How can one detect a broken piston ring (high oil consumption)?

Nobody sells dipsticks but I managed to find an item with the right thread on it

for 50 quid, so I will machine it down to make a plug, and run a pipe from it to the pitot tube so I can use the ASI to gauge the crankcase pressure.

Then I will do a quick local flight, using the GPS for the speed (no, only kidding it will be a ground run).

No; I have been looking for a used ASI but can’t find anything that looks like it works for under 100 quid… maybe there are homebuilder instruments which are cheap?

I can also get an electronic pressure gauge…

However, the pitot system is a closed-end. Nothing will travel into it, after the pressure has stabilised. One would use a very long hose – say 5-10m.

Not sure I understand the question, but one still has the crankcase breather in place and open to fresh air.

If one closed off the crankcase breather, the pressure in the crankcase would build up massively (via the cumulative effect of small combustion product leaks past the rings) and would pretty quickly blow out the seal at the front of the crankshaft.

So the crankcase pressure is a sort of dynamic equilibrium between the piston ring leaks and how fast the stuff can escape from the 20mm diameter breather. That’s my understanding, anyway.

Following the discussion here I think Vic found the issue.

The two coldest cylinders are the two which produced the oily plugs and these are going to be the two with the biggest piston-barrel clearance. This cannot be a coincidence.

This is because the aluminium piston expands 2x as fast as the steel liner (22ppm/K versus 12ppm/K) so the gap is bigger at lower temperatures.

At low MPs, the compression ring doesn’t seal as well (because it needs gas pressure to help push it against the cylinder wall) and this causes problems:

• it allows combustion products to get past it and contaminate the oil rings with lead deposits
• the combustion products pressurise the crankcase and cause a lot of oil vapour to come out of the breather, causing an elevated loss of oil
• it allows oil (which is now getting past the oil rings) to be burnt (and cause oily plugs)

It doesn’t happen noticeably at low altitudes because the MP is higher, the engine is hotter and any low-MP time is short (descent and landing).

The engine is probably designed with piston/cylinder clearances sufficient for a CHT of some 600F so at 315F (my coldest one at FL210) there will be tons of clearance, the engine is relying on the MP to push the compression ring out, but it isn’t achieving it.

This fits the known facts.

As to what can be done in the future… after a high altitude flight (which is done anyway only to get above wx so not all that regular):

• do a low level flight at a high power
• put in 1qt of modern detergent oil (for example Mobil 1 0W20 motor oil)

Obviously the latter option is not recommended by Lyco… but it does seem to work to clean the oil rings.

2-3 years ago I reworked the baffle seals and this dropped the summer-climb CHTs from 420F to 390F, but it also made some cylinders run a bit cold. The 315F CHT on #6 at FL200 (OAT say -25C) is too cold. I should create a gap behind #6 to let some air escape.

Adjusting the oil cooler bypass valve to increase the oil temp from 165F to maybe 175F would also help. Of the energy generated by combustion, around 44% goes out of the exhaust, 8% is lost via the oil cooler, and 12% is lost directly from the cylinders to the airflow (references here).

You did Michael but the entire picture cannot be disposed of with one-liners