Did it get better when it cooled down?  I have some iron piston engines that lose compression when hot.

As has been said, check for leaks.  Glow plugs can blow the seal, head gaskets blow, bolts come loose, back plate can leak, etc..

BIG Bear
RNMM/AMM

Put some light oil in it and make sure you didnt blow a head gasket or the plug is leaking

    Or it doesn't have a hole in the piston. If it was fine through break-in but failed in one flight, there's something interesting going on.

     There's a reason they are Hurled(tm).

    Brett

I recommend not lapping in your Enya. They can take some time to break in as well but resist the urge.

I would bet on a blown glowplug seal if it's not a lapping related problem. I would expect a lapping compound contamionation to show itself progressively during the break-in period rather than all at once in the first flight.

Back in the mid to late 60's, I bought a new Enya 35 stunt, based on recommendations from Jack Sheeks.  It was a precision motor that came with two heads and three venturi inserts.  One of the advertised features was the Enya was a hand lapped engine.  It was, without a doubt, the smoothest rotating engine I ever felt.  I don't know if the ones sold today are lapped at the factory.  Perhaps improvements in manufacturing and materials have done away with the need for lapping.

Do you recall if the castor oil residue was clear or dark colored? Clear = Good, Dark or Black = Bad.

I would never recommend lapping a piston into a liner. On all my engine builds the liner is lapped first, then the piston lapped to fit. The means a liner lap and a piston lap. If you think about it, the fits we require in these engines is VERY close, in the 100 thousanths or even millionths of an inch clearance regions. Consider the size of a 1000 grit particle, its about .0004 inches (1500 is a bit smaller) that times 2 is .0008 inches, nearly a thousandth  of an inch. What we want is about a decimal point further to the left! In order to get the piston to move in the liner with lapping compound in it and feel "just right" you've already made the piston undersize. The seal you had was the oil in the peaks of the surface finish and with very little running the peaks were worn away and could no longer hold any oil, your fit was gone. Typically lapped engines are fitted tightly so that as they are run the piston and liner wall "seat" together, this cannot be accomplished if they are lapped one to the other, they will always break in to a looser then desired fit. Further, well done fits are tapered, liner tighter at the top then the bottom and piston larger at the crown then the skirt. This does 2 things, it accounts to some degree for the thermal expansion of the liner which is hotter at the top then at the bottom and with the piston it minimizes the skirt friction that does nothing but sap power. A piston and cylinder lapped together will lack both of these very desirable characteristics. They will conform one to the other and the total surface area of the piston circumference will contact the liner, if the liner is tapered in the wrong direction due to wear it gets looser at the top rather then tighter and you will not be able to maintain compression.

Flame suit on.

you can use a pipe cutter to make a small groove in the piston.
I have seen some motors running very fast with this groove.

Greetings Robert-Jan

Indeed they will, but its short lived

My favorite nobler with a fox .15 "wore out" pre-maturely.....melted a hole in the piston. I reckon the glowplug was wrong for the engine. 

Oh, Oh!  If you used 1500 grit lapping compound, you might have gone too far.  That stuff is OK for automobile valve grinding, but our engines work with very close piston/cylinder fits.

Floyd

I use DuPont White polishing compound. I lap to a drop fit from the exhaust port down, a light drag fit from the exhaust port to a little before the top, then leave the top untouched.

   Note that this might - repeat might - be tolerable on an old-technology iron-piston engine that is exceptionally too tight, but you should not ever do anything like this on a current-era ABC/ABN/AAC/AAO engine, ever!  At the very best it would be pointless, and at the worst it will mean getting a new piston/liner.

   Brett