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to the Archive indexI'm sure many of you have read the Mini-Motor articles in this Moped Archive. I read them all but still had a few problems.
My late model Mini-Motor was running poorly and I decided to check the engine over and fit new crank seals. On stripping the engine I noticed that the crankcase was chipped and decided to use an earlier crankcase from a box of bits, being a Mark 1 it has the two long studs protruding from the base of the crankcase, and its number had an A prefix meaning it was a 1949/50 model (I've got to get a dating certificate and an early year would be nice). No problem here, the studs were easily removed.
The other change was that the early crankcase halves were secured together with four studs and nuts and I needed bolt fittings to suit the bracket on the base of the petrol tank. The studs to bolts was a known change from Mark 2 to Mark 3 Mini-Motors.
That's OK, I'll remove the two studs that are in the position of the mounting bracket and drill through the crankcase so I can fit two new bolts.
No, that wont work as the crankcases for the stud type are a lot wider where the studs go through. I'll spotface each side of the crankcase to narrow the bolt bosses, plenty of meat on the casting, so the width is the same as the bolt together crankcase, easily done...
But the bolt holes in the later crankcase are in a different position (or one of them is) and if I had continued to do all this work and machined down the width of the Mark 1 crankcase bosses and then tried to fit the whole lot in place I would have found that the cylinder head will foul the bottom of the petrol tank (it would be tilted upwards instead of sitting level) due to the different position of the holes in the crankcase. Moreover, there's not enough room to redrill the bracket that's on the base of the petrol tank.
So it's back to the original Mark 3 crankcase, and a little weld repair done by my local engineering shop...
So, if you are building a Mini-Motor from a box of bits, have a good look round first ... and take a few measurements.
First published in Iceni CAM Magazine, January 2010.