Recent owner of a 1958 C172 here so be patient...

I was talking with my A&P the other day (who is very supportive of my participating in "owner assisted" maintenance) about pretty low cylinder compression on our tired O300A. (#1 Cyl 35/80). Borescoped the suspect and the valves look real tired. I start to talk about replacing the cylinder and maybe (since this is a "new" to us airplane) maybe a top overhaul all together.
He said... Its one thing to do a top overhaul on a engine that has a reasonable amount of total time but to replace 6 cylinders with brand spanking new ones you're adding over 100% rated power per cylinder on a bottom end that has 4000+ hours total and 1600 smoh, which was done in the early 80s. You're putting a lot of stress on an old bottom end.

Is there validity to this?

Thanks!

Yes I believe there is, I was faced with a very similar situation. Fortunately I was able to locate a reason 0-360 that priced out at very close to the anticipated top overhaul, so now I’m 90% completed on an engine upgrade.

There is so much opinion involved with stuff like this. IMO you should find a mechanic you trust and go with his opinion unless your an engine guy and you trust yourself more.

My thought is it doesn’t make sense to put new cylinders on an engine that is already past TBO. The O300 has a 1500hr TBO, right? But that’s only from a financial perspective. Some future buyer will look at it and say, ‘the engine needs overhauled so deduct $X for that cost.’ Having recent cylinders won’t matter because they’ll need to be replaced or overhauled for the engine to get a real TBO reset.

As for the bottom end being weak, nah. Had you wanted a ‘zero timed’ engine from the factory you very well could end up with a crankshaft, case, or whatever with thousands of hours on it. As long as your current bottom end has good oil pressure and doesn’t leak like crazy then there is no reason to think it would have any issue caused by new cylinders. Though a mechanic can screw up a cylinder replacement in a manner that would cause bottom end failure and blame it on the high time bottom end.

Depending on the other cylinder compressions I’d just repair/replace the one cylinder and keep flying.

Not really. Your engine has 1600hrs on a bottom end. The 4000hrs plus has zero validity as factory reman engines have "unknown" hrs on the cases, crank and cam.

Over 100% power? No, many tests have been done by the factory on an engine with very low compression versus new compression - not a huge change in power whatsoever.

An economic issue? Yes, why put all the money into a new top end on an old bottom end overhaul. Plus a huge risk of buggering up the bottom end (dislodging a bearing) by changing all cylinders at once. These mechanic induced failures are then blamed on too much power on an old bottom end.

Overhaul your cylinders has needed and keep flying or spring for a proper engine overhaul from an reputable engine shop.

So it’s over 4K TT

1600 TSMO

How many hours on the bottom end? Was it replaced back 1600hrs ago? Or does it have over 4,000hrs? Bit of a difference


Need more info, but end of the day it’s on the AP ticket, if he ain’t comfy inking the log book, ether go with what he says, or shop other APs

If that bottom end has over 4,000hrs, like that crank has been in its current home for that time, I’d overhaul the whole thing.

In my case, in 2014 I had about 1540 hrs on a Pponk 520 engine (2000 hr TBO) done by Pponk in 1994. I had 35-45 compression in a couple cylinders and 50s in a couple others. Compression trend had been lower for a couple years. I decided to do the "all new cylinders" option - also replaced the lifters and had rocker arms ground. Cam looked fine. Cylinder walls were found to be pitted (previous owner didn't fly much). I'm now at about 2000 hours and everything is running fine - good oil pressure, clean filter, no leaks, minimal oil consumption, compressions all mid to high 70s. Airplane is in heated hangar and I use pickling oil/dehydrator plugs in the winter. I plan to keep running it until there's reason to overhaul it.

whee wrote: Depending on the other cylinder compressions I’d just repair/replace the one cylinder and keep flying.

^^^^^ This is the correct answer

Rickshaw84 wrote:He said... Its one thing to do a top overhaul on a engine that has a reasonable amount of total time but to replace 6 cylinders with brand spanking new ones you're adding over 100% rated power per cylinder on a bottom end that has 4000+ hours total and 1600 smoh, which was done in the early 80s. You're putting a lot of stress on an old bottom end.

Is there validity to this?

Thanks!

No.... but it may just be that he (having an appropriate certificate, and more knowledge of what's in the logs, chose his words poorly)

The business about adding more than 100% rated power to a weak bottom end is definitely a case of poorly chosen words. Weak or worn? weak infers it pending breakage. This means it needs fixed regardless of what the top end is doing. Worn infers normal wear and tear, which may be nearing the end of it's normal life expectancy, or may just keep on trucking for another 1600 hours and beyond. Wear is black magic that has things like how previous owners flew, treated and maintained it (something you'll never get a truthful grasp on) in it's formula.

Total time in and of itself, can be totally irrelevant. Dive in to your logs and research each and every time the engine received medium to heavy maintenance, and what was replaced, or reconditioned at those times. Based on the little information so far, everything else is an assumption. Your entire bottom end may be 1600hrs since new, or it may actually be older than 4000 hours as components may have been replaced at the OH with serviceable units that had much more calendar time on them than the ones they replaced (very common).

As a similar but very different example, I once flew a turbine thrush (for about 10 years) which had over 29,000 hours on the engine (3,500 TBO) when I got in it. I doubt there was a single piece of that engine other than the paper logs that had even close to half of that time on it. And I currently own one that has only 1,700 hours on the engine, that also has virtually nothing but paper from it's original state.

Going back to recips, my personal philosophy, is that TBO, SMOH, etc are great guidelines, but provide less than accurate meaning of an engines actual health. Bearings, seals, NDT and mechanic time are a relatively cheap way to assign actual first hand knowledge to that bill of health. Were it me, I'd fix any sagging cylinder and save for an appropriate handling of the entire engine. Without more data or a crystal ball, it's pure speculation wether that means a complete OH, splitting the cases and IRAN, or just topping it and flying it till there's a better reason to throw cubic dollars at it.

Take care, Rob