The process of reconditioning Piston Crowns
Each Piston Crowns which enters our workshop for reconditioning is treated as discribed in our General Procedure:
– Preliminary Inspection
– Heat Treatment
– Chrome Plating and Grinding
– Final inspection
Each piston that enters our workshop is:
1. stripped and grit blasted
2. measured; the measurements are recorded for comparison with the original drawings. A dimension sketch is made
3. pressure tested using hydraulic equipment, capable of pressures op to 2.5 MPa
4. inspected on the inside and outside for cracks using ultrasonic crack detection equipment.
During the complete premachining of the piston, all cracked and burnt areas are machined away. Lifting holes are drilled out. We try to keep as much as possible landing material between the grooves
The grooves, crown and diameter are welded semi-automatically using the submerged arc process.
1. If the piston crowns are badly burnt away, thery are rebuilt using welding material compatible with the original base matal. This ensures that the heat transfer properties of the original design are retained. Application of a special thin heat reflecting layer of high chrome alloy steel completes the welding of the crown.
2. In the case of all-steel pistons the ring grooves can be reconditioned by one of two methods. a) Hard face welding b) Welding using the original base material, followed by chrome plating.
Every piston receives heat treatment prior to and following the welding process.
It is preheated to a temperature of 200 C Before it is built up by welding.
During the entire welding process the correct temperature is maintained (around the clock if necessary)
Finally, the piston is stress relieved in a automatic tempreature controlled furnace at a temperature appropriate to the material used for welding
The piston and seating surface are finished in accordance with the specification of the manufacturer. Numerically controlled vertical lathes, controlled by an MDSI computer, guarantee that the exact profile of the crown, and the correct diameter and goove area are abtained. If the piston is to be chrome plated, the grooves are machined oversized. New lifting holes are drilled and tapped.
Chrome Plating and Grinding
The ring grooves are hard chrome plated by electrochemical depositing of metal. This is done by rotating the piston grooves continuously between specially made anodes in a bath containing chromic acid.
Precision instruments are used to check the thickness of the chrome plating during all stages of the plating and grinding processes.
After refurbishment every piston undergoes the following tests:
1. Inspection of the inside and outside for cracks,
- 2. Pressure testing in accordance with the specifications of the classification societies.
All items are checked on the Master Check List.