Hi Ken,
Over the past few years, I've been asked about building such a bike and have always taken the easy way out and replied that I've never been interested in building one, which is true. However, I have good friend (that collects and restores only Yamaha's) who has thrived on asking me about building "that" dual shock/air cooled/open class/CR on every given chance for the past 7 or 8 years. The first time he asked me about building such a bike, I told him that I had no interest in building one and would never do so. A few weeks later, he asked me again or made reference too "that bike" which kind of got under my skin at that time...I guess that is when became persistent about asking me what it would take and how far along I was with "that" project. Knowing that I had the parts and the means to build one, my friend Ted found it intriguing to ask me (hypothetically) what parts could be used to build one. I can honestly say that I have actually spent the time to see what it would take (in general) to build such a bike, just so I could tell my friend that this is what needs to be done...but I'm never building one. To build/restore one of these in a professional manor, will not be easy or quick.
With a '81 CR450R motor (or with the '82 or '83 CR480 motors) one would want to use an '80 CR250R frame. All three of these CR motors use center port exhaust, a '78/'79 frame would need to be cut at the down tube and cradle, then split/fabricated into a double down tube. Using the '81 motor would allow a little more clearance area between the pipe, frame and tank then the '82 or '83 motor. Though I'm not all that familiar with the '82 and '83 CR480's, it's well known that they were better then the '81 CR450 motor (which had clutch problems and a rather strange power band). However, when it comes to parts, they are all 1 year motors; the '81 and '82 were 4 speeds while the '83 was a 5 speed. The '82 and '83 480 motors share the same crank, but aside from that, they are more different then alike. Internally, the '82 CR480R motor is closer to the '81 CR450R, and the '83 CR480R motor is closer to the '81/'82 CR250R's. Clear as mud? Also, you should keep in mind that because of their popularity, the '83 motors (and parts for them) are very sought after and somewhat harder to come by.
Regardless of which motor used, the following parts would need to be customized and/or made: front engine mounts, rear engine mount/swingarm spacer, lower engine mount spacers, exhaust pipe, air box, kick starter shaft and/or arm joint (with use of an '81 motor) and rear wheel spacers. Depending on how the air box is made, an alternate intake manifold may need to be located as well. The CDI and coil for the year of motor chosen would also have to be located and adapted to the '80 CR250R frame (the '81 and '82 CR450/480R coils are the same, but each CDI for the '81/'82/'83 open CR's is different).
If someone just wanted to have an open class CR to ride in the dual shock/air cooled class, I'd suggest they find an '83 CR480R and modify it into a dual shock machine; similar to what Johnny Slomp of Holland has done with his bike (look at ad #2787 on Nathan's swap meet page...nice work Mr. Slomp). That could be done a lot easier (and probably with less money) then fitting the 450/480R motor into the '80 CR250R frame; or cutting the engine cradle from an '83 CR480R frame and fitting it to a '80 CR250R frame.
One other option would be to locate one of those rare RC400/450 motors and adapt it to the respective CR chassis. This would be my personal choice if I were to ever build such a bike.
Take care,
Robert
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