In the workshop, at the moment we have some cylinder heads from a Paxman 12YH engine which are being overhauled in preparation for use on an upcoming engine work scope.
The cylinder head overhaul consists of our standard process and following this meant we were able to highlight a couple of issues which would have caused major problems if any corners had been cut. This is the procedure we follow:
• Dismantle
• Clean
• Inspect including visual and dimensional
• Refit Injector tubes with new seals
• Pressure Test
• Rebuild
• Vacuum Test (to check valves sealing)
From the visual inspection, the heads looked in good condition, however when we measured the valve depths (critical on YH engines), some were out of manufacturer specified tolerance and were sitting too low (0.080 – 0.090 instead of the specified 0.055 – 0.070), so they were changed to bring back to required spec.
Before pressure testing, the injector tubes are re-fitted with new seals to ensure the heads are sound before being pressurised.
During the tests, we hydrostatically test each head to 100psi and hold it there for 30 minutes, but one of the heads was unable to get to the required pressure, and it became apparent there was a crack in the head.
Due to the size and location of the crack, the cylinder head is very difficult to repair, so a replacement head will be used.
| As the location of the crack was hidden by a screwed plug it could easily have been missed, but the pressure testing meant it was picked up. We hear too many stories of companies skipping checks to save time and money, but we take our responsibilities seriously and would rather be thorough to give reliability on all work we carry out. |
