The manufacturing process of pistons has changed considerably since the
inception of the internal combustion motor. Modern piston manufacturing
is fully automated with little or no human intervention.
This is not the case with JP Pistons.
JP Pistons strength lies in our ability to manufacture very low numbers
of pistons at a time (for example, 10-20). This ability means that we do
not, and cannot, compete with the massive automated facilities of the manufacturers
of mass produced pistons for modern vehicles. It also means that they cannot
do what we do either.
What is laid out below is the way that we make pistons. We use many
modern manufacturing processes, but also older methodologies which ensures
our position as a niche manufacturer.
NOTE: The images here are all available in larger format, just click on
The foundry is the beginning of the piston. At the foundry the die is prepared
by heating it to operating temperature for approximately one hour. This
process allows the die to readily accept the molten material when it is
The material used is a 10% silicon content aluminium.
The dies used are 5 piece and three piece. These dies are made from
cast iron with steel inserts for the gudgeon pin holes and the cores. The
cores dictate the placement of the gudgeon pin and can be located to give
offset pins or square pins.
The process starts by heating the material to 700 degrees Celsius.
This is well above the melting point of the aluminium, but below its boiling
point. The material is then scooped up with a ladle from the crucible (the
pot that holds the molten material). This is then poured into the die through
the sprue. The material is then allowed to cool before it is removed from
the die and placed into a bin of hot water. This water is used to facilitate
a more even settling of the hot metal.
After the castings have had time to cool they are placed into a heat
treatment plant overnight. This process tempers the casting and ensures
the piston will have improved qualities.
After it is removed from the heat treatment the casting has its runner
This process takes little time and is fully automated.
At this stage of the piston manufacturing process the casting has the gudgeon
pin hole rough machined and the locating bung machined.
This process is where the casting is machined on the base to allow
placement of the casting in other machines. This is carried out on a simple
The pin bore
Pin boring is done in conjunction with the bung turning, as one casting
is removed from having the bung face machined it is placed on the pin borer.
The pin borer is only a rough machining process which allows the reamer
to enter the gudgeon hole later.
Turning of the casting is carried out on CNC (Computer Numeric Control)
machinery. This equipment is the most accurate and fastest available for
this application with very tight tolerances and extremely fast spindle
The castings are placed in the lathe on a bung and held in place by
a solid rod through the gudgeon pin hole. A draw bolt is activated in the
chuck which draws the rod toward the chuck and holds the piston in place.
The lathe is then started and the machining cycle begun. This cycle
is programmed into the lathe in a basic language called G-Code (this code
is not the only one available). G-Code has basic commands to tell the lathe
to move to certain positions (X,Y,Z co-ordinates), at particular spindle
speeds (eg S2500 means spindle speed 2500rpm), at particular feed rates
(eg G01; rapid traverse) and other commands such as M01 (repeat programme)
As you can see this is a simple system to learn and implement.
After the piston is machined it is removed from the lathe and the part
number stamped on the crown (top) of the piston.
The piston is now ready for the finishing processes.
The first stages of the finishing process include drilling, slotting, valve
and crank relieving.
Drilling includes all oil holes in places such as the gudgeon pin bosses
and oil ring grooves.
Slotting is where slots are placed in the skirt or in the oil ring
This process is done on a mill and invloves setting the machine up
for the process, choosing the correct cutter and completing the job. Since
there are so many different types of valve reliefs it is impossible to
have a specialised machine set up to do one job.
Crank releiving is carried out on a specialised machine which scallops
the skirt of the piston to the required shape and depth by using two opposed
cutters placed on a common shaft.
process involves the final size being machined on the piston. The grinder
machines the skirt of the piston only and in the majority of cases is cam
ground. Cam grinding ensures the piston will "grow" evenly in the bore
of the engine. A perfectly round piston will expand unevenly during use
because of the uneven placement of material in the casting (gudgeon pin
bosses and ribbing used for strenghtening).
final machining process for the piston is that of reaming. This process
involves the piston being placed in a bath of oil and reamed at different
sizes to reach the final size required. Since the pin boring process is
only rough it is necessary to ream the pin bore a number of times to achieve
the surface finish and size required. Reaming is not a fast process and
is only partially automated (there are automatic feeds on the reaming machines).
Tolerances achieved on the finished reamed surface is 0.4Ra.
Pin Fitting and Final Inspection
At this stage the piston is cleaned, fitted with the appropriate gudgeon
pin, stamped with the pistons' oversize and any other markings, and then
sent to despatch.
Finally, the piston is wrapped and placed in the shipping container with
the ring set and sent to the customer.
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