Hi guys, call me an old fool if you wish, but I've used the 5mm Allen key manual method on Dukes I've had and had no problems at all. I question why some of you guys go to the expense of some of these special tools instead of using others experience. Here are a few comments from other sites about tensioning manually. Make your own mind up of course but if you tension by 5mm do it cold, not hot.
California Cycleworks has the tool for $300.same as factory tool but much better price.
I purchased a gauge from Chris Kelly, Ca-cycleworks.com. After doing it several times I can do it closely enough without the gauge. One indicator is to twist the belt(on the longest span of the belt between pulleys)45 degrees with only finger strength. (45degrees, not 90 degrees as is common for autos).
The hex key trick will work on a 4v, only you measure in a different place. Across the top of the belt, at the top of the head, between both cam sprockets, scribe a line where the belt sits. Then, using moderate pressure, take a 5mm key and push down. If you can get the top of the hex below the line you scribed, it is set good. Granted, it's not as accurate, but it is good for checking every now and then, and will even work if you don't have the $300 to pony up.
Another method is between the cam pulleys, with moderate pressure, push up in the center of the belt to the underside lip of the backing plate.
The last 2 responses work the best and you dont have to spend $300 on a tension tool that doesnt work worth a crap!
If you loosen the pulley nuts and apply light finger pressure to it while tightening it up you'll be just about right and be able to then slip a 5mm wrench in the gap. The tension on the factory spring tool is about 10 ft. lbs if I remember correctly from the Haynes manual. I use a digital fish scale too - and pull to 10 ft lbs then tighten the screws - magically the 5mm wrench slips in the gap when done. AND the old timer method of twisting the belt to 45 degrees with little (no) effort is a triple check on the work. I always use two or three methods as described above to make sure I'm not too tight on the belts.
I too was perplexed by this! Very unscientific but quite workable. Hold the 5 mm (long) key by the end and push against the belt with moderate pressure so the key won’t twist out of your hand. The key should just make the fixed roller turn from the dragging. When in doubt go looser. I have logged more than 30,000 miles using this method with no trouble. With new belts go back and recheck after 6-800 miles, they will be loose again but settle in from that point.
Check out this site for maintenance with pictures. http://www.ducatisuite.com/belttension