Replacing a Piston
An account of how a faulty piston ring was discovered and replaced and 50007 returned to service
In September 2022, 50007 Hercules was to have her final run in celebrating the 30th anniversary of 50’s at the Severn Valley Railway, prior to scheduled tests and work being undertaken. However, it was noted that 50007 Hercules wasn’t 100% fit and was starting to show traces of blow-by from the crankcase breather.
The crankcase breathers’ purpose is to remove unwanted gases which are referred to as “blow-by” from the crankcase. These are gases that have leaked past the piston rings and have entered the crankcase area of the engine, instead of being forced out via the exhaust system. This is often the first tell-tale sign that a piston ring has failed or is excessively worn.
Preparations
Preparations
A compression test was performed which identifies what the compression levels are like within the combustion chamber. This isn’t full proof as there are many contributing factors such as the sealing of the valves on the valve seats etc., but it always gives a good indication if something is amiss. The compression test was performed and as expected cylinder B6 was considerably lower than the other 15 cylinders which indicated that a piston ring had potentially failed. The decision was made to strip down the power unit and to have a deep look at B6 with the cylinder head removed.
The team promptly started the dismantling procedure which involves removing all the coolant from the loco, various pipes, fittings, hoses and fastenings prior to the cylinder head being removed. This was conducted over several weekends including during the SVR October Diesel Bash, whereby traincrews were helping with the dismantling process in-between turns.
The damaged piston
The damaged piston
Once the cylinder head had been removed it was clear to see that one of the piston rings had failed and the piston and conrod assembly would need removing. The big-end bolts were removed, and the piston and con-rod assembly was lifted from the crank shaft journal by means of the overhead crane in Kidderminster TMD. As the piston was lifted from the cylinder liner, the signs of blow by were clearly apparent. This is where the piston ring has failed, small particles of the ring have made their way between the cylinder liner and the piston itself onto the piston crown and have eroded the top part of the piston away, and the combustion seeps past the rings eroding the ring groves and the side of the piston whilst the combustion chamber gases enter the crankcase, producing the blow-by evident in the crankcase breather.
As the issue had been diagnosed, Hercules would require a replacement piston, new cylinder liner, new piston rings to suit the piston, and new big end bearings.
With blow-by having taken place, there was a more than certain probability that fuel contamination of the oil had taken place so because of this the engine oil was to be replaced also. All oil was drained and filters renewed as a matter of course.
Putting it all back together
Putting it all back together
Once all the new parts had arrived, the ovality of the conrod was checked to ensure that it was within its specified limits, this is completed by fastening the big end connecting cap onto the connecting rod to its specific torque/stretch and then measuring in several specific places. Thankfully 50007’s B6 con-rod was well within limits so was perfect to be reused. The faces of the con-rod were all dressed and prepped for the new bearings to be fitted. Once happy, the replacement piston had its new piston rings of the correct specification and size fitted, and the piston was fitted to the con-rod with the gudgeon pin securely located in place.
The cylinder liner at this point had been removed and replaced for a new item.
The piston and con-rod assembly were then lifted and lowered back into the engine with the con-rod locating back onto the crankshaft journal and fastened firmly back into place. Once back in place the engine was rebuilt starting with the cylinder head and brand-new gasket. The cylinder head was also inspected and had no damage whatsoever. All the associated pipes, fittings, hoses and fastenings were reconnected with new gaskets. Fresh engine oil was pumped into the lubricating system and the coolant system refilled.
After priming and barring the engine over several times to ensure everything was fit and well, the engine was started. Periodic checks were made on the power unit to ensure that all was well and that the piston rings were bedding in properly. Following on from this, thorough checks were made over the power unit and all subsequent systems to identify any leak, high temperature spots of which there were not any, deeming the piston change complete and a success.
In December 2022, 50007 was pressed back into service when operating the UK Railtours “A Beverley Christmas Carol” along with sister 50049 of which both locos performed faultlessly.
The replaced piston
The replaced piston
This nature of this work can only be successfully undertaken due to the fantastic team which we have along with their unwavering support, and also with thanks to the fantastic facilities we have at our disposal at Kidderminster TMD at our home, the Severn Valley Railway.
A huge thanks to everyone who was involved in this task.
Images courtesy of:
Anthony Middleton
James Gregory
Class 50 Alliance
