kyosho optima mid ball diff
 

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regarding this instruction for the rear of the optima mid diff.it says to install spacer 14.mmy question is..is this spacer really necessary? I built the rear diff like the fronts and the spacing is cortect. So my question is..what is the purpose of this odd looking spacer ?

I have asked myself the same question
The only thing I can think of is that it provides some form of stabilising effect and prevents the main gear from moving off centre in relation to the axles and bearings
However to do that would require it to be a tighter fit than it is and also would incite some friction when the diff is working i.e. when cornering as the spacer is not static relative to the main gear

Its the only purpose I can think of

If that is the case..if i can find a bearing with 8mm id and the od same or slighty smaller than the ball diff gear inner dia...perhaps it could work better than the spacer.

It supports the belt side of the diff gear. With all the others the torque load is in line with the ball diff, on the Mid while all the torque from the gear drive goes through the ball diff, without the spacer supporting the pulley the belt will pull on the pulley side and try and twist the gear on the shaft. The spacer supports the pulley on the inside like a bearing so the gear stays centred. As the only time

A better solution would be to use a bearing, or a small bearing in a larger ring that's a good fit into the pulley. As there isn't a bearing in the centre of the diff it would also help support the diff itself.

Thanks terry and hks. Makes sense. I am now working on a fix for this with a 8x16 bearing and a sleeve. Will post pics when done..which should be this weekend.

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Done..bearing was sleeved to increase outer diameter to 19.8mm.