Old Ned wrote:
The dimensions I gave (30mm/120 for 5 and 35mm/125 for 6) were taken from an article by Mike Mullett in an early 70's 'Cycling' when Regina blocks were the norm for most riders. Mike most probably used the dimensions required for these as his 'standards' as in the accompanying article he describes the various Regina body/sprocket types and possible combinations to suit varying requirements.
Are you saying that if using another brand of block these dimensions may not necessarily be correct?
120 mm outside width between outer surfaces of the locknuts were more or less standard until the 1970's. Generally speaking this was designed for freewheels with a maximum of 5 cogs. Suntour, and later others too, however made a "narrow" version that allowed one to fit 6 cogs in the space of 5 "normal" cogs of for example Regina. Likewise it was possible to play around a bit with cones, washers and locknuts to sometimes, with some frames, fit a "regular" six cog freewheel into a frame spaced for 120 mm spacing. By the end of the 70's virtually everybody had switched over from 120 mm spacing to 126 spacing. This was ostensibly to allow for 6 cog freewheel spacing, but again here, some manufacturers were able to fit in an extra cog in the same space, likewise with some fiddling, you could also fit in extra cogs by switching out the cones, washers and locknuts. Towards the end of the 80s everybody was passing over to 130 mm spacing, which has since become standard for racing bikes (MTB and tandems have their own sizes).
Each of these measurements is made up of the distance between the flanges, plus the space on each side of the flanges and the outer surface of the locknuts. The more cogs you would like to fit on the hub, the more space you need in the space between the right side flange and the right locknut. If you keep the total distance between locknuts unvaried and move the flanges closer in, you will be able to have more space available for cogs. If you measure vintage hubs, you will see that many have been messed around with. Some formerly 120 mm hubs are now 123 mm with 6 cogs. Some are now 128 mm...