Composite-two piece pistons are normally constructed with a steel upper part bolted to an alluminium skirt.

This type of construction makes up the majority of DMI's reconditioning.

The piston ring grooves in the steel crown section normally sustain the majority of the wear and the lower grooves in the aluminium skirt are frequently un-worm.

The steel upper part is normally detached from the assembly and shipped to DMI factories. Because they are relatively light, transport over long distances is not normally a problem.

The piston ring grooves are machined oversize to remove wear, chromium plated on specially developed rotary plating rings, and ground back to standard size.

Internal dimensions are checked and, if necessary, corrected by grinding. Where necessary flash chrome plating is applied to the inner landing face to prevent fretting.

Although certain designs use chromium plated piston ring grooves from new, many such pistons are flame hardened or induction hardened and after Chromegrooving the surface hardness of the grooves is increased from 300/400 Vickers to 800 Vickers. This results in a better wear performance than a new piston.

The lower aluminium part of the piston, if damaged, can be reconditioned by DMI's highly specialised weld procedures, approved by LRS, which provide an aluminium surface with similar mechanical properties to the parent material and with minimal distortion.

After machining, the aluminium surfaces are anodised by the DMI oxygrooving process and impregnated with graphite to assist in running in.

DMI Chromegrooving is applied to piston ring grooves in the Alfin insert. This may be a single insert containing only the top ring groove or in some designs the top two ring grooves are contained in the Alfin insert.

DMI Chromegrooving is applied to these grooves and wear in the excess of 1mm can be recovered.

Wear in the lower grooves is eliminated by DMI's sophisticated aluminium welding techniques, approved by LRS, followed by anodising using the Oxygrooving method.

In this way it is usually possible for all grooves to be returned to standard size.

Aluminium pistons without inserts are reconditioned in the same way.

Specialised DMI Chromegrooving techniques have been developed to enable chromium to be deposited onto the piston ring grooves of cast iron monobloc pistons.

Chromegrooving techniques are available for the processing of pistons manufactured from a wide range of cast irons including flake grey cast iron and high grade nodular irons.

The same reclamation methods can also be applied to the cast iron skirls now being fitted with steel crown sections in the latest generation of engines with uprated specific power outputs.

The advantage of obtaining a fully reconditioned piston for a fraction of the cost of a new one is undeniably attractive, but add to this the advantage of chromium in the upper ring grooves, thus increasing the hardness of the groove surface from less than 400 Vickers to 800 Vickers gives technical and commercial advantages that can only be obtained from DMI's reconditioning techniques.