Piston Crown Machining

Call us for expert advice and a quotation for your engine machining requirements.

There are several reasons why piston crowns may need machining.

In diesel engines, the piston protrusion from the cylinder block controls both the compression ratio and squish clearance between the piston itself and the cylinder head.  Both these factors are critical to the clean and efficient running of any diesel engine so it is particularly important to ensure that these dimensions are correct.

Older diesel engines were supplied with “pre topped” pistons which would require machining to comply with the manufacturers specifications and equally, modern diesels which have had machining work done to cylinder blocks and heads may also need to have their piston crowns machined to compensate for material which has been removed elsewhere.

Compression ratio is also vitally important in petrol engines with turbo chargers fitted.  In order to prevent compression rising beyond acceptable limits which can result in detonation and ultimately engine failure, material removed from the cylinder head or block mating surfaces can be compensated for by machining the piston crowns.  H T Howard are able to machine your pistons to prevent these issues occurring.

When fitting aftermarket turbochargers or superchargers the manufacturers of such kits will often specify a reduction in compression ratio to achieve the best reliable results.  While some prefer the easy option of fitting a compression plate, this method detracts from the efficiency of the combustion process by increasing the squish clearance between the piston and cylinder head which in turn reduces the efficiency of the flame path.

As an alternative to a compression plate, H T Howard can machine piston crowns to incorporate a bowl.  This adds the volume needed to reduce compression but leaves the overall piston height intact so maintaining the squish clearance needed for efficient combustion.

 Procedure

 All piston crown topping or bowl machining is done using our lathe, and regardless of which operation is to be carried out the procedure will be much the same.

Most pistons have a machined area inside the piston skirt which can be used as a datum point when setting up.  A machined ring of the correct size is located on this area and this is used to support the piston on the lathe face plate.  The piston is secured in position against the face plate using a draw bar.  A pin through the piston boss locates in an eye at one end of the draw bar which in turn runs through the rear of the lathe spindle where it is secured with a wheel nut which in turn pulls the piston against the face plate.

Once lightly secured, the piston is set in a concentric position with the aid of a dial indicator gauge before being finally tightened against the face plate.

A further check is made to ensure that the piston crown is also running correctly at 90 degrees to the piston axis.  Once the lathe tool is set at the correct height and approach angle the machining operation can begin.

Piston Topping

When machining purely to remove material from a flat piston crown (when machining older style diesel pistons for example) a series of small cuts can be made across the total diameter of the piston until the correct amount of material has been removed.

Bowl Re-profiling

However, when re-profiling existing bowls or machining in new ones, the machining operations are slightly more complex.

Firstly a cutting tool which will give the final bowl shape needs to be selected.  This tool may be profiled in such a way as to create the correct radii within the piston crown and ultimately achieve the final bowl shape.

The cutting tool is set at the diameter of the bowl required and used to cut into the piston crown in a series of cuts until the finished depth of the bowl is reached.  These cuts are relatively small in depth and the automatic cross feed is used to machine across the piston crown until the final shape is achieved.

Call us for expert advice and a quotation for your engine machining requirements.