Have any of us really considered the design of the clack. Many of us, I am sure, either make one from a drawing, or buy one ready built, but are any of them the optimum design?
It is considered that the lift of the ball should take account of its duty: -
Feed Pumps or Hand Pumps having1/8 to 1/6 of ball diameter,
Injectors having 1/3 of ball diameter.
So how does our bought clack satisfy both of these conditions unless we fit a spacing washer to lift the top fitting?

In designing a clack, it is a little bit like designing a safety valve, the passage limitation through the clack should be the entry area at the bottom. Likewise the ball must be prevented from impeding the flow out of the clack. The top fitting on most designs is flat like the design on the right, even if there is some relief to ensure a full flow.

So let us go right back to basics and work out what should happen to meet the design criteria. I will consider two design using 5/32” and 3/16” balls, which are probably most common.

The 5/32” ball sits on a 1/8” seat and the 3/16” on a 5/32” seat. So how far does the ball sit in the seat? Below is a table to cover a few sizes.

	Ball Size
Hole size	0.250	0.218	0.187	0.156
0.078	0.006	0.007	0.008	0.010
0.093	0.009	0.010	0.011	0.016
0.109	0.013	0.014	0.017	0.022
0.125	0.017	0.020	0.024	*0.029*
0.141	0.022	0.025	0.032	0.044
0.156	0.027	0.033	*0.042*
0.171	0.034	0.042	0.056
0.187	0.042	0.053

For the designs we see 0.029 for the 5/32” ball and 0.042 for the 3/16” ball.

Going back to the original design criteria of 1/6 lift for the pump and 1/3 lift for the injector give us the following lifts
5/32” ball – 0.026 and 0.052 3/16” ball – 0.031 and 0.062

As we can see, for the pump the ball should not completely leave its seat being 0.003 and 0.011 engaged respectively. For the case of the injector the ball will clear its seat by 0.023 and 0.020 respectively.
Now if we design the top fitting such that it has a recess for the ball to enter, this will restrain from moving sideways and enable us to fit a spacing washer for the injector case.
This is the design on the left.

The table shows that a 0.109” hole will allow a 5/32” ball to seat by 0.022, a 0.125” hole will allow a 3/16” ball to seat by 0.024. This allows us to make the end diameter of the top fitting 1/8” and 5/32” respectively so that we do not have a sharp edge at the recess.

Another factor to consider is the size of the tubing we use to connect to the clack. If we look at a typical pipe (22swg wall), we see 3/16 tube has a 1/8” bore and 5/32” tube a 0.100 bore. So this is our limiting feed size into the clack. If we go for 26swg wall tubing we see that the bore rises to 0.1515 and 0.01165 respectively.

Now certainly for the pumps we usually have the bypass open, but if we need maximum feed the larger bore will help. Certainly in the case of injectors, the less restriction the better. 26swg tube is a stock item and is only slightly more expensive.

Now we come to the design part. For a 5/32” ball we can use 5/16” OD material the total length being 0.75” with the bottom 0.250 being screwed 1/4x40. Drill through 3.0mm and ream 1/8”. Bore down by 3/8” with a 7/32 D bit and tap the top 0.0250 1/4x40. Drill a 3/16 hole 0.250 fro the top for the side fitting. Braze in the side fitting and clean up the inside thread. Seat a spare 5/32 ball inside using one tap on a guided rod.

Make up the top fitting with 1/4x40 for 0.250 length and turn the last 1/16” to 1/8” dia. and drill a 7/64” hole by 1/16 deep. Leave top as hexagon or make 6BA square for a scale look.

For a 3/16 ball it depends what taps and dies you have available. If you have 9/32x40 then use 3/8” dia. for the body as before and drill/ream through 5/32 and 1/4 bore by 3/8 deep. If only 5/16x32 tap and die available, then use 7/16” for the body, drill 3/8” deep with 1/4 D bit and drill top 0.250- 9/32 for 5/32x32. Fit side-fitting 5/32x32 as before, but drill though 5/32.

Make top fitting as before but length 0.230 reducing end to 5/32 then drill 1/8”x 1/16” deep.

To use the clacks with injectors fit 0.020 washer under top fitting with 0.030 for the larger ball.

You should now have enough information to make new top fittings for your own clacks. Measure down to the top of the ball to obtain the length of the fitting inside and make as above.

Periodically remove the clack, soak overnight in a weak citric acid (1 teaspoon of crystals to ¼ pint warm water). Use non-magnetic ball for the clack, magnetic ones due suffer corrosion over time.

The Humble Clack by Bill Perry.