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Merlin Update
Story added 22nd November 2007 by Fred
It is nice to report at last that the light at the end of the tunnel has finally turned into full daylight. The Merlin 45 is a complete engine at last with only a few ancillaries to be added.
Since the last report Toby and I have completed the blocks, pressure tested them and fitted them to the engine. The inlet manifolds have been fitted followed by the priming pipes and coupling to the supercharger outlet. The camshaft assemblies have been installed, clearances set, timed and hidden under their covers. The coolant pipes that connect the blocks to the pump and to the supercharger casing have been added together with the volute drain pipe which runs from the starboard inlet manifold to the base of the supercharger. A new ignition harness which serves the exhaust plugs is fitted using a dummy magneto to position it correctly, the inlet harness will be checked out shortly. Finally the dynamo and starter have been removed from the old engine. The dynamo has been overhauled and fitted with new flexible drive couplings and currently the starter is under overhaul which means in pieces on the bench.
From the above you would be excused for asking why the above took us from February to November. Firstly we must use the standard excuse of being roped in for other jobs on other aircraft like helping to take the Spitfire apart, permit renewals etc and of course during the flying season there is plenty to do getting aircraft ready for displays and doing minor repairs as the season progresses. All very well, but it is still 10 months so what else have we been doing. Well we must admit that the above synopsis gives the impression that all went without problems which, as anyone involved in restoration will know is not usually the case and it must be said we have had some problems.
The main problem we had that took a fair time to resolve was the O seals on the cylinder blocks or putting it another way getting them watertight and through the pressure test. It was the seals on the guard tubes that caused us most problems. These thin wall Tungum guard tubes surround the main block retaining studs which stretch from the crankcase up to the top deck of the heads. The tubes are in two pieces per stud, one in the head and one in the cylinder skirts and they pass through the water jackets in both castings. Hence there are four sealing O rings per guard tube assembly at the top and bottom of each wet section plus one non sealing O seal that is used to locate the very bottom of the tubes in the skirt. All the seals are the same size except the top head seals which are slightly larger than the others. These upper seals were modified in service by an increase of size of +0.040inch due to leaks. We also had seals which were specified as being to the +0.040 standard but they leaked. We had in stock a few leftover seals from the last block overhaul on the Spitfire’s original engine back in the early 1990s also to the +0.040 standard. So after assembling the blocks with our new seals, testing them, getting leaks and stripping them down again which is not a five minute job (about a weeks work per block in fact) we found these old left over seals were bigger than the new ones we had just fitted by, and you have guessed it, 0.040 inch. We then had to source some seals to the correct size from an aircraft supplier as our main supplier did not have the correct +0.040 seals we required. With these fitted we were pressure tight on the guard tubes, but then we ran into a smaller leak which had been masked by the greater guard tube leaks. These were small, but totally unacceptable leaks on the bobbins that join the coolant passages in the head to those in the skirt. These bobbins have two O seals each, one in the head,one in the skirt and we found that the new seals supplied varied in outside diameter very slightly when installed on the bobbins. Again we had to re-order these seals from another approved source, check the installed diameters, re-assemble once more and test yet again. All was then finally well. The two pressure tests one with air and one with coolant both at engine temperatures each take a full day to set up and carry out. The blocks are then left to cool overnight.
With the blocks finally pressure tight they were fitted which is at least a three man job if it is to go easily. The induction trunk is placed in the V between the cylinder blocks with the supercharger coupling loosely installed in the end. Ring clamps are installed on all the compression rings of the pistons for one block. The end pistons are raised to near top dead centre and the centre four are arranged such that two and five are slightly above three and four. The block is installed on the studs with the two block jacks that are used to lower the block in a very controlled manner fully extended. The blocks are lowered on the jacks until the highest end pistons are about to enter the cylinders. The block is then carefully levelled just above these end pistons and from then on the jack handles have to be moved precisely the same amount to keep the blocks perfectly level. The end pistons are then entered until all the compression rings are fully within the liners and then the clamps are removed. Very gentle rocking of the block assists the lowering process considerably. Lowering continues until the oil control rings are about to enter when the ends are pulled together in to the piston using two pairs of special callipers in the end oil holes. Lowering continues very carefully until the liners just starts to nip the callipers when these are removed and the block is then lowered fully over the oil rings. The process is repeated on the other pistons, but great care is needed as four sets of rings are being entered almost at the same time.
With the blocks fully lowered they are torque tightened in steps to 90 foot pounds per stud and installation is complete. The inlet trunk is then fully attached to the inlet manifolds on both blocks leaving the bolts loose until all are installed. The bolts are evenly tightened and then the coupling is made to the supercharger outlet.
From this point on it was really just a case of installing the already completed assemblies in the order shown in the second paragraph and this was achieved without further problems. Care does however have to be taken to space pipe assemblies and harnesses in the rather crowded area above the magnetos so no fretting can occur.
Once the starter and inlet harness are complete and installed the only outstanding items are the magnetos and constant speed unit (CSU) which are currently away for testing.
We are now so close that Toby has made a start on our next project which will be a Bristol Mercury as a flight spare engine for either the Gladiator or Lysander. We are also investigating as a back burner exercise parts availability for a similar project on a spare Kestrel for the Hind.
Steve McManus.
THE MERLIN IS ALMOST COMPLETE NOW AWAITING FITTING TO THE SPITFIRE, BUT FOR THE REPLACING OF THE TWO MAGNETO'S.
WE ARE NOW GOING TO DO FROM STEVE MCMANUS A COMPLETE REBUILD OF A MERCURY ENGINE STARTING SHORTLY