Yamaha RD1200 V6

Classic Motorcycle Mechanics - - WORKSHOP CMM -

He’s back on with this amaz­ing project – prob­a­bly the ul­ti­mate two-stroke build seen in the UK.

Iam build­ing what I hope will be the ul­ti­mate two-stroke, a 1200cc V6, based on one and a half RD500 en­gines us­ing Yamaha Ban­shee race bar­rels and heads. A good cus­tomer of mine, Ian Bux­ton has com­mis­sioned me to build it and he in­tends to have a bike built based on CMM’S Retro Re­boot Yamaha RD500. In Jan­uary 2015’s CMM (is­sue 327) I cov­ered the ini­tial cut­ting and weld­ing the crankcases and in the fol­low­ing is­sue I cov­ered ma­chin­ing and weld­ing the bar­rels to suit the dif­fer­ent stud spacing of the Ban­shee bar­rels and the RD500 cases. This month’s mis­sive con­tin­ues with the project, so let’s get on, shall we? One thing that I have to think of when build­ing this en­gine is that it must still be ride­able on the road: i.e. the Ban­shee bar­rels don’t have pow­er­valves. With this in mind I am us­ing 8mm longer con rods on the cranks – this will give more torque. The stroke of the RD500 is 50mm but the stroke of the Ban­shee is 54mm so the cranks were mod­i­fied to 54mm to get the port-tim­ing right and also the longer stroke will give more torque. To achieve the longer stroke was tricky: the crank-pins that the big-ends run on are part of the fly­wheel webs on the RD500 cranks. If the crank-pins were bored out to fit off­set pins the holes from bor­ing them out would be too close to the hole through the cen­tre of the fly­wheels. The crank-pins had to be ground off and the fly­wheels bored 2mm fur­ther out to take the new pins (it is 2mm, be­cause to get a 4mm longer stroke, the pis­ton will have to travel 2mm fur­ther up and 2mm fur­ther down).

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