Saturday 19 November 2022

The AJS 33/7 Trophy Model Restoration: The Cylinder Head Steady

I haven’t posted anything on the restoration for about 6 weeks, mainly because the dry build is nearly complete and there have been many little “tidying up” jobs that are not really worth reporting. One sub-project that has come to fruition is remaking the cylinder head steady. This is essentially a clamp on the front frame downtube that is attached with a 5/16” bolt and eye to the cylinder head.

This is what it should look like (picture from my R10):

And this is the clamp that came with the bike:

Someone has made an effort to make this clamp but, whilst I am not a stickler for total originality, this clamp is pretty ugly and would not be out of place supporting a car exhaust system.

I decided to make new clamps using the originals on my R10 as patterns. The process for the castings follows identically that used for the exhaust rings on the V-Twin and other small components that I’ve made in the past.

The starting point is take one of the clamps (it doesn’t matter which one is used – they are both the same):

and to build a small wooden box with 2BA screws around the clamp.

Short lengths of 5/16” diameter aluminium are then attached to the top of the clamp to act as a feed sprue and riser and the arrangement is then suspended about 1cm above the bottom of the box. The 2 pieces of brazing rod are to keep it level. The sides and bottom of the box and the screw threads are coated with Vaseline.

RTV silicone rubber is then mixed and poured up to the top of the part

and after setting, the RTV silicone surface is coated with Vaseline and more RTV added up to the bottom of the screw heads.

The mould can now be dismantled to remove the original clamp.

After reassembly and with the addition of a paper funnel for pouring, resin can now be poured into the mould

and, after hardening, a facsimile of the original component in resin can be removed. 2 were made and sent off to the foundry.

I used a local foundry, White Eagle, who cast these in “Naval Bronze”. Bronze is really a misnomer for this alloy as it consists of copper and zinc – which should really be referred to as brass. Anyway, this is more than adequate for these clamps. After a short time, I picked up 4 newly-cast clamps.

and, after some machining, these were ready for fitting.

There is one deficiency in the original AJS design: the threaded end of the connecting piece cannot be inserted into the cylinder head when the head is assembled on the bike – there is simply insufficient space to avoid the front frame downtube. The picture below is from the R10.

I therefore made the connecting piece as a turnbuckle – a RH thread in the cylinder head and a LH thread into a separate connecting eye.

The advantage of this design is that it can be positioned accurately and attached with the cylinder head in place.

There are many smaller jobs on which I haven’t yet reported but the dry build is essentially complete and the bike will shortly be stripped back to every component for painting and plating. I have also not yet stripped either the bottom-end of the engine or the front forks so we’ll see what surprises they yield.