Con rods

Hello now your rods are the black one is on the fragile side if pushed hard .the one with the piston  on. is the more robust types and there are two types ones with oil holes and earlier ones with out oil holes but both are good the earlier ones I have fitted in my own Norton Manxman ,650 

Yours Anna J

The 650 & 750 engines  used the same design of conrod. Both of these had 1.75” crankpin sizes and un-bushed, plain metal small ends. However, there are three versions of this particular rod to be found. The earliest is recognizable by having an un-drilled ‘U’ shaping to the top shoulder of the crankpin end.

The mark 2 version was also un-drilled in this area but the ‘U’ had now changed to a stronger ‘V’ section.

Finally, to help overcome a cylinder bore lubrication problem, the last version of the rod had a ‘V’ section and an oil spray hole in the top shoulder. This later rod was even used on the 850 engines so is the best to obtain and fit when rebuilding an engine. However, while the spray holes help lubricate the bores, they take away some of the pressure that keeps the crankshaft big-ends happy when under load. So it is fairly important to have an oil pump inside the timing cover with the 6 start gearing necessary to keep the oil feed pressure high. It is, however, permissible to blank off the lubricating holes in the conrods by reversing the position of the shells. Thus keeping the original oil pump system.