Cylinder Head Torque

When you replace the cylinder head, it is necessary to tighten the bolts to a predetermined torque. For the C15 it is 28 ft/Ibs. Easy to say, but not easy to do, as BSA for some reason of insanity, placed two of the four head nuts under the rocker cover, and they can not be accessed by a socket attached to a torque wrench.

Special Tool?

You could manufacture a spanner that will fit on the nuts with a hexagon head to take a torque socket. Not bad if you have a workshop and do not mind wrecking a good spanner.

An alternative, is to take a spanner with a 15mm end and fit a nut and bolt in the other end to attach the torque socket. Lock the nut and bolt together and tighten in a vice to 40 ft/lbs. Now you will need to calculate the correct torque to use for these two nuts. The other two are quite accessible by the normal socket. Place the spanner on the nut and the torque wrench onto the bolt on the other end and apply the amended torque.

It is important to tighten the bolts diagonally and in increments.

Calculation

When an extension of this type is used, the mechanical advantage is increased and it is therefore necessary to calculate a reduced torque loading (because of the increased leverage) to avoid over-tightening the nuts and the corresponding consequences. A torque wrench graduated in ft/lbs usually gives readings obtained with a leverage of 1 ft and this will be referred to as the “original length” in calculations for levers of a different length and the extra length is assumed to be 3"

HOW TO CALCULATE THE REVISED TORQUE SETTING

Original length 1ft x required torque 12 x 28 336
------------------------------------- = ------- = --- = 22.4
Original length 1ft + extra leverage 15 15

Consequently, when using a 3” extension with a torque wrench graduated in ft/lbs it would be necessary to set the wrench to 22.4 ft/lbs, in order to obtain a torque of 30 ft/lbs at the nut.

The extension and the torque wrench must be in a straight line.

Note:- When the torque is quoted as a number of ft/lbs the leverage must be measured in feet, otherwise the result of the calculation will be incorrect.