Story added 19th January 2012 by Fred
Mercury 20 update January 2012
and Hurricane Merlin III problems - yet again!
By S. McManus.
It seems to becoming an annual event for the engine department as just towards the end of the flying season when we are thinking about an uninterrupted spell of restoration work one of our current projects, which for me is the Mercury 20, that something else comes along to upset our best laid plans. In 2009 it was the Kestrel in the Hind with block liner top seal coolant leaks, then at the end of the 2010 flying season the other currently flying Rolls Royce engine in the Hurricane, not to be left out, suffered exactly the same problem, not uncommon with the earlier single piece blocks. This year at the end of the 2011 season it was the Hurricane Merlin again wanting attention. However, more of the Hurricane later as quite a substantial amount of progress has been made on the Mercury in between the more pressing flying aircraft problems.
By the time work restarted on the Mercury the large reduction gear front bearings had been returned from SPL bearings and we could start on rebuilding the reduction gear. Assembly is carried out in distinct stages where clearances and backlashes are set firstly on the planet gears followed by the main drive and driven gears. The replacement carrier for the propeller oil feed rings on the main shaft was fitted with the best piston ring type feeder seals available (well within new limits). Then the whole gear was mounted in a press to simulate propeller forward load and the final backlashes checked. Amazingly it was found that all parts showed virtually no wear being in new fits, clearances and backlashes so we ended up with an excellent reduction gear. This was duly fitted to the engine marking a finish to the front end.
Also returned from SPL with the reduction gear bearings were replacement bearings for the non-serviceable units in the wheel case and accessory drives mounted behind the supercharger. The wheel case itself was a magnesium alloy casting and during the engines many years out of action probably on the Canadian plains moisture had dissolved a small area at the base of the casing. Similar problems were noted in the sump so two of each of these units were sent to specialist welders for repair. Once we had a full compliment of parts the wheel case and accessory drives were rebuilt and fitted to the engine. So in a few months both the front and the rear of the engine were completed. We finally fitted one of the rebuilt sump units and were at last ready for some cylinders.
When our attentions switched back to the cylinders they turned out as to be quite a marathon. Though, in the last report I said we had nine serviceable cylinders some were near the top of the maximum wear limits. Though, you can use these well worn cylinders on a zero time rebuild as max worn limits allow a full engine life to be run, we also had some potentially better cylinders within new clearances, but with limited light corrosion in the steel barrels. It was desirable that we should use the best cylinders available so we purchased some diamond stones for our stroke honer. The diamond stones were needed because, although standard stones were suitable for glaze breaking the Mercury’s nitride hardened cylinders, they would take for ever to clear out the significant thickness of material needed to clear corrosion. We cleaned up four cylinders with the diamond stones and found that although we had removed a few thousandths of an inch and the corrosion, they were still better giving lower bore dimensions than the well worn units.
Next came the valves and we had from the Lysander engine top overhaul of a few years ago a set of new exhaust valves as we had bought two sets. There is, as with the Merlin, a mismatch of the seat and valve face angles (0.5 degrees) to allow for angular movement as the cylinder reaches running temperature. We, therefore, lapped the exhaust seats with and old valve re-faced (regularly) to the correct seat angle rather than the correct valve face angle. This does not apply to the inlet valves which are lapped with the valve to be used. The inlet valves were not new, but we had plenty of serviceable valves which were subjected to a light reface in the valve service machine bought for us by the SVAS some years ago and then lapped to their respective seats. Some valve seats required more than just lapping and on these they were first re-faced with a seat facing grinder before final lapping. All valves were checked for seat fit with engineers blue.
The cylinders were then thoroughly cleaned and painted and the valves finally fitted. The used valve springs were tested for compression against load and thoroughly inspected before fitting. Two inlet elbows were fitted to each cylinder with in house manufactured gaskets. The cylinders were then fitted to the engine with new base O rings. Finally the inlet trunks were connected to the outlets from the supercharger again with new O rings. So by about the end of the 2011 season we had fitted all nine cylinders and had an engine looking like a real Mercury at last.
Attention has now turned to the rocker gear. Firstly each set of previously overhauled push rods and rocker gear has to be set up to provide the correct “angle of attack” for the rocker assembly. This is the way Bristol radial engines cope with the effect of cylinder expansion on valve clearances. Typically other manufacturers of big radial engines use large valve clearances which close as the cylinder reaches temperature, but with the inherent wear problems when cold. Bristol use a rod attached to the crankcase at the base and the front of the rocker head at the top. This pivots the whole rocker head on two bearings as the cylinder expands maintaining valve clearances as set to the correct clearances whether hot or cold. The initial setting of the rocker head is, however, critical and time consuming. So far 6 cylinders have been completed and once all nine are done the assemblies will be removed one at a time and the push rod cover tubes installed along with the felt oilers and rocker covers. The ignition harness, oil pump and fuel pumps are also being worked on as time permits to complete the engine later this year, with luck, and assuming we do not get too many interruptions like Merlin IIIs.
The Merlin III currently powering the Hurricane was fitted last winter after the old engines problems. It has run well all season with pilots agreeing that it was much smoother and apparently more powerful than the original engine requiring lower boost settings for a given display routine. The old engine was tired! However, (there’s always a “however” isn’t there) after the last but one display Toby noticed an increase in oil under the engine during pre-flight checks and investigations showed a leak round the supercharger to wheel case joint. Further investigations showed a small number of broken studs on this joint. Though the casings had probably only opened up by a few thousandths of an inch, hot oil does not need much of a gap to get out. The problem we now faced was that to access these studs we needed the engine out.
Therefore, after the end of the season out came the engine again and off came the supercharger unit. All the mounting studs were replaced as there was a suspicion that they may have been over tightened in the past as there was slight stretching on other unbroken studs when viewed with magnification and there was also slight corrosion on two of the broken studs where they had broken. Stock studs from another casing were used after inspection and crack testing.
The supercharger has now been replaced and the engine is being refitted. Then perhaps after runs we can get back to the Mercury rebuild! With thanks for their continuing help to Toby, Barney, Jim, Phil and Adam.