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| The First Crankshaft: 90 Degree Offset Crankshafts for Triumph Twins |
I became interested in building a 90 degree crankshaft for my Bonneville after reading various Classic Bike and other articles published through the 90’s on the benefits of both 76 degree and 90 degree crankshafts. In these articles the crankshafts were built using custom machined components and high-strength fasteners bolted to stock or custom components. As a toolmaker, I looked at these constructions as something that could be improved on to make a stronger, all-welded crankshaft from stock components with some custom machining. This description does not go into the benefits of any configuration, but describes how I have built 76 degree and 90 degree crankshafts for Triumph twins. With slight modification in the process, Norton and BSA cranks can be modified as described too.
It should be noted that I did little of the actual machining or welding because I don’t have the equipmen Bushings to hold the crank ends more securely were machined as a slip fit on each end of the crankshaft. They extend the holding power of the clamps and prevent damage to ends of crankshaft during handling. The use of bushings was also part of the plan – in this way Norton and BSA cranks can be handled by swapping bushings instead of building a fixture that only suited Triumph bearing journals. Stops are used to set big end journals for accurate 76 or 90 degree offset. Norton cranks can use alignment pins based on the location of bolt holes in stock crank halves. Figure 1 shows the resulting fixture with clamps, bushings and stops. Once the fixture was built the modification of the crankshaft could begin. First, the flywheel was removed. Scarborough Engines arranged to have the crankshaft cut in half through the centre web. Each half of the crankshaft was machined on its cut face to true the surface and allow for the addition of a spacer plate. A spacer plate was originally used to carry oil between crankshaft journals (Figure 2) but newer cranks use a special flywheel. Spacing between drive and timing side journals is maintained. Scarborough Engines indexed the first crankshaft halves in the fixture, with spacer plate installed, then welded the two halves to make crank whole again. Since then stops are used to accurately set journals at correct 90 or 76 degree offset. At this point the crankshaft rotated freely in the fixture indicating that no distortion of crankshaft had occurred. Figure 3 shows the welded crankshaft sitting in fixture after centre-section was turned to re-fit stock flywheel. Flywheel is a slip-fit the same way stock flywheel is mounted. Flywheel is test mounted with counterweight opposite the journals. A conservative estimate of the material to be removed is marked and removed on a vertical mill using radius cutter so there are no sharp corners. Flywheel is ready it fitted to crankshaft and tack-welded in position. Both sides of flywheel were welded around complete joint between flywheel and crankshaft while in the fixture, rotating crankshaft to get at all sides. This also adds strength to narrow centre web. If crank is to be lightened, material is removed from flywheel after welding. Once welding is complete the entire assembly is stress-relieved. Crank is bolted solidly in the fixture using anti-seize compound and entire assembly is slow cycled up to 1500F degrees then back to room temperature in the furnace. This ensures that all welding stress’ have been removed. Weld slag is ground off, welds are ground smooth, then crank is Magna-fluxed to check for cracks. Cranks are now Nitrided after balancing for greater strength, wear resistance and some stress-relieving.
Drive side and timing side bearings are checked for true alignment. The smallest variation indicates that these journals must be ground undersize, hard-chrome plated oversize, then reground. Big-end journals are ground .010 or .020”” undersize to clean the bearing surface with Triumph-specified .090 radius at the edge of each journal. Figure 5 shows the first crankshaft with extra balance weights added to pear-shaped counterweights to compensate for material that could not be removed from the flywheel. Dynamic balancing was used with an RPM range for best smoothness. The first crank used a 60% balance factor based on a comprise between V-twin and typical Triumph twin balance factors. Subsequent cranks use from 40% to 50% balance factor depending on intended use and the RPM range where smoothest performance is required. The balancers are provided with the weight of one set of connecting rod, bearing shells, piston, wrist pin and circlips as they can balance within one-half-gram to specified balance factor. Camshafts There are two ways to make camshafts; by modifying existing Triumph cams or by having someone build Timing There are three ways to do it, each one has its advantages. It is possible to modify a Trident points plate to Other Cranks & Cams Since I first started this project in 1997 I have developed other methods for building cranks, such as building a |
t needed for a production-quality job. For example, once the fixture was cast the only machining I did was to modify the fixture to make the drive and timing side journal clamps. It was also part of the exercise to have others build the crank to determine the price to contract the entire job out. As well, any fixtures, drawings and specifications I developed along the way meant that skilled trades could build a crank without me being there to watch each step.From the start a fixture was required to hold crankshaft components while welding and maintain alignment during stress-relieving. With scrap wood, glue, screws, wood filler and a belt sander I built a pattern that was sent by mail to Lake Foundry near Grimsby, Ontario. They also received a drawing showing a line-bored hole between the two webs that would hold the crankshaft journals. With basic research and the foundry’s advice the fixture was cast in Class 45 iron, an alloy that would remain stable during the heating and cooling of the stress-relieving process. At home, I drilled and tapped holes for bushing clamps and hand-sawed the end webs apart to open the line bored holes, creating two clamps. 
them. If I modified stock camshafts another fixture would have to be built so I could saw camshafts between lobes, then weld them back together (this has since been done including heat-treating). It might take four cams to make two good cams using this process and an experienced micro-welder to put everything together without damaging cam lobes. The second alternative is to have custom cams built from billets. I had MegaCycle Cams make a set to my drawings but other cam vendors can do the same modifications. Numerous advantages came from this decision including the choice of cam grinds, cams that were known to run true and are harder than stock Triumph cams (510-05 grind used for the first engine). MegaCycle was provided with instructions for cam modifications using drawings showing the offset required. After engine was assembled, and cam timing checked, lobe positions were within 1 degrees of cam specification, which is more to do with variations in Triumph vernier timing then the cams themselves. Figure 5 shows camshaft lobe offset in comparison to the standard, a critical instruction to the cam grinder. 
get the required offset between points, modify a Boyer-Bransden ignition system or buy a Lucas Rita system that is designed for a 90 degree offset crank. The first way is cheaper but false economy with the variations in fuel quality and auto-advance units. With so much spent on building the crankshaft and cams I chose to modify a Boyer-Bransden system, using two separate ignition units and a modified pickup plate with four pickups. Lucas Rita systems were once available from SRM but they are no longer being manufactured by Lucas. At first I built a unit with two pickups, one in the normal position and the other 225 degrees later. This system did not produce a large enough current to trigger the Boyer ignition unit so another stator was built with two pickups in the standard position (timing side) and two other pickups mounted 45 degrees clockwise from the timing side pickups. Figure 6 shows the orginal modified stator with 4 pickups, each pair feeding a separate analogue Boyer ignition box. This method is no longer used! I keep planning to build a new system using Dyna ignition components but need someone to assist who is familar with these systems. 
complete new flywheel to suit any British twin with a central flywheel. Tooling has also been constructed to modify stock cams and heat-treat them prior to grinding. After building many Norton and Triumph cranks the first all-welded BSA cranks be came available in the winter of 2004 at the same cost as similar Triumph and Norton cranks. A “monster” unit Triumph/BSA crank is under development using an 89mm stroke and custom connecting rods supplied by MAP Cycle Enterprises in Florida. Its possible to buy billet offset cranks for BSA unit-construction twins from suppliers such SRM in the UK. If building a racing motor, or tuned to the limit hot-rod, I would recommend a billet crank. In everyday use, even in a mildly tuned 850 Commando engine, I rate the strength of a welded and Nitrided offset crankshaft as being equal or better than a stock because it is less-stressed then heavier 360 degree configurations.