Pushing on

(This one is out of sequence as I forgot to post it, sorry.                                                                 It should appear between “Sod’s Law Strikes” and “On Tap”.)

Once I had the clutch assembled it needed adjusting and I ran into another snag, there was no free play in the mechanism.

The linkage consists of a lever inside the gearbox outer cover, a short connector piece it bears against, a 1/4 inch ball bearing and the pushrod itself.

It is not possible to mis-assemble at the lever/connector as if you do the outer cover of the gearbox will not go home and omitting the ball would leave too much play so suspicion fell on the pushrod.

Checking the books showed there were two slightly differing lengths available for the pushrod across the period when my bike was built but I found no reference as to when the change was made.

So it was a case of take off the clutch spring plate, remove the pushrod and check which one I had.

On removing the pushrod it was obviously a home made rod and rolling it on the bench showed it was bent so I decided to make up a new one to fit.

To do this I got a length of 1/4inch diameter silver steel rod. This is readily available, I got mine from Cromwell Tools

It comes in a 13 inch length and is diametrically accurate to within 0.00025 inch, not only that but it is easily heat treated to harden the ends.

First thing was to measure the old pushrod to determine its length, as it was very nearly correct in length I cut the silver steel to the same length and made a check assembly of the clutch.

Sure enough there was still no free play so I shortened the new pushrod by gripping it in padded jaws in the vice and filing the end back a bit.

Because it’s much easier to remove material than to add it I only removed a little then tried another check assembly then repeated the operation until I had the play I wanted.

The ends of the pushrod were then polished with a hand held oil-stone and the sharp edges taken off at the same time.

Now to harden the ends. Because of the composition of silver steel all you need to do is heat treat it, there’s no need for Kasenit treatment as it would if I had used bright mild steel to make the pushrod.

First stage was to heat one end until it was bright red-hot, about the colour of a cut carrot, and to hold it at that temperature for a couple of minutes.

The red-hot end was then plunged vertically into water and stirred round vigorously to quench it.

This hardens the heated metal to a “glass-hard” state, one where a file or a hacksaw will “skate” on the surface and will not cut it, downside is that at this state of “temper” the metal is brittle.

So I next “drew the temper down” so it was no longer as brittle but was still a good bearing surface on the ends.

To do this the now discoloured end was cleaned with some fine wet and dry paper and given a polish. The trick now is to apply heat to the rod about an inch in from the end and watch the colour change at the end.

As the metal was heated, at the heated point it turned blue and was kept at this.

Colour could be seen to move from the blue area down the pushrod and at the polished steel end it first showed a light yellow and when it had darkened to a yellow-brown it was again quenched.

This drew down the temper to the level you would use for making a punch which is what I wanted for this job.

If you want more details as to tempering steels a web search will throw up a number of sites such as this one;( be careful, this site is very moreish!).

Having done one end of the pushrod I then hardened the other in the same way and I had my new pushrod ready to fit.