I've owned several motorcycles (all Asian), but in thirty years of riding I've never been inside a crankcase. After studying Draw-Tech's crankshaft drawing, I spent several hours watching Youtube videos to get some background and familiarize myself with Harley crankshafts. I developed a particular fondness for a series of videos (Youtube search 'Tatro Machine Knucklehead') dealing with an actual Knucklehead engine rebuild. Although it ended tragically for the owner, it was apparent that Draw-Tech's crankshaft was very similar to the one in the original full-size engine.
The full-size crank assembly has two shafted flywheels and a crankpin with ends that are tapered and threaded. During assembly, large nuts, tightened to some 100 ft-lbs, draw the pin into matching tapers machined into the inside faces of the flywheels. The assembly is then mounted in an on-centers alignment fixture so the runout's of the two outer shafts can be simultaneously monitored. What seems like a brutal alignment process is actually some intelligent hammering, spreading, and pinching of the two flywheels in just the right places to reduce the typical .010" starting runout to a couple thousandths or less. The connecting rods use roller bearings on their big ends, and so they have be installed on the pin before assembly.
Many of us have machined perfectly adequate crankshafts with simple cylindrical crankpins attached to the crank throws with pinch bolts. Several alignment issues disappear if the crank halves are machined from a single billet, and the crankpin hole is drilled and reamed before sawing the halves apart. Since this technique is time consuming and creates a lot of waste, some builders instead opt for a five piece assembly with separate shafts.
The tapered crankpin is capable of handling the power requirements and allows the rods to be serviced. But it adds significant complication and risk because tapers must be machined in four individual parts, and any imprecision in their machining can create misalignment in the final assembly. In addition, the major diameters of the flywheels' tapers, being non-measurable two dimensional quantities, will determine the depths of the crankpin inside the flywheels and therefore the side clearances of the rods in the final assembly.
These concerns create reasonable arguments for tossing aside what will be a hidden-from-sight scaled-down version of the crank and instead fabricating a more conventional model engine crankshaft. Besides 'wimping out', though, this would require reshaping the crank halves to accommodate the pinch bolts which, in all likelihood, would significantly reduce the rotational inertia of the relatively massive Draw-Tech crank. An alternative would be a conventional crankpin held in place with taper pins, but I was concerned about being able to remove them if the rods ever had to come out.
All engines, big or small, benefit from some amount of rotational inertia to keep them rotating between plug firings at low rpm's. One and two cylinder four-cycle engines benefit the most. (It turns out that Harley flywheels are so effective that some owners reduce their older engine's idle speeds to the point where the oil pump can barely function, just to show off that world famous staccato.)
The Draw-Tech Knucklehead includes an additional external brass flywheel, reminiscent of a steam engine, that adds additional inertia to the engine's rotating mass. I plan to replace this with a faux roller sprocket or a belt drive that will look more at home on a motorcycle engine, but it won't be as an effective flywheel. I don't want to give up the rotational inertia of the Draw-Tech crank assembly, and so I decided to stick closely with its design. I had enough Stressproof on hand for two attempts. The drawing called for a finished crank flywheel diameter of 2.83", but since my material was only 2-3/4", my crankshaft flywheels ended up a bit smaller.
My plan did not include turning the crankshaft between centers as one might expect. The modified cross slide on my Wabeco lathe won't allow a cutting tool to access the tailstock end of a workpiece supported by a conventional dead center. The stock cross slide will allow this, but its lightweight construction greatly limits the lathe's precision and surface finish quality. When using the tailstock, I often have to use an expensive long nose live center that can add up to three tenths of its own runout. At the end of the day, turning between centers can be somewhat counter-productive on this lathe.
Construction began with skimming the workpiece o.d. to a consistent diameter in my Enco lathe where I faced both ends and center-drilled one. The workpiece was then moved to a three-jaw chuck in my little Wabeco where, in conjunction with the tailstock, the o.d. was taken to its finished diameter over as much of its length as I could access including the portion that would eventually become the flywheels. The outer shaft was then turned and fitted to the outboard crankcase bearing. At this point, the o.d.'s of the shaft and the flywheels were concentric, and what will become the outside face of the outer flywheel was normal to their axis. A dial indicator showed all three TIR's were less than two tenths with the tailstock engaged and three tenths with it disengaged.
The chuck, with the workpiece still attached, was then taken to the mill where the crankpin through-hole was drilled and reamed. The flat-back chuck insured the hole ended up parallel to the axis of the bearings and identically drilled through both flywheels. A recess for the crankpin nut was then machined into the face of the outside flywheel, and material required for balancing the assembly was removed. Some practice material in a vise is visible in the photo of this operation. A 3/8" end mill with more than two inches of stick-out was required to work around the shaft, and some fine tuning was required to determine the speed, feed, and doc for minimum chatter.
The workpiece was returned to the Wabeco in a six-jaw set-true chuck so the shaft on the cam box side of the crank could be turned. The workpiece was gripped on the finished flywheel diameter which I was now only able to indicate to what would have to be called +/- three tenths because the TIR reading had picked up a second bump in an opposite direction to the first one. The Wabeco and tailstock center bearings left their runout imprinted on the workpiece when it was turned the first time, and now with the workpiece back in the lathe but oriented differently on the spindle, a second copy of their runout was now being indicated. I tried alternate orientations, but differences between the chucks I was using prevented me from improving it.
The fresh end of the workpiece was then center-drilled for the tailstock center, and the shaft diameters for the crankshaft and cam box bearings were turned and fitted. The measured TIR of the shaft had returned once more to two tenths with the tailstock engaged and three tenths with it disengaged. Although it appeared that the second bump was gone, it really wasn't. A second copy of the Wabeco and tailstock runout was now machined into the workpiece, and both will appear when the runout's on the ends of the two shafts are compared.
The chucked workpiece was returned one last time to the mill where the second nut recess and the balance area were machined on the face of the cam box side flywheel.
I slid a couple ball bearings from my scrap collection onto the two shafts so a TIR baseline measurement could be made with the work in progress sitting on a pair of v-blocks. The result was .001" which included the blended humps in the workpiece TIR as well as the runout's of the two ball bearings. Just for interest, I also measured the three runout's with the workpiece held between centers on my lathe. Since the workpiece hadn't been turned between centers, I wasn't expecting stellar results. However, the TIR of the output shaft measured .002", the cam box shaft measured .001", and the flywheels measured .0025". If this had been an actual Knucklehead crankshaft completed at a Harley assembly plant, it would have actually (although just) passed their QC.
I suspect the first measurement is probably the one that's most important to me. After completing the machining and assembly, my current plan is to compare the runout's of the two flywheels while the shafts are in ball bearings on v-blocks to see how well the machining turned out. - Terry
