One reason why the involute form is almost universally chosen for gear teeth is that it is to a degree tolerant of small inaccuracies in pitch centres. That is not saying that it is entirely acceptable, just that mechanically it’ll still work OK. In a perfect world with perfect centres and tooth profiles the contact faces of the teeth will have a true rolling contact thus promoting minimum wear, constant velocity transmission and less noise.
In post #6 it is stated: “not all idlers are dead round”. Well if it ‘aint send the b****r back ‘cos it is N.B.G.! A properly cut gear will be absolutely concentric with the centre hole and the correct tooth profile is is built into the calculations when the gear is cut.
The old Toolmaker’s adage that; “The time spent making a jig is never wasted” is true here, it would be well worth the Club considering making such a jig and hiring it out so that members can the idler shaft perfectly positioned.
The correct centre distance between crankshaft and idler is 3.781” (96,04mm) and between idler and camshaft 3.031” (76,99mm).
However the expansion of the crankcase should not be ignored, if the crankcase gets to around 60°C then the crankshaft/idler centres will increase by 0.005” and the idler/camshaft distance by 0.004” when using an alloy idler that is mitigated somewhat as it too will “grow” but a steel idler will expand by considerably less than that. Also regardless of material for several reasons the idler is unlikely to attain the temperature of the crankcase.
A council of perfection? Well maybe, but it is worth considering the customary engine operating temperature and modify centre distances to suit.
For those who don’t want all that faffing about it is worth noting that the correct backlash for 16DP gear teeth is 0.0098” (0,025mm). Make that 0.009” (0,023mm) to allow for crankcase expansion and if feelers of that size will fit snugly between the meshing teeth the centres won’t be far wrong.
Finally, worn or not there is only one correct centre distance . . .