During a flight in the 150 in fall of 2023, I thought the engine sounded odd just a little, but all checks were okay, and I wondered if maybe it was just my imagination (kind of like when the engine sounds odd when flying over water, even if the ‘water’ is nothing more than a municipal swimming pool). Not my imagination, as my co-owner thought something sounded a touch ‘off’ when he next took the plane up. After landing, he taxied up to the flight school hangar (where we both trained at) and asked our wise sensei what he thought. Cocking his head to one side and listening to a few revolutions, the pronouncement of ‘exhaust gasket’ was given. (Sensei is no slouch when it comes to such things, having tens of thousands of hours, and being a well known columnist in an aviation magazine).
With that in mind, I removed the cowling and started looking around for issues. I thought I had first found it with the engine oil breather tube rubbing against a brace, but yanking on the right side exhaust stack resulted in a bit of wiggle that should not exist. Looking at things more closely, the exhaust manifold nuts on cylinder #3 were loose – Sensei may have been onto something! Seems simple enough, just tighten those little buggers back down. Only, when I tightened up the nut on the second stud, I kept tightening, and tightening, and tightening…. Uh, yeah, that should not be happening.
Backing out the nut, the stud came with it. Yep, there’s our problem. Not sure of what to do, we saw that our nearest A&P (himself also having a regular column in a different aviation magazine – we have no shortage of expertise on our field) a few hangars down was around and knocked on his door. “Uh, ice cream machine broken” we murmured while displaying the lonely stud and helicoil-ish detritus it had pulled out upon extraction.
With an experienced eye, he appraised the evidence and quickly assured us that it was easily fixable. He’d pop over to our hangar when he had a chance and see about installing a new thread insert. A few days later, metal shavings on the hangar floor and a text message indicated that he had indeed visited and cleaned the existing stud hole out with a drill bit, but that the baffles in place were too much in the way to easily tap new threads. If we could remove the baffles, he’d have a much better shot at being able to tap out the stud hole. Depending on accessibility, we may still need to pull the cylinder, though.
I’ll take “things that make my blood run cold for $600, Alex”. I’ve bought several of Mike Busch‘s books (and sat in on a few of his various webinars and presentations at Oshkosh), and one thing seems clear as a result: Mike Busch will not pull a cylinder even if you hold a gun to his head. Meanwhile, other mentors of mine have been of the mindset “pull it, it’s just like a Volkswagen”. These two diametrically opposed viewpoints seem to come from opposite ends of the power spectrum. Mike Busch appears to be dealing with engines like the (comparatively) firebreathing IO-540, while I’m dealing with something closer to a garden tractor powerplant (at least in original form – see Formula One air racing for the other end, as I’ve personally witnessed a [highly modified] O-200 powered aircraft out-climb a MiG-17 jet fighter in full afterburner, which gave me immense appreciation for this tiny little workhorse).
And even having done a top-end overhaul on this very engine (replacing all 4 cylinders), I’m still loathe to pull a cylinder for the simple fact that it’s simply a giant pain in the butt. (Hmmm, maybe that’s the underlying hesitation that Mike Busch has had all these years? Eh, that’s between him and his therapist) Even after removing the baffles on the right side of the engine, the accessibility to tap new threads was still slightly awkward, and I had a sneaking suspicion that our A&P would say “nope, not good enough”. Really, it wasn’t so much being an awkward position (show me a position on an aircraft that isn’t awkward to wrench on) as ensuring that the tap is going in perfectly straight.
Somewhere in the shop I have a tap handle and guide bought from Enco decades ago (goodness, do I miss them – those catalogs were wonderful for amateur machinists), but I knew that the wide base of the guide was simply too large to not bump into something else. I needed a tap guide, yes, but one small enough to work in this constricted area. Maybe I could weld a piece of tubing to some bar stock and…. That’s when I realized that I was entirely forgetting that I have several 3D printers. And that “when you have a hammer, everything looks like a nail”. 3D printed tap guide, here we come!
A bit of caliper action on an O-200 cylinder I had sitting in the basement (I’ve since learned that pretty much every aviator has a similar cylinder pile) allowed me to cobble together a solid model of what was rattling around in my tiny smooth cranium. Behold!
Mounts to the ‘good’ stud and provides a perfectly straight guide for tapping the errant threads. I printed off a few in various hole sizes, and showed up at the A&P’s hangar with the collection. As he himself has an Ender 5 in his shop, he took a look at what I had concocted, made a big grin, and pronounced “that’ll do!”
It still took a fair bit of wrestling (those helicoil taps are taking some very heavy cuts), but we were able to finally tap deep enough to install a helicoil and a new stud! Woohoo, put the exhaust stack back on, and we’re back in the air!
…fast forward to 2024…
Did my flight review with sensei in September of that year, and the plane was climbing like a dog. I was absolutely white knuckled on the yoke, and sensei was cool as a cucumber – he may be the most unflappable instructor in history. We certainly cleared the trees beyond RWY 36, but it took a really long time to get to pattern altitude. After my review was complete, sensei suggested I check the compression on the engine. Normally this is done with a compression tester, air compressor, and willing accomplice, but sensei is ‘old school cool’ and suggested pulling the prop through by hand and feeling how the compression felt. Sure enough, I pulled the prop through 180 degrees at a time, each pass being a single cylinder’s compression stroke – one such stroke felt particularly soft.
My co-owner and I wound up doing a full compression test on each cylinder, and dear reader, take a guess as to which cylinder wasn’t a happy camper. Yep, #3. Oh, and did I mention that the stud that we had so lovingly re-installed the previous year was now straight-up MIA? Yep, it was time to… [suspenseful music] pull the cylinder. With right side engine baffles removed, we gingerly extracted #3 and I took it down to the best cylinder shop in the region, Poplar Grove. Cylinder whisperer Bryan McKiness himself had a look at it, but there was simply not enough material left to do a double helicoil repair (one of the several possible FAA approved repair methods, and the one best suited to our situation). We ultimately opted to just go for a brand new Millennium cylinder instead rather than overhauling another Continental cylinder (our original 4 were a matched set, and if we couldn’t repair #3, a brand new cylinder was around the same price as overhauling another one from the accumulated (and ever-enlarging) pile).
Fortunately, Aircraft Spruce and Specialty (not to be confused with that other AS&S) has their newest location just outside of Chicago, so I stopped by and purchased a brand new Millennium O-200 cylinder. This was in itself an eye-opening experience – imagine an automotive speed shop… …but for airplanes. Aisles have various cleaners, engine oils, etc., but for airplanes. Assortment of headsets on one wall, a bunch of aviation brand banners on the other. Walk up to the counter, ask for an O-200 cylinder, boom. Apparently we pilots go through enough O-200 cylinders for it to be worthwhile enough to have such an item available at a retail storefront.
(Side note for anyone needing to visit ‘Spruce’ at their IL location to get parts in a hurry: Fly into KDPA, stop at the DuPage Flight Center, talk to Ellen – she can get you the courtesy car keys for an hour. The store is only about 2 miles away from the end of RWY 20)
Now for the ‘fun’ part – installing the new cylinder. I took the time to check all the ring gaps by pushing them with the new piston into the cylinder bore to check with a feeler gauge, and all were entirely within spec. Honestly, installation of the new cylinder was pretty smooth, with the main hiccup being that we forgot to bleed the valve lifters before installing the new cylinder. Which required removing said cylinder, pulling the lifter cover, digging out the lifters, bleeding them, putting it all back, and finally installing the cylinder again.
The final quest was finding the wisened greybeards of the airport, keepers of the treasured shoebox that contains the heavenly assortment of O-200 pushrods of all lengths and varieties, beseeching them to borrow said holy vessel and sacred contents, assuring the deities that we would return said shoebox with no fewer pushrods than when loaned to us. (As we had donated a couple of purchased extra pushrods to the holy shoebox years before when we did the full top end overhaul, the Olympians were kind and benevolent, and allowed us to utilize this most sacred of airplane part stashes)
We did the cylinder installation as part of our annual inspection, and our IA checked our work and pronounced it flight worthy. Once putting the interior and cowl back in place (oh how I hate having to reinstall the entire interior, but that’s aircraft ownership for you), we just had to break in the new cylinder. One exasperating bit of aviation is that you can ask 5 pilots a maintenance question and get 10 answers. Ask 5 A&Ps the same question and get twice as many answers. The simple query of “how to break in a new cylinder” has a myriad of answers, with anecdotes covering the full spectrum of outcome possibilities. For the rather pedestrian O-200 engine, we decided that our best procedure would be:
- Drain all the fuel (which was almost entirely ethanol-free mogas) and run 100LL for the break-in period instead. Running 100LL vs. ethanol-free mogas is one of those “get a thousand answers” questions, and is only getting more convoluted with the introduction of G100UL and 100R fuels in an effort to remove lead from aviation fuels. But for break-in purposes, nobody seems to think 100LL is a bad idea at all.
- Switch out our usual Aeroshell 15W-50 oil for Phillips X/C 20W-50 instead. Reason being, the Phillips doesn’t have any anti-wear additives, which are detrimental to the break-in process. Break-in is the only time when you want wear to occur so that the components, well, break in.
- Run the engine HARD. Minimal runup time, mixture and throttle shoved all the way to the panel. Babying it* during break-in leads to cylinder glazing, the warnings say. We figured we’d do a run to an airport about 2 hours away and back at full power.
*My TBM Avenger owning friend/mentor tells me that during wartime on long missions, pilots would pull the power back to levels that provided for remarkably low fuel consumption, but the correspondingly low cylinder pressures meant the piston rings would not stay seated properly and would cause excessive wear (but hey, aircraft lifespan back then was measured in missions, not hours). Piston aircraft engines are best run hard.
The climbout, even with 3 good cylinders and cold dense air was rather pitiful. But the engine quickly gained power and after filling the tanks back up at our chosen remote field, the takeoff had much more oomph. After 10 hours, we changed the oil, and again after an additional 25 hours. After another 25 we’ll switch back to our regular Aeroshell oil, even though it seems many people just stick with the Phillips for their normal lubricant. Once again, opinions are more than plentiful.