The DT175 has a gear change mechanism on the left of the motor, which fits under a small cover, which is secured with 4 bolts.  I wonder if anyone has come across the boss in the cases which  No5 in the parts drawing fits into, getting worn oval? 

TY175 gear change mechanism is pretty much exactly the same, and many of them are badly worn.  Wonder if wear is common on the DT's as well, and if it is, does anyone have any good ideas on how to repair them? 

www.cmsnl.com/yamaha-dt175-1975-usa_mode.../partslist/B-03.html

As I've mentioned before, that 74-76 design is not one of Yamaha's better ideas. They are almost always sloppy. I'm not sure if true today, but not that many years ago the covers were readily available and swapping the cover before it gets really bad does help a lot... for a while. Machine shop could possibly bore and bush the case.

No its not great, and wear is something that means gear changing is often poor.

The covers are still available, but the gear shaft that fits into the boss isnt, and in most cases thats worn as well.

I suppose the point of the design was to make the cases smaller and more compact?

My guess is the change was to move the shift lever location. 

A trials bike has the footrests about 4in further back than a trail bike, so you are probably right about the reason for the gear change mechanism? Yamaha then used the cases on 100/125/175 trail bikes, as well as trials.

Some of the old Spanish trials bikes, which use motors derived from road bikes, have gear levers which are a long way forward. In the days when they were competitive, though, changing gear in a section was not common.

Have made a bushed housing that fits into the stock gear change mechanism cover, but need to work out some sort of cutter, so I can machine and bush the crankcase, without a complete strip down.