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Below you will find some of the finer points I have picked up from working on Merlins. |
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The first year I had the Merlin XX running, both front cylinders leaked water at the head-joint, as can be seen. The engine had been in a crash, and the impact on the front of the cylinder blocks had caused a misalignment of the combustion chamber, which is separated from its neighbour by a saw-cut, for heat expansion.
This engine has single-piece blocks, ie the head is not separate, so the trick is to ensure all the liners pull down with equal loadings. On a Merlin two-piece block, the top of the liner is flanged, and is nipped between the head and skirt castings, a much better joint. |
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The solution was to set up the blocks on a large surface table, and make a gauge block to enable the relative depths to be clocked without lifting the clock off the table. Get this: the manual states that the seatings, measured at four points must all be within .0005" along the entire casting. (this casting is so long that the weather can be different at either end.....especially here in Cumbria!) Anyway, I found the front to be over .020" out, and managed to get them all within about .004". I made a cutter and spigoted guide-bush, and cut the first two joint faces by hand, and also lapped them all in, then checked the liner fits with blue. No leaks!
This info does not apply to most Merlins in the air today, as only the Sea Hurricane currently uses single piece blocks, in its Merlin III. |
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Towards the end of 2001, the engine began to flood the carburettor. The first thing I did was to strip and overhaul the carb, as I had never had it to pieces. all seemed okay, and sure enough, the engine was no better. It was running rough, and some cylinders would literally crack into life only above a given RPM. On removing the rocker-covers and camshafts, the picture above is the sorry sight that greeted me! One missing valve and half the springs broken.
The moral here is not to allow any pitting at all on valve springs. I have subsequently had to acquire a full set of good springs, but I will still need to check them. |
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This is just some of the damage. To correct it I had to bore the head for two oversize cast-iron inlet valve guides, and get
all the seats recut/valves reground and then lapped in.
The piston tested okay,and so I refitted it!
An interesting point regarding these valves is that they incorporate a snap ring to prevent the valve falling into the cylinder. MERLIN OPERATORS BEWARE, they don`t necessarily work! |
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