Engine Issues

History

I originally purchased a refurbished Lycoming engine from an engine supplier who is very closely affiliated with the Bearhawk brand and design. Many Bearhawk aircraft are known to have an engine sourced from the same supplier. The logbook that accompanied the engine states that the engine is built from parts, "per Lycoming O.H. Manual".

Issue

At approximately 360 tacho hours (470 Hobbs), while crossing a large body of water, the engine began to vibrate significantly with a reduction in power output. Engine Failure checks were accomplished, but with no improvement. After landing we found that the #4 cylinder had sheared 3 of the top cylinder studs.

Cause

During the diagnosis, the engineer (mechanic) found paint between the cylinder flanges and the cylinder hold-down plates. The practice of applying paint (usually corrosion proofing) in this area is expressly discussed in the Lycoming Maintenance Manual.

It was explained to me that a number of best practices may not have been followed while refurbishing the engine.

The paint wears quickly in this area and removes the torque from the cylinder studs, which then begin oscillating and quickly fail.

Symptoms

There was a small oil leak that I had attributed to the prop governor. The governor had recently had a new seal installed to fix an oil leak there, so it seemed natural to think that the new seal was at fault. In hindsight, it was a separate issue altogether.

Repair

Compression was still good, so the cylinder was crack tested, broken studs replaced, exhaust repaired (the exhaust had multiple cracks from the vibration) and after 10 days of being isolated at a remote airfield the aircraft was flown to it's home base.

We then boroscoped the other cylinders and had a diagnosis done by Savvy Aviation. The oil filter was removed and checked for metal.

We have been advised to replace all the studs on the number 4 cylinder, so have obtained a full set of studs and will shortly be replacing the remaining ones . We will also remove the other cylinders and cleaned the paint from the hold-down plates to prevent the same issue occurring on those cylinders.

Inflight while experiencing the engine issue, the number 4 cylinder can be seen with a very low CHT and EGT.

The cylinder nuts, with broken studs inside.

Paint found on the cylinder hold-down plates.

This is a very well known cause of engine failure.

The sheared cylinder studs are seen here at the top (1, 2 and 4th from the top left).

Replacing the number 4 cylinder. To change the cylinder studs we also had to remove the number 5 cylinder.