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Mercury 20 update and Hurricane Merlin III problems.

By S. McManus.

Looking back to my last update on the Mercury 20 I note that about this time of year I was saying that I had been significantly interrupted for quite a few months by the Kestrel cylinder block problems.  Now it seems like a touch of deja vu with the Hurricanes Merlin III, however, more of this later.  The Kestrel problems occurred early in the 2009 season and took me off the Mercury for about 4/5 months over last winter.  The work on the Kestrel was complex and a steep learning curve, but was completed in time to test fly and have the aircraft available well in time for training week.

So with the Kestrel repaired, in April it was back to the Mercury where Barney as well as helping me with some aspects of the Kestrel work had completed the required nine sets of rocker and pushrod assemblies.  This was no small task as each rocker head has 8 ball bearing races to be cleaned checked and in some cases replaced (luckily from stock).  Even the push rod assemblies are more complex than one would expect as they are mounted in a frame with return springs on the rods and close fits within the frames.  Before the Kestrel interruption I had been overhauling a carburettor for the Mercury and just about managed to complete this by the start of the flying season.  These carburettors are complicated beasts as they control both boost and mixture with altitude and power requirements as well as all the normal carburettor functions all in a very compact and busy unit.  On the Merlin boost control functions are carried out in a totally separate unit mounted well away from the carburettor.

Having completed the leftover tasks from 2009 we started on two new jobs, nine cylinders and the reduction gear.  The reduction gear was finally stripped after Dave had made us a robust puller to extract the front bearing / oil sealing ring assembly.  The oil sealing assembly consists of 4 piston ring type seals running in a very hard steel carrier and bearing on a finely machined bore in the nose of the reduction gear casing.  The high pressure oil feed to the prop pitch change cylinder is fed between rings 2 and 3 into the prop shaft and hence to the prop cylinder.  The front bearing showed considerable corrosion on the tracks and the seal carrier was badly cracked resulting in a small piece dropping off which we found in the reduction gear casing.  I wonder what the last pilots of the Canadian Bolingbroke would have thought if they new things were starting to break up in the engine.  Luckily no damage was caused to any other parts, though we did need another carrier.

We raided our stocks of ex Strathalen spares and stripped another part reduction gear and found a serviceable carrier and a much better bearing though still with a few fine spots on the tracks.  We have sent both the bearings to SPL bearings for expert attention and currently the prop shaft is away for crack testing.  All other parts are fine and in new fits and clearances.

The cylinders have proved quite a marathon for Barney and to a lesser extent for myself, but we now have nine serviceable cylinders plus an extra completely built up cylinder (with valves and rocker gear ready to fit) as a flight spare for our running engines.  Our stroke honer bought second hand by the SVAS a year or so ago has earned its keep on these cylinders.  This combined with the new honer cabinet including lighting and cutting oil circulation made for us by Pace Precision engineering in Harlow to our design and f.o.c. have worked well.  We have honed all cylinders to glaze break the bores and remove light corrosion, but as at least 3 are near the top limit of maximum wear for a full life (serviceable as per manual).  We are currently looking at supplementing these cylinders with some slightly more corroded units.  We have about six more cylinders that have some bore corrosion, but are in new bore dimensions.  As the cylinders are nitride hardened our existing honer stones, though fine for glaze breaking and fine corrosion cleaning are not capable of removing significant material.  We have, therefore, purchased a set of diamond honing stones to see if we can clean up these six cylinders and possibly end up with lower clearance units.  We were about to start this when along came the Merlin.

The Hurricane completed its very well publicised stint at the last Duxford 2 day air display flying with 3 other Hurricanes and putting on a fine show without problems.  However, after the steam fair where it gave a short display Toby noted that the aircraft seemed to be manufacturing coolant with the level higher than at the pre-flight check.  This is not usually a good sign as it implies gas is getting into the cooling system.  As the rise was not great we decided to bleed the blocks and found that the A bank (starboard side) had a considerable amount of air in it, but the B block had none.  After bleeding the coolant level was back to normal not requiring topping up.  The aircraft was scheduled to do an away display as well as running straight into our next OW display on 25^th September.  We decided to let it have one short display flight at Old Warden only, where we could monitor it carefully and planned to send other aircraft for the other display.  No problems were noted by the pilot on this display.  In the following week when all had cooled down we checked the coolant and again the level was up, but now to the top of the filler neck and over flowing when the cap was removed.  We repeated the bleed not because we wanted to fly it again, but to see if we had lost coolant and it was found that we had probably lost about ½ a pint.  We also noted coolant stains on the cowling behind the header tank vent.  This was enough to ground the aircraft and it subsequently missed the last display of the season, not that we could fly much that day due to heavy winds.

Once the season was over we removed the outer A bank plugs and Toby (young eyes) used our very ancient bore scope to look inside.  There was definitely a crack in A4 cylinder, but no liquid coolant in any cylinders; however, there was a coolant smell somewhere near the middle cylinders of the block.  We decided to pull A block the following week and retired for the weekend.  After the weekend much to our surprise we found coolant droplets coming from A 3 cylinder, but A 4 dry!

At this point a re-appraisal of the engine was carried out.  It is possible that due to the Piston Provost engine fire and crash partly blamed on internal corrosion in an engine last overhauled many years ago that the CAA may well introduce a calendar overhaul life to piston engines.  It was known that as the engine had completed a considerable number of hours in the 16 years of use and 20+ years since the last strip, so we decided that this was a good time to do an engine change to the spare completed in 2000.  In addition, to the block problem the engine was showing signs of significant sludge build up inherent with using straight oils.  The decision to swap the engines proved to be good when on a subsequent scavenge filter check before the engine was lifted out a tang from a lock tab was found.  This would have meant a certain strip without the block problem.  The tang looks like the size of lock tabs used in the wheel case and as expected for the wheel case it as found in the rear filter.

The engine has been removed, though; we have not pulled the leaky block for the time being.  We have, however, well soaked the cylinder with inhibiting oil and we have removed the cam rack assemblies.  These particular cams and fingers are now about to be fitted to their third engine having started life some years ago in the Spitfire engine when low oil pressure was causing it to eat camshafts.  They are the original “Merlin Fingersâ€? manufactured on the other side of the Atlantic and have tungsten carbide cam follower pads to replace the chromium plated items.  This can only be carried out on the thicker Packard and very early RR fingers as later RR items have insufficient material thickness to be ground for the fitting of the new pads.  When the Spitfire engine was taken out in 2006 Toby removed the cam racks and fitted them in the Hurricane engine after a couple of examples of chrome break up had been noticed on the Hurricanes annual checks.  Luckily only tiny spots had lifted and no damage had been done to the camshafts.  The fitting of these fingers ensured no further problems with the chrome fingers and the potential camshaft damage.  They had run for nearly 4 years in the Hurricane without problem, therefore, it was decided to transfer the whole rack assemblies to the replacement engine which had been left without camshafts to keep the valves closed and prevent bore corrosion during storage.  However, before they can be fitted they needed stripping and cleaning to remove any sludge from the oil ways as the passages are long and narrow.  Toby has completed the marathon of stripping and cleaning both assemblies (48 fingers) and they are now ready to fit.  (Incidentally a new set of English manufactured “Merlin Fingersâ€? were obtained and fitted to the new Spitfire engine when this was built up a couple of years back.)

The replacement engine, an identical Merlin III is now being prepared for installation.  We have removed the blocks for an engine internal examination and carried out pressure tests on one of them to date in order to ensure the serviceability of the seals and this block has just been replaced.  We now have one more to do plus a number of pipes also require pressure testing, then we can install the engine, hopefully before Christmas.

One day we will get back to the Mercury again on its protracted rebuild, but that’s how things go at Old Warden with so many flying aircraft.  It looks as if that rebuild will now be running parallel to a Merlin strip and clean.  As the engine was running well with good T and Ps prior to the block problem we are not anticipation that it will need too much work, mainly a thorough clean out and finding the source of the lock ring tang found in the filter.