Tuesday, February 26, 2013


Recently I have been messing about positioning the big bit of the Overland to get a feel for what the frame will be.  In the process the gear box and clutch basket and the drive cog on the engine needed to be lined up for the first look.

All went smoothly until I got to the chain cog on the engine main drive.  I had a splined stub and a nut but no obvious way to secure the cog.  Well at least obvious to this novice but aspiring garage monkey.

I couldn’t even figure out what type of fixing I should be looking for in the spares.  So I grabbed a photo and put out a query to the JAP engine experts who read a JAP engine facebook group.  Sure enough I got several quick replies regarding how the splined stub, nut, cog and the missing fixing, a circlip, should work.

Well I found the packet of circlips in the spares and fitted one up.  Now here is the explanation of this arrangement for the JAP.

Speedway bike frames are designed with a bit of flex to them.  Something about the torque, no gear box and turning left.  Well that’s what the experts said.  Anyway so as a result of the frame flex the main chain cog splined stub was design such that the main cog could float a bit on the splines as the frame flexed and keep a straight chain line with the jack shaft.  The purpose of the circlip is so you can quick change main cogs rather than having to deal with dismounting the rear wheel to change the hub sprocket.

It sure makes sense to me once it’s all explained.  Never would have thought of that but I’m not a speedway person; just barely a motorcycle guy.

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