How to R&R cylinder on Lycoming

[Dan Thomas]

Not quite sure whats going on in the 3rd pic towards the base of the cylinder.

That's the bottom of the ring travels. You can see where each ring stops. The most wear is where all the rings travel, the least is where only the top or bottom rings travel, at the top and bottom of the travel range.

During ground-running moisture can build up between the piston and cylinder walls. After shutdown, this moisture causes corrosion of the cylinder wall, pitting it and reducing the ring contact area. The rings then eat away at the cylinder wall, and leave a sharp ridge at the bottom of the ring travel. That ridge chips away at the aluminum piston pin plugs that are designed to center the pin by running on the cylinder wall. You start finding aluminum ships in the filter. Just one more way of shortening the life of an engine.

Some old engines had five rings: three above the pin, two more beneath it. This moved any ridging below the pin button travels and avoided chipping the buttons.

The piston pins in both 1 & 2 were about the same. Perhaps slightly more pressure was required to start movement on #1. This is the first time I have removed a piston pin so I don't have a point of reference.

Those pins float in both the rod and piston. They aren't pressed into the rod as was common with auto engines. Only if there's serious varnish buildup on the short sections of pin between the rod's bushing and piston bosses will you have some shoving to do.

 

[FORANE]

That's the bottom of the ring travels. You can see where each ring stops.

I was referring to the seemingly random scuffs just outward of the ring travel limit.

Any idea what they used on the pins? There was an off white milky substance on the pins.
The assembly manual advises STP and 50w oil mix if I am not reading it incorrectly.

 

[Dan Thomas]

I was referring to the seemingly random scuffs just outward of the ring travel limit.

Those nice straight wide lines are from the piston pin buttons. Other scuffs are probably from carbon loosened from the ring grooves, marking the cylinder walls as you worked the piston out of the cylinder. Carbon is hard stuff.

Any idea what they used on the pins? There was an off white milky substance on the pins.
The assembly manual advises STP and 50w oil mix if I am not reading it incorrectly.

That would work fine.

 

[Gary Ward]

I stopped using STP in the 80s. Nasty to use while building an engine.
Reg motor oil on the wrist pins.
ONE drop of oil on the piston skirts and no oil on the rings or cylinder walls.
WD40 on the piston rings and cylinder walls and not much of it.
I have rebuilt a few hundred blown hemis and many other street, work engines and my own 0-320 with only WD 40 on the rings and cylinders. I didn’t leave the engine sit around for months.

Defiantly don't use motor oil on the rings. All that does is create carbon and burnt oil on your new rings and dirties up the ring grooves on your new engine. More importantly it pro longs how long it takes for your rings to seal up. They may not seal up at all with motor oil. With STP mixed in you are making it even harder for the rings to seat right away on start up.
I shipped out that wire UPS on Weds this past week.

 

[Magman]

To my knowledge no one has mentioned “ Choke” or “ Straight Bore”.

This is important when checking ring gaps.

There is no ALWAYS. Or NEVER with aircraft engines.

Don’t rely on the “ Big Guys” to necessarily do this correctly.

This is what Service data is for and is the responsibility of the

Installing Agency to verify.

Not a big deal; just very important.

 

[FORANE]

Oops, put the cylinders on today. Went by the manual. STP and 50w oil mix used on rings, cylinder walls and pins.

Checked ring gap at a piston depth into the cylinder as per a mostly cfi / licensed a&p but doesn't do a lot of maintenance. He works at our airport, attends our families church and graciously offered to assist us.

 

[FORANE]

Update on my mechanic. He's still missing, but they caught the other SOB last night.