Piston Fitting

All pistons have a wrist pin or gudgeon pin which fits through both the piston and connecting rod linking them together.

In all cases there must be free movement on at least one of the parts to allow the piston to swivel on its connecting rod as it moves through an arc during the engine stroke.

There are three main methods used for securing these pins to their respective connecting rods.

H T Howard have equipment and procedures which enable us to carry out any piston fitting operation.


Probably the most common and recognised method of retaining piston pins employs seeger or round wire clips which fit into grooves in the piston pin housings.  These clips retain the pin in position while allowing free rotational movement of the pin in both the connecting rod small end and piston.  For this reason, pins retained using this method are referred to as fully floating.

 Press Fitting

Originally introduced by vehicle manufacturers to reduce the cost of making connecting rods, this method sees the elimination of the small end bush completely.  Instead of the pin sliding through the small end it is fixed in position by being a tight fit in the connecting rod itself.  Removal of old pistons from con-rods of this type requires the use of a hydraulic press and special mounting jigs.

The pistons are supported under the press using one of the special jigs and a punch of suitable size and length is selected and used to push the pin out through the connecting rod using the hydraulic press.

Fitting pistons onto connecting rods using this retaining method also requires some specialist tooling.

Because the piston pin will be a tight friction fit in the connecting rod the easiest way of fitting the two components together is to expand the small end of the rod using heat.  This operation is carried out using a hot plate on which the rod is placed until it reaches the correct fitting temperature.  Meanwhile, the piston is again supported on a special jig which is used to set the position of the piston pin in relation to the piston itself as it is fitted.  Once the connecting rod is at the correct temperature, it is moved to the piston mounted in the jig and positioned in such a way as to allow the piston pin to drop through both the piston and expanded connecting rod in a single movement.  The pin drops on the jig and stops in the correct position.  As the con rod cools, the small end shrinks and grips the piston pin in this position.  Friction alone holds the pin in place and prevents it coming loose in service.

This fitting method only allows for the piston to rotate about the pin while the pin remains firmly fixed onto the connecting rod.

 Pinch Bolts

As with press fitting, this method of fixing also results in the piston being able to move while the pin remains fixed to the connecting rod.

The small end of connecting rods employing this method are split horizontally then drilled vertically and threaded to accept a fixing bolt.  This bolt passes through part of the diameter of the small end and when tightened has the effect of closing the split which in turn reduces the diameter of the small end and clamps the pin in place.  Because the bolt also passes through the diameter of the small end a small groove is machined into the piston pin to accept this.  The bolt is then also locating the pin in the correct position laterally.

A common problem encountered when fitting a piston to this type of con rod is that the piston pin is very hard while the fixing bolt is relatively soft.  It alignment between the pin and bolt is not exactly correct damage to both parts will occur.

At H T Howard we have a specially designed jig to successfully fit this type without damaging the components.