Not many Aveling rollers were supplied with diffs . , and those that have them will be keeping them . You could get one made , but it would be very expensive indeed , because bevel gears are awkward things to cut or to profile . What sort of an Aveling roller are we talking about ? Most of them are not worth the great expense of a new diff .
Remember that diff gears were cast from patterns which were probably supplied to Avelings by specialists who made gear patterns . Fowlers seem to have bought in gear patterns . Added to this is the cost of a diff centre pattern , and different final drive gear ring to bolt to the diff centre . Once Avelings had the patterns then the cost of a steel casting for an individual gear would not have been inconsiderable , depending on whether Avelings did their own steel castings , many engine builders did not , including Burrells pre WW 1 . By and large bevel gears on our engines could only be made economically by casting them , tho' diffs for double drive engines would have been cheaper .
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