I know that most everyone flies turbines (I will soon also) but I still have a D/F plane and I was taking apart an engine yesterday that had the JMP tuned pipe on it with the big fat orange silicone seal. When I took the head off it looked sand blasted, like little while spots (thousands of them) on the head and the top of the piston. I have heard that this can be a problem if the critical length of the tuned pipe is too short, but this tuned pipe was made for this motor and there is no way to lengthen it. Just curious to know if anyone has had this problem or knows how to fix it. I'll attach a few pics, but the didn't come out all that great. Thanks, Mike L

Another pic.

Well, sad to say, I've seen this a couple times. The usual cause is a piece(s) of schrapnel bouncing around in the cylinder. The usual source is the rear bearing cage or a piece of a bearing. The failure mode is rust. I've also had a piece of the rear rotor break off and do a similar thing and I've even seen pieces embedded in the cyclinder head. It's unlikely anything to do with the pipe unless a piece of metal came from there or the header.

I suggest you completely tear down the engine and particularly check the rear bearing. Also look for another source of a broken off piece of metal. I'm betting you find a slightly rusted bearing and rust on the crank under the bearing. Very common.

Unless the piston/sleeve have bad scoring, you can use a fine needle file to clean up the head and top of piston and reuse the parts. Check for compression and do a ground engine run for confidence. Bad deal to do that??? Possibly, but I do have one engine running right now like that. Cost of head, piston & cyclinder = cost of new engine if you could find one.

I'll look at the bearings, but this motor is brand new. It could have been the element in the glow plug when the motor quit, but I have a hard time believing that this much damage could have been caused by a piece of schrapnel when the motor killed. I'll look. Thanks, Mike

Glow plug element is pretty soft and hard to believe it could do that, unless it was an idle bar that broke loose. Others have made similar claims about the element but I've always taken it with a gain of salt.

New engine or not, something beat up the inside of the cylinder head and piston.

I'd say get the crank out of it and get your finger in the the rear bearing and rotate it. You will be able to feel it if its rough. Better if you can get most of the oil out of the bearing with some solvent. Roughness really shows up with a cleaned out bearing. Any little metal particles will make their presence known by the feel. If you can't find anything, clean it up and run it.

Anyway, that's my best guess.

Mike,

You might want to check the engine clinic on this one but I beleive it is possible that what you have was caused by pre-ignition. To high a compression or the wrong plug can cause this and do the same type of damage. What plug and fuel were you running the engine with? What nitro content?? Todd

Roger that, I will give it a try. The article in RCJI I was refering to earlier mentioned the cause as redetonation of the unburned fuel being pushed back into the head due to a short critical lenth of the tuned pipe. They claimed that the shockwave came back too quick and pushed unburned fuel into the motor at the wrong time. I can't see why that would cause a sandblast effect, I can see how that could cause a hole in the piston though. Anyway, I will give her a go. Thanks again for the info., always better to have two heads on a problem. Mike L.

Todd, I was using 5% sig champion fuel with a McCoy/hangar 9 #9 ducted fan plug. I have used this setup for every ducted fan flight I have ever made (for 2 years) with no probs until I switched the pipe to the JMP system. I switched so I didn't have to keep replacing heat gaskets as often. Strangest thing I have ever seen (at home, at work I see some wierd stuff). Thanks, Mike L.

That looks exactly like a detonation problem. In pylon racing we mess around with head shims quite a bit and one thing we watch for is the "sand blasted" look. That tells us the head clearance is too small and we need to back off. I'm not saying that head clearance has anything to do with you problem but I have seen that look many many times.

Mike,
If you are missing the element in your glow plug it is what did the damage. I did the exact same thing to a KBV .82 after a too lean run. My first deadstick in over 100 flights. I sent it to Ackerman and it was only $85 for the repair, I don't know if he replaced the head or just polished out the surfaces. He also replaced the bearings.
Yes, your glow plug element did the deed.
Sparky

Well, I know one sure way to figure it out, I just need to polish the head and run it again. If it comes out sandblasted again, then I know it isn't the glow plug. Regards, Mike

I have been flying a Starfire with a Dynamax Fan and JMP pipe on an OS 91 for two years with no problems at all. I find it hard to believe that your pipe is too short.
Roger

I know it, I refuse to believe that it is my pipe! (I love that pipe) I will do a few tests and see. Never surrender! Mike