I just opened up my O.S 46AX to clean it as I run it on pure castor and methanol so it gets pretty gunky. I discovered that the piston and piston liner are clearly marked by elements that have been through the cylinder and I don't know up to which point are these damages tolerable and when should I think about replacing both parts. Doing a test with a tacho is enough to know if the engine is losing its power or should I seek other pointers?

I would replace it if there are noticeable marks on it, even I would replace it if those marks are not very noticeable but I could feel them with my fingers.
Doing a tach test will not tell you much but the sound of your engine is quite different and tuning is hard as it tends to overheat.
It is going to be a good idea to change the liner, piston and bearings too, because the most common problem resides on the bearings. (they expel the most particles that commonly ruin any engine)
Also check the retainer of the piston rod (A tiny wire that keeps the tube in place) as this wire has happened to broke and cause the ruin of the liner and piston.
I hope this could be of help.

Alfredo

Marks are noticeable just by looking at them. Is this a definite need to change both parts? If indeed I need to change piston, liner and bearings, its cheaper to buy a new engine.

Aside from losing power, is there any other problem with running the engine like this? What symptoms should I look for to determine if the engine is definitely in no condition for flying?

Here's a picture of the piston.

is this engine still under warranty? if it is i would check into sending it back that is some major scouring... did you find anything in the crankcase when you dissassembled it?

You have what the japaneese call severe touching on that piston. as a stop gap you could take some emory cloth and clean the piston and liner. i would do that and run it rich utill it explodes and get another engine.

There was nothing in the crankcase, I figure this is due to the fact that I blend my own fuel and use only castor and not synthetic oil so I get a lot of carbon buildup inside the cylinder which I think eventually breaks off the head of the piston and gets caught between it and the liner causing this amount of damage.

Look guys this engine doesn't go on a full scale airplane.....something did indeed go through the engine..a chunk from inside the engine like a chunk of glow plug or part of the bearings...perhaps something like a pebble of sand...if the engine isn't under warranty...and you were making power before you tore it down..clean it re install it and fly it..make sure the piston goes back in the sleve the same way it came out...in other words don't turn the piston around......once you stop making power or dead stick alot I'd replace the engine..after all...piston, piston sleve, and both bearings is a rebuild...whats left? case and crankshaft...IMHO might as well buy new for 150 bucks

That's not damage from carbon breaking loose. Either you ran some sand or dirt through it, or lost the glow plug element or have the rear bearing breaking up the ball retainer. That piston is toast.

Those deep scores were not from anyting from within the engine, and they're not a product of your fuel.

It's obvious from the baked on castor that you've flown it way past any warranty responsibility on the part of OS. It's also enough buildup that it needs to be removed. There was a really good article in one of the magazines about how to restore an engine that's seen a lot of castor fuel. Can't remember which magazine, but it's probably the AMA one since it's usually the only one with articles beyond ARF reviews. You basically have to manually remove the buildup.

Castor is magic but it's not without it's cost. You're seeing the major one. It builds up and will start to cause heating problems. Today's fuels that are a mix of oils are really, really good fuels. They have enough castor for the overheating protection, but not enough to cause buildup at too fast a rate. Those of us who flew a lot in the old days would have to cleanup an engine at least once a season. Back then, the piston/sleeves weren't too expensive too.

Tower has the 46AX for $106. They have the piston/sleeve for $70. I wonder if it'd be priced like in the old days if more modelers today built their own kits and maintenanced their own engines.

Those scores look bad, but what really matters is how many are there and far more important, how good the piston/cylinder fit is. You won't know that until you've got it back together and running.

The OS.35S used to last through 2 or 3 cleanups before one would need a new piston/cylinder. And then it'd need a rod too. And next time it needed the piston/cylinder, the shaft bearing would have opened up enough that you'd not bother.

I use commercial 10% with a blend of oil, and I've got a 46AX that's seen 3 hard years. The piston is about as dark as yours, but there isn't the varnish buildup. Think about changing to fuel with a blend of oils. I'm one that used to run nothing but 25% castor with 7% nitro and if we'd had this modern oil back then, I would have jumped on the commercial oil-blend stuff in a heartbeat.

Thanks to all of you guys. I'm probably gonna replace the piston and liner and I'm gona find the bearings from bearingsdirect.com since they come a lot cheaper than buying them directly from tower. They even cary the same references but made of ceramic, I wonder if that could make any difference. It most definitely would in terms of rust.

That's not baked on castor, that's just varnish which is the last line of defence in a horribly lean run and it'll come off quite easily. But the piston...oh wow...do you fly off a beach or something? Some of those scores are very deep, is there still any plating left on the liner? Either way, you're up for a new piston/liner set unfortunately. If the liner hasn't peeled then I'd suggest you use a BruLine air filter (green filter) to try to stop that happening again.

You've nailed it downunder. I've never used an air filter which I'll be getting with the new piston and liner plus a fuel line filter which I didnt use either. I hope this prevents this type of wear of happening again. I should note that this engine was not properly broken in and I don't know if this might have affected the final result.

Is it cost effective?

Well, the new engine costs $104 and the piston and liner go for 65, and the bearings I can get for $8 so I'd say it is.

We called the varnish, baked on castor. It's a result of castor and heat. And it's part of the defence line for sure, but it builds up to a point where it starts to cause heating problems. And must be removed.

The scoring does look pretty awful, but it really depends on the piston sleeve fit and since the parts cost about 70% of what a new engine costs, it'd be worth de-varnishing and running to get a bit more use out of that engine.

As for the liner peeling, that really seemed to be an era that is well past. How many years ago?

Another vote for the air filter!!! Those scores look like sand. Was the engine crashed recently? The scoring looks recent. The varnish builds up in scoring just like everywhere else. Matter of fact, the varnish buildup can lead to overly hot engine runs, that will setup a worse environment inside the engine that'll make scoring a bit easier. (the trash that normally passes through has more clearance to get caught and has varnish buildup to stick into)

BTW, when you install the new parts, look very carefully at the rod-to-wristpin and the wristpin-to-piston fits. The grainy looking pockmarks in the piston wall often happens from too lean running. And that can wreck the wristpin fits.

BTW, it is a fact of life with 100% castor fuel, that varnish buildup can bite you someday. It makes the needle setting you've learned to trust a bit of a risk. The buildup will eventually cause a problem. The needle gets overly sensitive as one symptom. Usually is the first symptom. Engine goes lean "for no reason" is another. It doesn't go real lean to begin with, but does give you pause if you're paying attention.

Well, at $73, to repair and $104 to replace, your $30 away from a new engine and just use yours for parts. That would be a better move.