Chain Tensioner

The last piece of the jigsaw is the chain tensioner.  This simple blade tensioner was originally developed by John Weller (who also played a significant role in the early days of AC and, indeed, AC patented the chain tensioner). In addition to early OHC AJS motorcycles, this design continued to be used on post-war AJS 7R and Matchless G50 racing engines and was adopted by Jaguar for the very successful XK 120/140 and Mk 7.

You can see the crumbling remains of the K7 tensioner blade and spring below.

There are actually 3 springs, 2 pins and one sliding blade support that make up this system. The 3 springs are, of course, the blade itself that bears against the chain and the tension spring that is stretched between the 2 pins at either end, but also a thin sheet steel spring that retains the sliding blade support in its slot but allows its lateral movement. All of these parts (new ones!) are shown below.

By an amazing piece of luck, when I phoned the company that I have always used for making springs, namely Alberta Springs in Colchester, Graham, who owns the business and is springmaker-in-chief, told me that not only had they had made these before, but even had drawings! Sure enough, a couple of weeks later a new set of 3 perfect springs turned up in the post. Since the time when these springs for the K7 were made, Alberta have also made further sets of these chain tensioner springs for the subsequent AJcette and V-twin projects ….but more of these at a later date.

The picture below shows the new installed complete drive system for the overhead camshaft.

Camshaft Drive Outrigger Bearing - K7

The last piece that needs making is the bearing housing for the camshaft drive. On the original engine this is an end-cap that is screwed down onto the 3 pillars. Here, it needs to be a more substantial structure with 3 “legs” to support the bearing as the original pillars no longer exist.

This was made from a solid piece of high-strength aluminium alloy, 7075-T6, on the lathe and milling machine. This is one of the strongest aluminium alloys known and is substantially stronger than mild steel.

The series of picture below shows various stages of the machining on the lathe and milling machine through to completion.

With the lip to ensure positive location and using high tensile (EN24T) 5/16” studs this arrangement is substantially stronger and more rigid than the original.

Heat Treatment

In addition to having good machinability, the advantage of using O1 tool steel is that heat treatment – hardening and tempering - is straightforward.

Some years ago I purchased two small furnaces on ebay. They are quite old but were really cheap and have proved very reliable. The furnace that I use for hardening, shown below, has a small working section because it needs to be heated to around 800 ⁰C but it is quite large enough for the gears, shafts, bearings etc that you find on motorbikes.

The other furnace has a much larger working section because it only needs to be heated to around 250 ⁰C maximum. It is also used for heating castings for removing and fitting bearings and for preheating prior to welding. My missus would be really unhappy if I put large and, when heated, smelly chunks of metal into her kitchen oven!

The basic heat treatment process is:

1)    Heat gear to 820 ⁰C in the small furnace and oil quench. Cheap vegetable oil from the local supermarket is used.

2)    Temper at 230 ⁰C to give a HRc (Rockwell Hardness) of 60/61, which is a quite sufficient degree of hardness with good toughness for a gear like this.

A few practical points:

Before heating to high temperature, stainless steel lock wire is used to make a little “package” so that the gear can be easily taken out of the furnace when hot. An anti-scaling compound (see picture below) is also used to avoid a black oxide scale that would otherwise form after high temperature heating. Finally, the larger furnace is preheated to the correct temperature so that the gear can be put into an already warm environment for tempering immediately after quenching.

After heat treatment, the anti-scale compound can be easily removed because it forms a baked hard shell that peels off to leave a clean surface. The picture below shows the gear and the pinion after heat treatment but before cleaning up.

Keyways need to be cut in the gear (and also in the sprockets). I leave this to after heat treatment because we have a local engineer with 50+ years of experience in spark eroding and it makes no difference whether the material is in the annealed state or has been hardened. In the absence of a spark eroder, the keyway would need to be broached before heat treatment.

Finally, a tumbler using ceramic chips is useful both at the end of machining and as a final step to smooth any rough edges and to give the gears a uniform and clean appearance. 

Cutting the Gears

Now we know the main details of the 2 gears to be made, the next step is to select a material and cutters before machining.

Bearing in mind that the gears need to be hardened and tempered, it is by far the easiest for the home restorer to choose a steel that can be through-hardened by simple heat treatment rather than case-hardening or other hardening methods such as nitriding. I have chosen to use O1 tool steel  for making all my gears, including those in gearboxes, and so far this material has performed extremely well and can be finished to size before heat treatment. It is also readily obtainable.

The first step is to make a gear blank by cutting off a suitable size piece of steel from round bar and bore it to the finished size that will eventually be a press fit on the shaft. A mandrell is then made to support a gear blank and the OD of the gear blank is machined to its final size (1.375” or 2.625” in this case) on the lathe. The whole mandrell + blank assembly is then then set up in the dividing head on the milling machine in preparation for cutting the teeth.

Selection of the appropriate gear cutter depends on 2 things: the DP of the gear and the number of teeth of the gear to be cut. Here, we need:

20 tooth pinion: 16 DP, #3 cutter (for 17 – 20 teeth)

40 tooth gear: 16DP, #6 cutter (for 35 – 54 teeth)

These cutters can either be bought individually or form part of a set of 8 cutters. It is also worth pointing out that the gears in these older motorcycle engines (and gearboxes) invariably have a gear form with a 14 ½ degree pressure angle, but you don’t need to know this to cut the gears.

It is useful to use a small end mill first to remove material to save wear and tear on the gear cutter. The picture below shows the setup ….but be careful not to remove too much!

The next step is to cut the gear. The cutter is set up exactly in the centre of the gear in the vertical plane and then each tooth is cut in turn. The depth-of-cut is given by (D + f) and can be calculated by the formula:

D + f  = 2.157 / DP  =  0.135”

It is also engraved on each cutter, so you don’t need to calculate it but it does provide a check.

Each tooth is then cut to within .005” of the final depth, ie 0.130”, the tooth depth is checked with a vernier and then the milling machine table is fixed at the final depth of cut and one last cut on each tooth is made. The setup with the gear and the pinion is shown below.

Any final machining, such as oil transport slots, can be put in before removing the gear from the mandrell and mounting it vertically in a rotary table.

The gears should now be assembled on dummy shafts to ensure that the machining has been carried out correctly and that they engage without backlash or binding before heat treatment.