MX250 Engine Reassembly

Basically, the best order for rebuilding the engine is probably as follows;

– heat the cases [ I’ve never tried the oven approach, though my [ kitchen is still fairly new .]] and drop in the new bearings, I used SKF 6306/C3 bearings, they are both the same size.
– place the left hand (LH) crankcase (ie flywheel side) face down on some blocks (I’ve made a simple frame which works for most the two stroke engines I’ve worked on
– slightly heat the bearing in the LH case and just drop the crankshaft in; this saves any need to tap it into place and therefore damaging the threads on either end (it isn’t a good feeling if you do mash these with a hammer, which is possible even with the nut on the end.
– place the new seals in the right way round over each bearing. The LH seal is 32-62-10 and RH seal is a 40-62-10, the only thing to note is that the OEM Yamaha seal has a castled top, to hold it clear the bearing, whilst an ordinary seal won’t have that; not sure what impact this has on assembly.
– drop in the gear cluster; you can put in both shafts together at the same time and put in the selector forks and drum in afterwards. Slacken off the selector both through the case and make sure the shafts are seated correctly. Put in the selector forks, but pull the shafts for the forks out, allowing the drum to be put into place, before swinging them in an locating the shafts. Put the drum into the neutral position and you can then check if the shafts run correctly before proceeding.

I wrote these notes a few weeks back and haven’t had a chance to run the engine in a frame yet but should be all okay.

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