Only a bit of progress in the last month. I made a tube to squirt oil sideways at the cylinder bores. Cylinder 3 compression dry 105psi and with oil 130psi. Seemed to show a difference but I was expecting it to go right up if the rings or bores were shot. I tried to convince myself I had put too much oil in and probably reduced the compressed volume. (1 cylinder = 400cc, compressed volume about 40cc, if I added 10ml of non-compressible oil... 33% higher maybe....?). I tried using my £10 USB bore-o-scope and the cylinder walls looked fine, all I could see was the reflection of the mucky piston top.
I toyed some more with the idea of a replacement 2nd hand engine and came down the side of stripping this one down which at least wouldn't cost me anything even if it came to nothing (which it still may).
So, after spending only a few hours across a couple of weekends stripping down it was time to take the head off.
Along the way I noticed that some of the Allen head bolts in the cam housing had been rounded off before - so someone has been in here before, assuming they didn't routinely round off the bolt heads at the factory...
I was slightly heartened to see the camshafts, roller followers etc without the slightest sign of wear, no sludge and everything shiny.
The exhaust manifold flange onto the starter catalytic converter bolts came off easily and so did the bracket onto the back of the bell-housing, though it was a wriggle to get it free.
The head itself came off quite easily after loosening the bolts in the order specified in the workshop manual. No bore has any wear ridge at the top.
All the bores show scuffing rather than scoring on the rear side (which takes the sideways part of the reaction to thrust load on the power stroke after ignition). The worst is on cylinder 3, the one with the low compression. The scuffing consists of really fine vertical lines. I can't feel them with my finger tip but they are perceptible with a fingernail, they don't drag with a fingernail so I wouldn't call them scoring.
I decided to turn the crankshaft to equalise the cylinder top levels and poured about 100cc of paraffin into each one. Cylinder 3 took about 45 minutes for the paraffin to drain into the sump. I got bored (so nearly a pun) waiting for cylinders 1, 2 and 4 to drain and went to bed but they were a long way behind.
What about the head? I've not removed any valves yet but the combustion chamber tops of cylinders 1, 2 and 4 are sooty whereas 3 has less soot but is wet with condensation... so maybe the head gasket was leaking a little too. The gasket itself is metal with solidified black goop on both sides and I have a job to do now scraping the remnants off the head and the deck before I can use a straight edge and feeler gauge to see if they need skimming flat.
Really I don't know yet whether the block is scrap. Over-size pistons aren't available so far as I'm aware so a re-bore is out of the question. I'm going to get my trusted mechanic neighbour to take a look and see if he thinks anything can be done with a very light re-bore or honing and new rings. I don't think sleeving the would be economic. Anyone got any other ideas? Anyone got a complete block and bottom end in great condition they'd like to flog to me for something less than the LOL prices on eBay?
So, what caused this bore scuffing the first place? I can think of some possibilities:
1) Pipe of death bust and it got really close to seizing.
2) Improperly filtered air. I have never liked K&N and I like them less now.
3) Wrong oil, oil starvation or filter failed - but the camshafts etc are lovely and the oil pressure lamp hasn't come on.
Enjoy the photos below and all the best.
Matt