What would cause this?

It certainly looks like it either ingested something or something broke loose inside the engine.

Is your plug element still intact? Is the cup in the ball socket still intact? Did you remove the cylinder using your wrench in the exhaust port? If so, you might have rolled a burr up on the port and it broke loose while running. It's difficult to see in the picture, but at the top of the piston where the scoring is worst, does the score appear to have been caused by an external object or from a portion of the piston flaking off -- perhaps from a void on the original bar.

cribbs74 wrote:That is strange, I don't know if just regular dirt would cause it to throw sparks. Were you running it close to your milling equipment? Maybe a steel shaving got thrown into the exhaust port.

I suppose it's possible that the piston had a manufacturing defect.
Is the Medallion cylinder Single bypass like the Sportsman?

Major bummer, sorry that happened. Can I assume that one won't make the tach race?

Ron

Ron,

I have more medallion 15's I can send. It was a single port cylinder. Was running out in the shed on a flat clean bench. Could have been a defect but there was no binding or tight spots.

The plug is intact and was new. The cylinder is galled also but not at an edge of the exhaust port where a burr would have occurred.


Jason

Appears to be a lack of fuel lubrication to me.

Jason, this isn't uncommon. What happened to you happened to me. This discussion came up a few days ago with a friend of mine who said he could tell me the damage sight unseen. It's due to improper piston to cylinder fit and the piston is rocking at the bottom end. On the way up it catches the exhaust port and stops like it seized. I can tell you when I had to replace mine a few years ago, the cost wasn't cheap. These are certainly becoming harder to locate. My engine when it stopped had 3 runs on it when this happened. I was flying mine when it happened and the plane almost flipped over. Ken

SuperDave wrote:Appears to be a lack of fuel lubrication to me.

20% nitro and 22% oil content of which 1/2 is castor. It was running rich so I doubt is siezed due to lack of lube.

Jason

Ken Cook wrote: Jason, this isn't uncommon. What happened to you happened to me. This discussion came up a few days ago with a friend of mine who said he could tell me the damage sight unseen. It's due to improper piston to cylinder fit and the piston is rocking at the bottom end. On the way up it catches the exhaust port and stops like it seized. I can tell you when I had to replace mine a few years ago, the cost wasn't cheap. These are certainly becoming harder to locate. My engine when it stopped had 3 runs on it when this happened. I was flying mine when it happened and the plane almost flipped over. Ken

Ken, are you saying this is a problem specific to the Cox Medallion .15, or all Cox .15 piston/cylinders?

Rusty, I was told that this was happening with the Medallion .15's. Mine came in the jewel case with the old 60's looking cardboard card inside. The problem with this engine is that even in this case and packaging it's hard to tell when it was really manufactured. These engines were produced up into the 1980's using that packaging. I remember Tower Hobbies selling Medallion .15's as a discontinued items sale for $25. When talking to my friend the other evening, he told me about this problem when I mentioned the engine on the Jr Satan which is going to be a Medallion .15. I told him afterwards that it did in fact happen to me. I bought 2 new piston cylinders about 7 years ago, These were not inexpensive then so I'm sure they're probably more and harder to find nowadays. Currently, I would think Dan Sitter would be a reliable source for these piston cylinders and he can be located on Ebay.

I was using 10% nitro with 25% all castor fuel when I was running mine. Just like Jason said, no binds, started right up and then it stopped. Pretty weird I must say. My piston looked scored just like his does with a matching cylinder to boot. Ken

This sucks about the Medallion 15 piston/cylinder fit. Seems like Medallion 15's are up in price since the last time I bought them. Was getting them for $35 and under but now seem to be going for more.

Jason, this isn't the situation with all of them. The new piston cylinders I received have been an absolute joy to use. I suppose a choice few have had this problem and were both unfortunate to experience this. I wouldn't give up on them. The new piston sleeve I've been using other than probably one of the loudest .15 engines I run is a very user friendly engine. Ken

SuperDave wrote:Appears to be a lack of fuel lubrication to me.

Since the fuel and lube weren't mentioned early on in this thread it seemed a good place to start. As details have now emerged the cause of the problem is now more apparent.

'Litho in USA ??/??' on the instruction leaflet should tell you
Within a few months the date it was put in the box Ken.

Ken Cook wrote:Jason, this isn't uncommon. What happened to you happened to me.... It's due to improper piston to cylinder fit and the piston is rocking at the bottom end. On the way up it catches the exhaust port and stops like it seized....My engine when it stopped had 3 runs on it when this happened.

Was this just due to excessive piston to cylinder clearance when the design was 'new'? Tightened up later in production run? Doesn't seem like this would be just the result of sloppy tolerances in production given Cox's qa standards for much smaller engines.

Wonder if this would be aggravated by cylinder indexing, with exhaust ports sitting perpendicular to crank shaft axis?

Good point, never thought of that. If the cylinder exhaust ports were facing opposite the thrust side of of the piston it may not have happened as it wouldn't have caught the lip.

I imagine even with Cox's good reputation for machining standards a couple bad ones slipped through the cracks.

I tried to clean up the cylinder where the groove was cut in. There is a slight ridge which tells me the piston galled to the cylinder instead of striking the edge. The inside exhaust port edge was beveled slightly and there isn't any sign it caught the edge. The ridge on the cylinder is extremely hard and cannot be touched with a xacto knife. Usually the cylinders are soft and can be scraped with a xacto knife. Maybe there was impurities in this blank and this led to unequal expansion when coming up to temp. This caused a tight spot and siezed the piston.

Jason