Spoke Too Soon…

What an idiot. Why did I have to go and say:

If all goes well, it’s back into the air. Can’t wait…

All didn’t go well. We ran the engine to set up the idle mixture on the carburettor and there was a lot of vibration. Cylinder head temperature (CHT) on number one cylinder was way down, almost as if it wasn’t firing.

Well, no “almost as if” about it. It wasn’t firing. We swapped sparkplugs around and checked ignition leads to try and isolate the fault, but it stubbornly remained.

Inspector Sandy started to suspect a stuck valve, which would be allowing the fuel/air mixture to leak away, so next step was a compression check to check how well each cylinder held its pressure…

With an input pressure of 80 psi, cylinders 2,3 and 4 were losing between 5 and 7 psi – those are pretty good compressions.

Cylinder 1 was losing 64 psi – that’s a pretty BAD compression. Obviously.

Cover off and inspection of the valve rockers revealed movement, so no stuck valve. We began to suspect worn piston rings. For this, the cylinder would have to come off. Time to get the tools out again…

A sorry looking engine. To get to this stage involved removing the inlet tube, exhaust gas temperature (EGT) probe, cylinder heat temperature (CHT) probe, exhaust, fuel prime pipe, oil return line, front baffle, lower baffle, spark plugs and ignition leads, cylinder and piston head. We found the problem.

Worn piston rings confirmed. The gaps were way oversize and had also migrated round so that all three were lined up, providing an easy wide pathway for the fuel/air mixture to just hiss away on the compression stroke, leaving nothing to ignite when the spark plugs fired…(two plugs per cylinder, it’s an aviation thing…)

New piston rings aren’t cheap…ask me how I know. While they were on order Sandy took the time to clean up the piston, cylinder and valves as they were a bit manky. All normal for an 1800 hour engine, but why not?

The piston crown before:

The same piston and one of the valves after cleanup:

We had a little trouble with the valve collets, they are the little semi-circular pieces by the spring:

So I took them over to Bob in the next hangar. We had struggled over a whole lunchtime to get them out, but Bob had them out in 10 seconds, plus another 10 seconds to put them back in a few days later…It’s amazing what the correct tool can do!

Sandy fabricated a valve ring compressor tool for getting the cylinder back on:

…and after about 10 minutes of huffing and puffing and maybe the odd swear word, we got the cylinder over the piston rings and onto the engine. Cylinder bolts tightened up to the correct torque, all I need to do now is reverse the process and get all those bits back on, then several ground runs (…including the one to set the idle mixture which we were half way through when this problem surfaced…) and a specific flight profile to get the piston rings bedded in. We’ll be flying again by Tuesday.

I’m not committing to WHICH Tuesday though. I’ve learnt my lesson.