PA50 Crankshaft in NU50 Case
If you've ever ridden an Urban, you're probably familiar with that screaming sound the stock crank makes at high RPMs (also, no thrust washers on stock). Or maybe you're not, but have a desire to upgrade to a stuffed crank anyway, only to find that such products (amazingly) don't exist (yet). Below is a synopsis of how I took an aftermarket PA50 crank (Treatland) and modified it to work with the stock Urban lower half.
First, the right (electrical) side of the crank is nearly identical between the two - which is great due to the high potential for error in cutting woodruff key slots.
The actual stock crank is about 1mm wider, but this can easily be corrected with washers, or simply left as is (as long as it's centered in the case; I also used a much thinner gasket paper on the rebuild, which about compensated for the lost width). Overall, not much to worry about.
Now the drive side. The taper of the PA is entirely different, save for the first centimeter or so (i.e. the part that the bearing sits on). After that point, three areas need to be modified; in all cases this involves adding to the existing diameter.
First, the area immediately adjacent to the portion holding the bearing should be brought to about an equal diameter so as to hold the NU50 seal, which is different from both it's own right side seal, and PA50 counterparts (i.e. 20x31x7 rather than 15x25.5x7). I choose to use brass for this fitting because of its naturally lower coefficient of friction (also, it's easier to cut, which makes forming an interference fit a bit easier).
Next, the kick-start gear. This is perhaps the most difficult part to manufacture (it's event a bit tricky to remove some of these from long-standing stock engines in the first place). The taper angle at which the inner diameter of the gear receded, and the taper angle of the replacement crank (for the latter ~14mm of the gears 17mm length) are both 7 degrees. This makes cutting a bit easier, although he small bore (i.e. starting as 12mm) makes cutting a challenge (I had to significantly trim down my best internal cutter to prevent unwanted contact even at that low angle).
The tolerances of the finished product provided a very light interference fit, but came out true, which I feel is more important.
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