thailazer

Had a great first flight with my 73 inch Flair Cub today but on the second flight it was not developing full power so I decided to land to see what might be up. Had a good landing, taxied in, and shut down. Realized that the prop would not move so decided to pull the head and this is what I found. There are bits of the rear bearing ball-separator all over the engine. It is probably repairable, although the scoring on the prop shaft under the carb looks pretty bad. (Is that a critical interface there?) The piston and liner look amazingly good, and it appears the metal was shredded by the crankshaft counterweight edge and the engine casing right next to the induction port. I have always had a strange grey substance coming out of this engine so that bearing must have been bad from the factory, but the engine has given good performance with the 4 gallons or so of fuel I have run through it. Still can't believe it kept on running with this metal storm inside!

thailazer

Anybody have any advice on how the scoring on the prop shaft might affect performance? This was such a good running engine that it might even be better than new with some higher quality bearings in it.

aspeed

If the scoring causes a leak, it may surge a bit on the top end, and idle a bit faster or run leaner basically on idle. Try blowing through the venturi while holding you fingers over the side holes. If it seals, you are good. If you don't want to kiss the motor, you can use a bulb, or syringe to pressurize one of the side holes. I guess it is a good idea to investigate the grey matter. It seems to spit out the bad stuff pretty good though?!! At least parts are cheap on those motors but I think upgraded bearings may be a good plan.

thailazer

Appreciate that advice and I will give it a try. Am going to knock out the bearings here this morning so your input is very timely!

thailazer

From that test, it looks like the shaft is fine. A bit of air leaks through but not very much. Now that I have it apart, the scoring on the shaft does not look bad at all. Had to take another look at the liner and piston and they are in fine shape. My bet is that the bearing ball-spacer was defective when the engine was built and that a bit of it must have been sticking out and rubbing, causing the grey matter that I had noted. Must have finally worn through causing the metal storm inside the engine.

thailazer

Have decided to rebuild it and have a new set of bearings on the way from RC Bearings. It has been a good running engine and ought to be even better with a solid set of bearings in it.

blw

My Feedback: (3)

Keep us updated as this is interesting.

thailazer

Will do, as it is interesting isn't it? I would never have imagined that a glow engine could pass this much metal and still be in a re-buildable condition. Have now triple-checked everything and the piston and sleeve look great, as does the rod end bearing. Not much more to worry about so it should run once the bearings arrive. Love working on these little gems.

speedracerntrixie

My Feedback: (29)

I'm wondering if you are concerned about the rust on the counter balance? Rust is very abrasive and you can bet that if the crank has this much rust then the bearings had rust as well. Could have been the cause of the bearing failure. Depending on the type,of oil used in the fuel and your humidity the steel parts in your engine can rust very quickly. You may want to get into the habit of pumping your tank dry and running all the fuel out of the engine and then lube up with ATF between flying sessions.

thailazer

I hear what you are saying there about the rust, but with the grey sludge occuring since new, I am thinking it was the ball spacer on the bearing not properly installed. The rust is there on all my engines and might be due to running 15% nitro Wildcat or Coolmax fuels. I always run the engine dry and then spin it a few more seconds after it stops firing. Also use after run oil about 80% of the time or more. We have high humidity here often and high temperatures so everything corrodes fairly quickly. I do find that planes I hang by the prop have bearings that last much longer than those I store horizontally. Might be gravity pulling the water lower in the engine case?

liquid_TR

this may sound a little out of fashion but 3-5% castor oil in the fuel mix really goes a long way for nitro engines to keep the rust away in humid places. ie using an 18% castor/synth oil blend fuel from the get go. By doing this you will get away with not using after run oil at all. I also never run my engines dry. tried and true in my own experience.

thailazer

Got a new rear bearing from RC Bearings.com in an amazing ten days and got the engine rebuilt. Great compression, still a tight piston fit at the top, and the engine is ready to be run in the Cub. Monsoon rains came today so will have to wait for a better day to do a test run and test flight. Found more bits of aluminum inside the muffler that I had to clean out to keep them from going into the fuel tank via the pressure line. Keeping my fingers crossed it will run okay, but don't see any reason why it shouldn't.

thailazer

Did the test run today and flew three times. Everything was fine except for two things:

1.) Had to replace the glow plug as the element had been pushed all the way up from the flying metal. It ran okay, but on the third flight start-up I noticed the idle RPM dropped a lot when I removed the glow plug driver. Took off the plug and found the problem. A new OS #8 and things are good again.

2.) There seems to be a new "rod end knock" right at TDC. I know the connecting rod was tight at the crankshaft, but I never took the piston off the other end so am wondering if that rod end took a pounding, or if it is just worn out. I have run into this on two stroke dirt bikes and that noise usually means the engine will blow itself apart soon.

At least I got the Cub back up in the air and had fun with the rebuild. Looking back at those photos in the original post, it is hard to believe it is running again. The only parts I replaced were the rear bearing and the glow plug.

thailazer

Maybe an engine expert could provide some advice. That rod end knock at TDC is completely gone once the engine cools off. I noticed it between flights and it is very obvious. Last night, working on the plane, no trace of play or knock at TDC. Is there anything else besides rod end play that can cause that knock? I am a bit mystified.

aspeed

Bent rod, wrist pin too loose on the piston or seized on the rod? (maybe) Oh sorry you wanted an engine expert.

thailazer

Hey, you are as expert as they come! I think I am just going to ignore it and see what happens. I only paid 30 bucks for this engine (wholesale price) so even with the bearing replacement, if it turns into a pile of aluminum like a lot of my old motorcycle engines, I am fine with it.

Seriously though, the things you mentioned should exhibit themselves hot or cold, right? Nothing is seized in the engine, and the only thing I did not check carefully was the piston wrist pin fit. I suppose the aluminum piston could be expanding with heat and that is the play that I am detecting at TDC. Performance is as good or better than before it ate the bearing spacer.

1QwkSport2.5r

The tolerances widen as the engine warms, so a knock will usually become more evident when the engine is hot. Keep in mind a lean mixture or too much nitro can cause a knock or ping as well. If it were me, I'd just run it and see what happens. Maybe add a little extra castor oil to your fuel.

thailazer

Wasn't able to find any castor oil so still using the straight 15% Coolmax. Got a third of a gallon through the engine after the rebuild and it is a real powerhouse with a MAS 12-4 on it. From what I can see, there has been no degradation of compression or power even with the metal shredding. I still hear the pre-ignition occasionally, but it only occurs in a very narrow throttle range. I am thinking the new OS #8 plug I put into it might be hotter than the original ASP plug and that is the cause of the knocking.

jaka

Hi!
Try a better prop like a RAM, Graupner G-sonic or APC and I promise you will see better performance.

thailazer

I agree, and the shop here used to have Graupners, but now all they stock is the MAS. Will have to take the time to try and order a better one from overseas.

thailazer

Got another month of flying in after the rebuild and the engine is running as strong as ever. Been getting the Cub up frequently even with all the rain we are having this Monsoon season. It appears that the knock I was hearing was from the fresh OS #8 plug as it went away after a half dozen flights or so. My guess is that glow plug elements oxidize fairly fast initially which makes them less "hot".