This is a faithful reproduction of the SOHC exhaust valve lifter arn and the special 'reduced head' bolt that is fitted with it
It slides over the eccentric exhaust valve lifter arm and can then be adjusted to give the correct level of tension.
An original Norton arm was used to produce the patterns for this arm, and they have then been cast using the Lost Wax process, which gives a finer finish than sand casting. The arm is identical to an original item, except it is cast from a stainless steel, so will not rust and comes in a satin finish almost identical to the original satin nickel finish.
The bolt is also copied from an original, and faithfully reproduces the reduced 'across flats' head (i.e. the hex width is less than normal for a 5/16" bolt size, so the head does not foul the cambox), but is also deeper than a stanard bolt head, so the spanner contact area is maintained. As a final detail, as per the original there is a slight taper on the base of the bolt head which corresponds with the countersink in the arm, which I think was intentional with the originals - as this was a commonly adjusted item
Again, the bolt is identical to an original, except manufactured in stainless steel. and as teh correct imperial Hex is now getting difficult to source - we mill the head of these bolts to the correct imperial spanner size.
These items are now getting very hard to find at autojumbles - and normally the hex on the bolt is chewed, as people use the wrong spanner.
Sorry they are not cheap, but the price reflects the cost of lost wax castings and CNC machining