Author Topic: Confused again, Big-End Nut Torque.  (Read 127 times)

Offline Steveba10

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Confused again, Big-End Nut Torque.
« on: 04.11. 2020 20:06 »
I’m aware I’m easily confused, but I do like to understand why certain thinks are done certain ways.
Better safe than sorry especially with torque wrench settings.

So my confusion is after reading both the BSA service sheet 208 and the Haynes manual for A10 Twins.
I’ve added pictures of the relevant paragraphs from both for comparison and hope to get some clarity.
I’ve searched the forum but have not come across the definitive answers amongst the talk centring on ARP.

My plunger is 1957, the drive side crankcase is date stamped 14-6-57. Engine number BA10-15XXX
The alloy rods have 26tpi cycle threaded bolts with castellated nuts with split pins.

From 208 It explains about castellated nuts and split pins then states the torque settings.
No mention of self locking nuts here.
Early bolts -  22tpi - BSF - Torque 10 lb/ft.
Later bolts -  26tpi - Cycle - Torque 8.5 lb/ft.
I assume both 5/16” diameter.
So no confusion yet, mine are the later type.
Lower torque setting due to greater clamping force with finer cycle thread, I’m happy with that.

But the next sentence says “ From 1956 onwards the torque spanner setting is 22 lb/ft.”  But mine is a ‘57 so have the bolts changed?
Two and a half times greater torque setting! 
No explanation!
I’m getting a bit confused now so maybe Haynes can sort it out.

I look to Haynes and it says “torque spanner set to 22 ft/lb.”
and then “( 8.5 ft/in engines prior to BA10-13830, ———)’
And then goes on to say self locking nuts are used.

So can you see my confusion?
First is it only bolts fitted with locking nuts that need 22 ft/lb, are these bolts still 5/16” Cycle thread?

Second are those bolts 1/2” diameter because a quick calculation says the cross section area of a 5/16” diameter bolt multiplied by 22/8.5 equates to 0.1985sqin. which is the cross section area of a 1/2” diameter bolt.  Assuming the torque setting relates to equal stress in similar material.

Am I happy, no, my Jag’s 700cc per pot and the big end bolts are only 3/8” diameter.
So 1/2” must be way over size for a 325cc BSA pot surely.
The only other notion  I can come up with is the use of a steel with a very much higher working strength that warrants the higher torque.

Do any of you have an explanation or can tell me the correct torque for my 5/16”. 26tpi Cycle thread nuts with split pins?
Thanks for reading,