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If two components have to be fitted together very precisely one way of doing this is by means of parallel pins. One example of this is where two castings have to be joined together like a car’s gearbox/clutch housing and the cylinder block.

If the two parts are held together by, for example, cap screws, it is not possible for these screws to define the location of the parts accurately. for the screws to fit easily –  some clearance is required.

The parts are located relative to each other by using two or more parallel pins.  The parts are then bolted etc together.

Parallel pins are simply metal rods which are hardened and ground to a very accurate diameter. As with most simple devices, for them to work well there are some other points that need attention. For the pin to enter a hole easily the corners must be round. Often the pin will need a tap with a hammer to get it in. If the end was flat and it was tapped it could easily damage the end so that it would not fit properly. To counter this the ends are usually domed.

fig parallel pin

The holes the pins are to be fitted in must be drilled and reamed to the correct size. nothing should be necessary to ensure the pin stays in.

In might seem that a tapered pin could do the same job. There are at least two good reasons why they should not be used.

Firstly they are much more expensive.

Secondly, whereas with a parallel pin the alignment is always correct once the pin has entered the holes in the two parts, this is not the case with a taper pin. A taper pin will only align the two parts if it is tapped firmly in. If there are any dirt between the two parts the taper will not fit the second hole perfectly. Of course, the whole point of a taper pin is that it is self-locking. But if it was being used to align two parts this property will make it difficult to get the two parts apart later should it be necessary.