OS 50 Hyper a PIMA - help!!!

[tailStrike]

Ok, this is my first nitro motor and I have had nothing but bad experience with it in my Raptor. This is my 2nd motor after the first was replaced by OS after the piston exploded - hole right through the center. After the incident, I found that the head bolts had loosened, and I'm fairly certain that might have caused it to run super lean and overheat.

I have been sure to run the new motor on the rich side, and my temps have been very low, but the thing died during flight a couple of weekends ago. I flew it very briefly last weekend, and landed it as I was tinkering with a newly installed governor and then was unable to get it started. I thought it was due to my field battery being low, and took it out to the field today after a good charge.

The think just wouldn't crank. Long story short, after wasting forever trying different batteries, starters, igniters, etc., I finally brought it home assuming it was a dead plug (forgot my wrench) although I knew I had just replaced it.

We had noticed a grinding sound when turning the starter shaft by hand while at the field, and after ruling out the glow plug, I opened the crank case to reveal the cause - all kinds of crud. Looks like metal flakes. Same kinda crap I found after I blew a piston, but this piston is fine and I don't really see any other destructed parts. Also, the engine seemed to create good compression, so hopefully the problem is localized to the crank case. The piston and cylinder sleeve look good.

I'd like to break the motor down completely and look it over. Does anyone know of a good source for disassembly instructions/tips? If the crud was pulled in from outside the motor, I don't really see how as i use an air filter (although no fuel filter at the moment).

[tailStrike]

Nevermind the request for disassembly instructions...I found some here and now have the engine broken down with the exception of the bearings:
http://www.raptortechnique.com/enginebearing.htm

Everything looks good. Now I just need to find something to clean it with.

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On a suggestion of a buddy of mine that works on small engines a lot, I bought some carb cleaner for use to flush the motor out, which I will be doing tomorrow. I'm still baffled buy the discovery of metal flakes in the crank case as no parts look damaged, including the bearings. Since this motor is a refurb, I wonder if the metal was something from a part that was replaced that just wasn't flushed out in the refurbishing process.

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Well, hopefully I have everything worked out. After flushing everything out, I noticed some rings grinded in the back plate as mentioned here:


If your engine is leaning out severely and no amount of tuning will stop it then an often overlooked cause is from a loose fan. Sometimes when you try to crank the helicopter and it is fuel locked then this can slightly unscrew the prop nut and fan. When this happens it allows some in/out play in the crankshaft. Then while the engine is running, the connecting rod pin grinds against the back plate. With the engine spinning 10,000+ times per minute this quickly builds up heat from the friction and overheats the engine.The above fix is the most common but I did get an email from someone that was having the same problem and it turned out to be that his muffler was full of carbon buildup. He tried two engines before figuring out the muffler was clogged. (Thanks Ian for the info)

[ghtracey]

I was just going to post to have a read at that site. Mike Prewitt is awesome. Another thing, don't use that governor until your motor is fully broken in and you are sure the needles are tuned right. A governor can hide problems until its too late. A motor that sags when its overheating will just be fed more throttle by a governor, and suddenly you have a hole in your piston.....

My $0.02 anyhow.

[tailStrike]

Thanks for the advice. The motor should be broken in by now, but it probably is a good idea to leave the governor switched off and run it easy until it has had time to run a while since being disassembled.