Big end torque settings

Can't answer your question as the factory manuals are sparse but this manual states for the Heavy twins it covers 1960 onwards (but not to when it stops sadly)

http://www.eurooldtimers.com/functions/get_file_manualy.php?id=3521

and this is from 68

http://www.inoanorton.com/Articles/1968_AllModelsManual.pdf

both have the big end nut torque as 15 ft lb (defined as 180 in lbs in the 68). I would have thought the higher torque used on a weaker con rod would have caused the bearings to bind on the crank but have no hard facts to support this.

Finally this may sort it out for you and looks to confirm the 25 ft lbs you used is correct provided your bolts were 3/8 and not 5/16.

To avoid distortion the following should be considered the maximum torque figures....

3/8â Big end nuts (all twins 650 onwards) 25 lbs/ft.

5/16â Big end nuts (pre 650 twins with steel caps) 20 lbs/ft

5/16â Big end nuts (all twins with alloy big end caps) 15 lbs/ft

Which is from

http://www.rgmnorton.co.uk/csi/1245184/f/pdf/bottom_end_overhaul_kit_notes.pdf

Looks to me like the factory manuals did not keep up or initially the 15 applied to all con rods, the Haynes then over simplifies it and forgets about the earlier 15 Ft lbs.

The 650, 750 and 850 engines essentially all use the same conrod, with or without oiling holes. Mick Hemmings recommends torquing the big end nuts up to 25 ft-lbs on these rods. Paul Dunstall goes for 22ft-lbs. Both of these people get my vote for knowing what they are on about, so I have always used 25ft-lbs on my big engine rebuilds.

The 15ft-lbs figures generally applies only to the smaller 1.50" journal engines with alloy rods and caps. It was probably also used on the early 650 prototype engines but then upped to 25 ft-lbs on account of the conrod caps being changed to steel. As there were now disimilar metals, in contact, with differing expansion rates, there was probably a need for a higher torquing figure to hold the chunks together.

An additional point worth pondering. I have never ever come across a well-used adjustable spring car torque wrench that gave an accurate figure below 30lbs. Just like cylinder head valve springs the internal spring weakens with aging and what has clicked at a 25ft-lbs setting is more likely to be just 15. Use a proper motorcycle torque wrench if possible.