Celestial Navigation – when the stars are misaligned

What? Strange title after my “100hrs/doldrums” – post, you say? Well, bear with me…..

No, I’m not talking about an extended cross country flight in the dark – it’s a metaphoric title, of course.

Yes, I’m talking about an extended operation in the dark, but in the shop – and although it happened under a solid roof, more often than not the gaze turned heavenward for guidance.

The sky went dark the afternoon Greg changed the oil and found stardust in the filter.

Well, actually it was a collection of small metal chips caught in the pleated filter paper. He dragged a small magnet through it and caught all of them, so they were not aluminum.

Where did they come from? 

Did something break and continued operation ground that something down to small pieces? 

Since the engine ran until I put the bird in its hangar it couldn’t have been a critical part. Did it come from an ongoing chafing process?

We sent an oil sample and the contaminated filter paper off for analysis.

Less iron than last time and a tiny bit of Nickel was the verdict, so cylinders and pistons were unlikely suspects. 

Where did all that metal come from – which the oil analysis apparently didn’t see?

Big pieces from the camshaft or the cam followers?

Heaven forbid! That would be really bad news and require a teardown of the engine – and big bucks to pay for the overhaul.

Whatever the source, what caused the problem in the first place?

“..you over-revved the engine!” was the experts first hypothesis and it triggered a close look at the engine monitor and its stored data.

At first sight, with data recorded at six second intervalls, there were two subsequent events with 2750 and 2801 rpm, ie ≤100 over max. So at most less than half a minute above nominal max rpm by less than 4% – could that do some damage?

We decided to take a closer look and Greg removed the valve covers for a visual inspection of cylinder heads – and, bingo, the exhaust valve spring on cyl 2 showed some damage. That could explain the metal in the oil – at least partially.

New cylinder ordered, installed, some additional adjustments made and test flown.

“ She flew, but we still have issues, unfortunately” was the verdict, vibrations the issue.

Oh man, this is going from bad to worse. 

We held an extensive powwow and went over a lot of potential culprits: propeller, engine mounts, magnetos, carburetor, spark plugs, valves and springs,…

Then back to the engine data: any anomalies there?

Well, to my eye there were a few, but Greg classified them as “merely interesting” and didn’t assign them much diagnostic value: “ don’t forget, this is an ancient engine-design under an old cowling – most components work in a symbiotic rather than carefully engineered fashion. An anomaly in the recorded data might just reflect a regular wobble given the wider tolerances.” he ventured.

Ok, but what else might give a hint on what’s wrong?

Was this the moment to ask the stars for help, despite their apparent “bad alignment”?

We decided to try the local specialists first:

Greg’s favourite engine-builder suggested to ream the valve guides, as a sticky valve would definitely cause uneven combustion resulting most likey in some sort of vibration. 

Some progress made, but still some shaking.

Now what?

More conference calls with different stars (mostly the ones in the magneto- and engine-universe): more reaming!

And while that is taken care of sent magnetos off for check and overhaul.

And looking at those valves: why not check the valve springs….

Greg found an exhaust valve spring to be just out of spec – one more part to replace.

A never-ending story? – like the one about the Universe, the Big-Bang one?

It was a star called Tim who came up with the final navigation-fix (we sincerely hope!!), after he saw that:

Burnt oil on an exhaust valve – how did it get there?  –  running down the valve guide!

Running down from where?  –  from a worn rocker shaft bushing!

Apparently those bushings hadn’t been replaced during the last engine overhaul.

Worn bushing and weak valve spring letting excess oil flow down the guide, caking up the valve and choking up the intake

and exhaust guides – as simple as that…..

Check/overhaul all rocker shafts, – bearings, -bushings…

 

Last hoop to jump through !?

Leave a Comment

Your email address will not be published.