I just built a custom freewheel with 12,13,14,15,17,19, 21 sprockets. This should be great for my high speed pedal assisted BionX hybrid road bike.
Last summer, when I built my high speed BionX hybrid road bike, I installed a 11-34T MegaRange freewheel (11,13,15,18,21,24,34). I've since found the gear spacing too wide. With the MegaRange, I'm only using the 13,15, 18 sprockets and rarely the 11. Here's a link to my post detailing my my bike build with photo showing the MegaRange installed..
For a customized freewheel, I'm using a used Dura-Ace MF-7400 and the sprockets from a new freewheel by Interlock Racing Design (IRD). Both are index shifting (SIS) compatible. I could have just installed the Dura-Ace unmodified, but some of the sprockets were excessively worn. Also, the IRD uses the later Hyper-Glide sprocket design, as compared to the Dura-Ace's older Uni-Glide sprockets.
The IRD freewheel is part number 23730, and costs $50 plus shipping. This is the 7-sp freewheel with 13-14-15-17-19-21-24 sprockets.
The used Dura-Ace MF-7400 that I bought on eBay for $40 including shipping, has the following sprockets: 12,13,14,15,16,17,18. If you buy one make sure it has a 17 on the 3rd or 4th sprocket as you'll need that to use as a template as shown here.
Basically, this customized freewheel was the Dura-Ace re-assembled using the first six sprockets from the IRD freewheel, while still using the Dura-Ace's 12 sprocket. But the individual 17 tooth sprocket takes some grinding to make it fit. The idea for this project came from this great writeup...
Notice the stepped arrangement. The three largest spockets have larger inner diameters. Take a look at the 17 sprockets from the new and old freewheels. Notice the inner diameter is larger on the Dura-Ace.
In order to enlarge the inner diameter of the new 17 sprocket, I used the old 17 sprocket as a template. Here they are stacked and held in place with screws.
I used a Dremel like rotary tool with carbide disk to do the grinding. If you do this be sure to wear safety goggles. I wore safety goggles and a respirator.
I also used a saber saw with carbide and hand files grind into the sharp corners and to do some finishing work.
Here's the finished 17 sprocket ready for assembly. Also on all the other IRD sprockets I needed to grind off the index notch on each, but that was really easy compared to the 17 sprocket work.
Here's the finished product. I'll install it on the bike when I get some free time.