You'd be surprised how easy it is to detect radial play using this technique -- especially if you're using a larger diameter prop. There should be virtually no perceptable movement on good bearings but once they start going bad the movement begins to increase quite rapidly and can usually be picked up before they start screaming or grenade.

Probably a combo of both, you'll feel it if it is bad enough, but won't if it isn't. Last time I experienced bearing failure, engine was running close to normal, just making a rapid but soft "kunking" sound, no play in the shaft at all. When I took the bearing out, found that the little fingers that sandwich the bearing cages had started to come apart ever so slightly.

Retainer?

Here's a pic of the cage,

Ok, but there was mention of fingers?

From NTN

Pressed Steel Cages

Pressed steel cages are the most common type of cage produced by NTN. In this process, steel is formed (pressed) into the proper shape and size to fit the rolling elements, their spacing, etc. Some steel cages are pressed as a one-piece retainer while others are pressed as two separate halves and assembled to form a two-piece retainer. The assembly methods used for two-piece steel retainers are spot welds, rivets, or bent finger joints. (The bent finger joints are designed into the stamped cage halves and are created when they are folded, or bent, over each other to hold the cage together).

You guys should borrow a page from the heli side. After the OS 50 SX-H and Hyper got the reputation for eating metal retainers, a lot of bearing suppliers started selling bearings with plastic retainers. VOILA! Problems mostly went away. OS finally caught on and began installing them as OEM. Tey still have problems with the OEM bearings but at least some of them can get more than 5-7 gallons through without destroying their engines. I have several different sizes of rear bearings with plastic retainers and some with stainless steel races. For those engines, problem solved!

>>> I periodically check my bearings by grabbing the tips of the propellor, alternatively pushing on one and pulling on the other to see if there's any radial runout. >>>

Gosh, I like that. That would be percieved as "wiggle" in the crank rather than straight in and out. I am really, really going to try hard to remember that, lol.

At what stage of wear would you guys say bearings are when you can put your ear down on the fuselage (great sound amplifier unless it's a profile) and hear a slight growl when turning the prop gently back and forth?

Thanks,

Ernie

I would say that those bearings are not long for this world!

If I ran the engine regularly the growl only gets so bad. I have let one in an OS108 go so long that the prop could be wiggled, the crank was riding against its bore. This took 110 flights over three years for that bearing to develop over .015" radial clearance. The bearing did not appear rusty just dull silver inside and out, clearly corrosion and use wore down the steel. If an NSK 6904 bearing can go that long without it coming apart they must be well made and they are my top choice as a replacement if I can't find a high end stainless one.

3 years and 110 flights is nothing. My saito 45 went 25 years and God knows how many flights before I had to change bearings. I have a Johnson 36 BB I won in 1962 and it was extensively in the 60's. Still has original bearings and they are fine.

**UPDATE**

I have been too busy to play with the hobbies lately. My wife gave birth to our first child on April 18th so I've been extremely busy. I was able to get down to the shop for a while this evening. I re-heated the block and the new bearings popped in very smoothly. I noticed about 2 or 3 thousands of an inch play in the shaft. I can push it in and pull it out by that much. My other GMS engines do not have this "feature", but my OS engine that runs great and has never been taken apart does the same thing. I had to replace the teflon pin retainers too...the new ones could have gone in a little nicer but they should be fine. I oiled it all up and it felt like it had good compression.

I pulled the GMS off my trainer and popped this one on. Got everything tweaked and tried to start it. At this point I wasn't expecting much since I have made several mistakes throught the process. No worries...It runs great from a nice smooth idle all the way to WOT. I didn't run it very long because it got dark. Weather permiting, I'll run some fuel through it tomorrow and get it tuned in. I'll leave it on the trainer for a bit until it proves itself. Then it's going on my World Models Ultimate 40S.

Thanks again for all your help.
-Rocko