Having seen a few CR500 hybrids (2T motor in an alloy frame) on the ‘net, Jeff decided to build one using a late model CRF chassis. Unlike those which use the earlier two-stroke CR frame, Jeff knew that there was a hell of a lot of work involved in getting the old stroker lump into the four-stroke’s frame, so he asked Phil if he was up for it…
Often, when building a hybrid, it’s hard to purchase just an engine and just a frame, instead you need to look at buying complete bikes. And that was just the case here. Jeff saw a 1991 CR500 advertised locally and snapped it up, before posting a ‘wanted’ ad on the web for a CRF chassis. This led to a 2007 CRF450 which had grenaded its motor. Twice. The owner had just spent £1400 having the engine rebuilt when it blew-up for a second time, and he simply couldn’t face that kind of expenditure again. Jeff ’ripped the bloke’s arm off’ when he offered him the bike for just £650 quid.
With the engine out of the CR, and just the rolling chassis of the CRF, Jeff took the parts down to Phil’s workshop and work commenced. Although it fired-up okay, the motor was fully rebuilt ‘you don’t do a project like this without stripping the engine’, noted Phil - and a good job too, as removing the head revealed a huge crack in the piston!
New bearings and seals were used throughout, the crank balanced, and once Stan Stephens had refreshed the bore and played with the porting a new, slightly oversize, Prox piston was slipped in. ‘It’s not in a mental state of tune because I wanted it to be reliable. Stan gave it what he called an “enduro” tune.’
As the bike was intended for road use, Jeff wanted to be confident that it wouldn’t boil-up in traffic so ordered a Coolhead - a machined aluminium cylinder head featuring improved cooling and interchangeable combustion chambers that allow you to alter the compression ratio. However this, along with the Boyesen clutch- and ignition-covers were left on the shelf and the original parts put in place so that the motor could be taken in and out whilst the frame was being modified without scratching the new components.
Phil worked a great deal of magic on the CRF frame in order to get the motor into place. Firstly, the 500’s expansion chamber necessitated the Y-piece on the frame’s downtube be moved 3.5in higher. In order to achieve this some people would simply cut it out and add some box section in the gap underneath, but not Phil. Instead he chopped it out, raised it and then created a pair of new lower frame rails. Leaving one original rail in place while he worked on the other side allowed him to obtain the exact same curvature, before replicating it on the other side. The result is a frame that at first glance appears factory standard… except with better welding!
Positioning the engine brought about a further problem. The CR500 motor is a sizeable lump, and on such conversions there’s often precious little clearance between the spark plug and the bottom of the fuel tank. One way to get around this is to gently heat the plastic tank and press it up out of the way. That wasn’t an option here, as Jeff would be using a carbon fibre tank, so Phil had to ensure that the engine sat low in the frame. A set of engine mounts were fabbed from billet and there’s now plenty of clearance for the sparkplug without resorting to a shorty plug.