D6948 progress update – March 2015

With much intense work having gone into Project D6948 over the winter period, the onset of Spring is an ideal time to take stock of what we have done and bring everyone up to date with the state of play. What everyone wants to know of course is....when will it be finished and a pristine green and steaming D6948 emerge into the daylight and back on to its first train? The owners have been careful not to set a specific deadline for the completion of the project, which commenced in October 2010 when 37248 was withdrawn from traffic at the GWR, given the unknowns involved. This included fitting a boiler to a locomotive that had not previously carried one and overhauling a 50 year old locomotive that has clearly had a hard life and has only recently come off the main line to name but two. 'It will be ready when it's ready' has been the mantra, and no compromise will be made when it comes to 'doing it right'. However, we are now entering what should be the final straight, and the intention is for completion to be 'some time in the second half of 2015', 5 years after the refurbishment project started. Progress report - general The whole body has had all paint layers and body filler removed and needle gunned down to bare metal following which many applications of body filler followed by sanding have taken place to achieve a smooth finish. The body has now been primed and undercoated. Bogies and fuel tanks have also been needle gunned and have had a coat of paint meaning just about everything now is in primer or undercoat. The cab rebuilds have continued and we are now working steadily on the woodwork of the floors and window supports. A new wooden support for the batteries has been manufactured and the area around the battery box refurbished. Other internal engine compartment refurbishment has included painting the electrical cubicle doors. The headcode mechanisms and blinds are being refitted and a lot of internal painting has been carried out in the nose ends, fan room and engine compartment. The faulty exhauster is disconnected and ready for lifting out, whilst the fire bottles are away being refilled and certified. The cab instrumentation is almost all refitted and the seats are away with our friends at the Winchcombe Carriage & Wagon works for reupholstering in original red leatherette. The main steam pipe running from buffer beam to buffer beam under the full length of the loco and connecting to the steam heating boiler outlet has been fitted and lagged - no easy task with the body in situ. The appropriate fittings have been added to each buffer beam ready for the fitting of the steam heat connection 'bags' and the entire pipe tested with compressed air to ensure it is steam tight. Two outstanding tasks to be completed just before the loco re-enters service will be an air receiver insurance inspection and a boiler inspection. Progress report - power unit repairs As part of the refurbishment and overhaul of D6948, the critical tension of the power unit timing chain situated on the generator end of the lump and linking the A and B camshaft banks to the crankshaft was checked. This was found to require adjustment and so a call to our friends at Bo'ness was made to borrow the necessary tools, a chain tension checking device and camshaft clamps. The flywheel, camshaft gear wheels and the camshaft journals are marked for timing purposes, when the valve timing is correct, the crankshaft is set at Top Dead Centre on the compression stroke for B1 cylinder. After stripping down the access panels, the existing timing chain was adjusted following the workshop procedures and a very large spanner, however during detailed checks on chain tension whilst barring the power unit around, the chain tension was observed to be tightening and slackening off, something was clearly amiss. After examining several theories and sourcing a replacement brand new timing chain from our friends at SDDT, the existing timing chain was cut and dragged out of the power unit and the problem became apparent. The existing chain securing pin was sheared in situ and chain links within the joining area were oval allowing differential tension as they passed over the various gears, this explained the variations in chain tension that we had observed. With the camshafts clamped in position, the new timing chain was man handled into position, not an easy task especially as it had to be fed over the numerous gears and the two ends joined together. With the temporary pin in place, detailed inlet cam lift timing checks were undertaken using workshop procedures to measure the exact 0.551” lift using a DTI Gauge of the inlet cam follower against the flywheel rotation of 80 degrees before Top Dead Center on the non firing stroke.  Tolerances are tight and the phase differences between A and B banks are critical. A timing tolerance of + or – 2 degrees can be accepted on a camshaft, B bank was greater than this.  When the phase difference between the A and B banks exceeds 3 degrees coarse adjustment is required.  A Bank measurements were very good in line with the spec. B Bank had an unacceptable difference in inlet cam timing between B1 and B6 Pistons. More measurements confirmed B1/2 to be suspected. The top end of the power unit was stripped down, rocker gear removed, push rods removed, fuel pumps taken off and various other parts to gain access to the various cam followers. An injection timing data sheet was completed for all 12 fuel cam followers, the fuel pump cam top clearance is critical to the power unit timing and each peak lift was measured with a DTI gauge to calculate the top clearance. This has to be between 0.040" and 0.100".  This is based on the A Bank injection point being 24 degrees before TDC and the B Bank being 23 degrees before TDC. D6948 is fitted with fixed length fuel pump tappets.  B2 clearance were less than required so the camshaft section had to be removed. More stripping down and separating the muff couplings between the B2/3 sections finally allowed the B1/2 camshaft section to be pulled out and inspected. This can only be described as open heart surgery..... A replacement spare section has now being fitted - more fine adjustment is required to set this up and requires the rotation of B Bank camshaft in chain relationship to the crankshaft.  Once this is correct, we can then check all 12 fuel pump top clearances, work out the required shims to set the pumps correctly, fit the timing chain permanent pin and rebuild the top end of the powerunit. Whilst this is a set back, we now fully understand the condition of the power unit in D6948 and this hard graft will serve well for the locos safe future. There is still a huge amount to do but there is light at the end of the tunnel, and we tentatively hope that the locomotive will be able to make its debut into service at the GWR's October Diesel Gala on 10th and 11th October, where it's newly fitted steam heating capability will be put into use!

Comments are closed.