Honda RC500 crankshaft
My latest find is this RC500 crankshaft. I never knew Honda made such beautiful and well engineered crankshafts until I saw the RC500 crankshaft of Terry Good (MX Works Bike) . I got interested in these cranks for my RC250M project and after a very long search I found a RC crankshaft for my 2001 RC250M. The RC500 crank is a lot different compared to the RC250 crank. The RC500 crank is stuffed with tungsten where the RC250 crank is balanced with only three plugs. Here is a quote out of Terry Good’s book; Legendary Motocross Bikes.
This is the HRC RC500 snap-ring crankshaft that is talked about in my book “Legendary Motocross Bikes.” These cranks are tunable for different power-band characteristic’s by altering the rotating inertia. Steel, tungsten and nylon inserts were available and gave the tuner the flexibility to adjust the power-band simply by changing the weights. The process to change the weights is very simple as they are held in by snap-rings located in the crank flywheels. This along with all the other available options that HRC offered, made the RC500’s some of the most competitive motocross bikes ever produced. Ask any factory Honda rider that ever used one of these crankshafts and he will tell you this was the biggest improvement on the bike. Even though the crank flywheels look like they are made from billet, they are actually made from several machined pieces that are hollow in strategic areas and then electro laser welded together. Very few of these were made and the cost was well over $10,000.00 in 1980’s dollars.
HRC 1997-1999 engine cases
I had the opportunity to take a look at a set of 1997-1999 Honda RC250 engine cases. These are the cases as used by Fred Bolley, Stefan Everts, Joakim Karlsson, Gordon Crockard, Sebastien Tortelli, Ryan Hughes and Steve Lamson to win many races and championships.
As you can see on the picture above, there is a black sensor mounted in the engine cases behind the shiftlever. This sensor is a gear position sensor which HRC linked to the ignition from 1997 until 1999. The gear position sensor features a pawl which fits in a modified gearshift drum and so a different ignition map for each gear can be achieved.
To fit the gear position sensor into the Honda CR250 cases, HRC modified each left engine case by welding an extra chamber on the outside aligned with the gearshift drum inside.
As you can see there is only a little weld around the area where the sensor is bolted on, this is because the rest is welded from the inside of the case and then milled down to provide just enough clearance for the gearshift drum.
The crank and transferports are machined too. Differently than most engine tuners would do, HRC modifies their cases on a CNC mill, rather than grinding them with a rotary tool.
Despite using the latest technology at the time, the CNC mill doesn’t exactly provide the smoothest surface I have seen, but I’m sure HRC had their reasons for it.