After many many hours of turnig cranks and watching clear space I realized a few things. In order to gain ground clearance without making the engine unacceptably tall, Yamaha mounted the crank high and ran rather short rods. As the crank moves toward BDC the rods are canted almost parallel to the sleeve bottom. They tug the pistons out of the center of the bore and into contact with the sleeve. Then they approaches BDC and straighten the angle of attack more to the bore center. For a few degrees either side of BDC the piston sits motionless then must accelerate up to where it again sits still at TDC. Stroking the beast would necessitate shorter rods and more severe rod/piston angles; more time motionless at TDC and BDC. The car tuners tell us that a low rod length - stroke ratio is a good thing for acceleration. After considerable thought and measurements, I hazard that the maximum reasonable stroke is 81mm with 3mm shorter rods with the bolts coming in from the top and into blind holes in the caps, 6mm longer sleeves, and considerable skimming of the flyweights. Reckon on $2000 plus for stroked and balanced crank, rod and main bearings, CP-Carrillo rods, a blue-printed oil pump (exchange), and a K&N oil filter.

Bore? The cam drive gears spin on rods lubricated by the oil scraped off the cylinder wall by the rings on the down stroke. Really broad (and longer) sleeves would block this flow. The studs are perilously close to the spigot holes in the case. I hazard that 105mm is the safe limit: it's a popular bore size with vintage racers in OZ. To bore thousand or eleven cylinders (exchange) to accept larger sleeves, commissioning a pair of these sleeves, pressing them into the cylinders, opening up the passages in the cases, acquiring oversize head gaskets, and o-ringing the cylinders would likely exceed $2000.

It would be a good idea to install an oil cooler (exchange on the oil filter cap); about $200. Note that I haven't touched on cams, carbs, pipes, or really anything else. What do you get for your (at least) $4000? 1403cc or 85.6 cubic inches.

To make it powerful, you are looking at cams and springs (exchange), rockers, valves, head-work (exchange), ignition, carbs, and pipes. Reckon on another $5000. What do you get for $8925 plus shipping and the effort or expense of the work involved? I'm considering an engine exchange: an additional $1000 and you pay the freight both ways. First consider more modern running gear: 'upside-down' forks, four live piston calipers all around, wider rear tire, whatever you can think of. Figure on spending maybe $4000 on this. Now you have a bike that can smoke the tire or wheelie in the first three gears. A bike that can run a quarter mile in the elevens. A bike that can't be beat out of the hole. A bike that can pull alongside a four cylinder (or ANY air-cooled twin), roll on the throttle and not get caught. Tons of fun.

For those of you that have discovered Project Silver Back on another of my sites, this configuration is the one likely that I'll use.

Last Modified:   Mon => 02:57:19 PM PDT - 18 Oct 2021 America/Los_Angeles