750 big pin cranks

[Basketcase]

OK ... purely from curiosity ...

Why do racing 750 motors use 1000cc inner flywheels for the big 35mm centre pin conversion ?

1) Do the 1000cc inner flywheels have integral big end crankpins so that there is more meat for the centre main pin interference fit ?

2) What is the big end width for the 1000cc crankpins ? If not 30mm, then how do they line up the outer axles for overall length ?

3) If 750cc outer flywheels are used, surely slightly unequal tolerance crank throws will result even if the stroke is nominally the same ?

4) Is it not easier just to make flywheels from scratch to line everything up rather than mix and match ?

So many questions, I am starting to sound like my kid.

 

[phil37]

1) - Yes - there is only 1.5mm of "meat" left if using a 35mm pin with all 750 parts.

2) Width of triple pin is adequate when using triple or late SFC rods with appropriate spacers and thinning outer 750 webs

3) Don't follow that - but I turn outer webs to same diameter as triple ones

4) If you have a full machine shop with correct heat treatment equipment - then maybe.

  Regards - Phil

 

[sfcpiet]

OK ... purely from curiosity ...

Why do racing 750 motors use 1000cc inner flywheels for the big 35mm centre pin conversion ?

1) Do the 1000cc inner flywheels have integral big end crankpins so that there is more meat for the centre main pin interference fit ?

2) What is the big end width for the 1000cc crankpins ? If not 30mm, then how do they line up the outer axles for overall length ?

3) If 750cc outer flywheels are used, surely slightly unequal tolerance crank throws will result even if the stroke is nominally the same ?

4) Is it not easier just to make flywheels from scratch to line everything up rather than mix and match ?

So many questions, I am starting to sound like my kid.

The factory achieved a surprising level of precision, given they just had more-or-less fundamental machinery, no CNC gizmos back then!

A crank built from spares usually ends up just as straight as one that was built as a unit originally.

piet