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Bristol Mercury Engine
Story added 27th May 2008 by Fred
From Merlin to Mercury.
Recent Engine work progress.
By Steve McManus.
The Merlin was completed, well almost, in March this year. I say almost as we are waiting for the magnetos to come back from overhaul and for a few external pipes to be made otherwise it is complete. Once the engine was structurally completed we inhibited it for storage until the Spitfire is ready to use it and while doing this an amusing thing happened for Phil and myself, if not for Toby. Phil was turning the engine slowly using a large hand turning ratchet spanner that fits on the propeller shaft. I was using an inhibiting gun to spray oil into the cylinders when a clicking noise started to come from the starboard rear side of the engine. We stopped mystified as to what it could be. We had not turned the engine using the side hand turning gear so it could not be the bendix starter gear
We asked Toby to try and track down the clicking as Phil continued to turn the engine. Toby was just saying it sounded like somewhere near the starter gear when there was a pop and the little pressed soft aluminium blank we had on the engine oil outlet from scavenge pumps blew off closely followed by a good half pint of clean engine oil all over poor old Toby’s lower regions. Toby stood giving good vocal vent to what had happened as Phil and I clung on to the engine hardly able to stand and laughing our heads off. This was not the first time Toby had got covered in oil by my hands and now I am worried he will get his own back soon. The clicking incidentally was the threads in the soft aluminium blank popping one at a time over the outlet connection threads.
Well at least the scavenge pumps worked well, though I had not thought the numerous oil flow tests during engine build to check oil was getting where it should, had not put enough oil into the engine to start it scavenging. I was wrong in this assumption. No damage was of course done to the engine, though an oil soaked Toby was a different matter.
T.H.T. had long held the view that when the Merlin was complete we should next look at the Mercury spares that came from the Strathallen Collection with the Lysander to see if a flight spare engine could be built for either the Gladiator or Lysander. We collected by road an almost complete front section of an engine nacelle (firewall forward) from what looked like a Bollingbroke complete with a Mercury 20 engine less one cylinder. In addition, there was another late Mercury core less cylinders (type yet to be confirmed as I have not climbed under the shelves in the back store looking for numbers on it, but probably a 20 or 30) and what appeared to be a complete earlier mark (maybe a Mercury 8) in total kit form. Numerous cylinders both early and late also came with the spares package, some in good order and some internally corroded probably beyond recovery. (The earlier Mercury marks had 8 cylinder holding down studs whereas the higher powered later marks had 12.) We also have on display a Mercury 8 that was removed as very tired from the Gladiator some years ago.
Currently we have a Mercury 20 in the Lysander and a 30 in the Gladiator. A universal spare would need to be a later engine like the 15 to 30 series, although, we could perhaps rebuild an earlier 8 specially for the Gladiator some time in the future as we seem to have a reasonable stock of spares for one. For our current rebuild we have chosen the Mercury 20 from the nacelle as it is the most complete and we are currently stripping the engine and cleaning parts, but few measurements have yet been made. In general bearings (roller / ball) and steel components seem in surprisingly good condition as are the main forged aluminium alloy casings, but the outer ancillary casings manufactured from magnesium alloy are a different matter due to patches of corrosion. So far trawling through the spares we seem to have one probable usable casing of each type so far removed, but some originals from the engine being stripped are very poor which is surprising given the otherwise good condition of the engine and the fact that it still had plenty of oil in it. Jim (Phil’s dad) is helping with the strip and also making the odd special tool when we grind to a halt. Barney is currently cleaning and checking the 18 cam follower assemblies recently removed, which are quite complex little assemblies in there own right.
In parallel to the strip we have taken every late type cylinder from the stores and are cleaning and evaluating each to see if we have enough usable cylinders to make an engine. We may have to go down the route of honing and chroming back to size to compensate for either wear or the lighter corrosion in some cylinders. Early days yet, but work continues as much as the flying season permits.