larger. Now is a good time to check for all that.
Now that I’ve cleaned up the handlebar tubing, the throttle slips right in and I don’t need the shim stock because the tubing I’m using is DOM with a pretty true .120” wall thickness. So, the next step is to double check how things will look with the grip installed. As you can see, the grip overhangs the internal throttle by a little over .5”. Make a mark where your grip overhangs to. You’ll want to locate your throttle set screws in this small area.
You can see the mark representing the outside edge of the grip and one mark representing where I’ll eventually drill 1 of 3 set screw locations around the circumference of the handlebar tube.
As mentioned before, the set screws that hold the throttle into the handlebar are 10-32. For a 75% cut thread, you will want to drill the pilot hole for the 10-32 tap with a #21 drill bit. The closest fractional drill bit size to a #21 is a 5/32 drill bit, which is .0028” smaller than the #21. This means that you can get away with using the 5/32 drill bit if you don’t have the #21. Always make sure any tap you use is nice and sharp, because you’re asking for the tap to snap if it’s dull and not lubricated. This is especially important when you’re dealing with smaller taps. You also get a better quality thread with a sharp tap, as it is cutting and not tearing the material. Whenever possible, I recommend you get and use the drill bit the chart calls for. I find it produces better results with less aggravation.
Your best bet for keeping a drilled hole location where you want it is to start with a center punch. Follow that up with a center drill if you have one and then once you’ve got a good pilot hole, move on to your actual drill bit. In this case, doing this will also help keep your drill from walking around on the curved surface of the bar. If you don’t have a center drill, center punch and then start off slow with your bit and make sure you’re applying solid pressure.
Here are the three holes drilled and tapped. Once I had the hole drilled to the proper size, I used a hand tap handle with the 10-32 tap and slowly and carefully tapped the hole. Do your best to keep the tap square to the area of the bar you’re tapping, so your new threads will be straight. This will allow the set screw to apply the most pressure to its contact point on the internal throttle sleeve. Once you’ve tapped all the holes, take a round file or a burr in an air tool and clean out the burrs on the backside of the holes. You should also grab a countersink and kiss the outside edge of each hole as well, or run a file over them.
Test each of the threaded holes for smooth action.
This next step is important for grip fitment because the grip will extend past the internal throttle sleeve, and if the allen set screw is sticking up, the grip will bind, catch or drag on it. The set screws these kits come with are always too long, as you can see in this photo. Because the set screws accept an allen wrench from the top, you don’t want to remove material (and therefore depth) from that end. Doing so will increase the likelihood of stripping the tiny allen set screw when tightening or loosening it up. So, remove material from the other end until the screw sits flush with the surface.
Shorten it up and then chamfer the edge a bit so it starts easy in the threaded hole. You may have to do this a couple of times for each one until they are dialed in.
A perfect fit!
This is a completed installation. When buttoning up a throttle install for road use, make sure you add a few drops of blue lock-tight to each set screw for good measure!
In the next segment, we will go over making and installing a throttle cable and also some different methods of grip installation on these throttles.