Okay, this is a new one on me.

I just assembled a CoMo 40 (SuperTigre) engine after it had been in storage. The bearings felt good enough to me so I re-used them.

When I attached a prop and tightened the prop nut, the engine got very tight to turn. Too tight to attempt running.

So blaming the bearings, I ordered a new set from RC-Bearings. They arrived today, and yes they felt much smoother. I figured I had solved the problem.

I re-assembled the engine, and have the same problem. If I tighten a prop nut so that the prop is secure, the engine is too tight to run.

I had thought it might be either the tapered cone jamming the front bearing inner race or the prop driver rubbing the outer race or the engine case itself so I made a spacer to go between the cone and the inner race. No joy.

I then removed the prop driver, and installed only the tapered cone, but backwards. Then I tightened the prop nut as much as I could, until the crank started turning because the taper cone was not locking onto the crankshaft without the prop driver. I was thinking this might eliminate the prop driver as the culprit. Even at this 'lighter' load the engine had become tight and hard to turn.

If I remove the taper cone, the crankshaft turns easily and has ~ .010" freeplay.

So I have run out of possible explanations.

Maybe the front or rear bearing is not seated in its bore properly. I have had this happen before. Try installing the prop as if you are going to run the engine and then heat the entire engine with your heatgun, the case will expand and the bearings will move to wherever they should be seated. If this don't work, I would suspect a crank spacer missing or a improperly machined case for starters.

Some Supertigre engines used a washer on the shaft that sat right behind the front bearing. One side is flat and the other has a taper on the hole that faces the larger shaft side. Usually the washer was made of aluminum and a few were brass. The washer takes the side load off the rear bearing and prevents the bind you have when you tighten the prop.

Thanks guys! I'll check into both methods.

I have found an exploded diagram with the crankshaft spacer:

http://www.supertigre.com/parts/supg...9-exploded.pdf

This spacer was not in the can of parts I had for the engine. Unless..................maybe it is stuck to the bottom of the can!

BUT, looking at this, I know I have seen this item in the past!

http://www3.towerhobbies.com/cgi-bin...0294&search=Go

Thanks again!

I found it!

LOL, it was stuck to the side of the can in a vertical position. That's why I didn't see it when I was fishing out the screws and stuff.

Thanks mucho!! [sm=thumbup.gif][sm=thumbup.gif]

I had a chance to tear the engine down and install the spacer tonight. Now the engine spins nicely with the prop very snug and everything is hunky dory. Thanks again MrMotor!

Kmot, I did the exact same thing with my ST S.61K. I might have even asked about it here a while back. I think when I took my engine apart it stayed in the case and when I cleaned the case it fell out. Luckily, it was in the tub I was using to hold the parts.

David

This is my one and only SuperTigre design that I have ever torn down so that is why I couldn't figure out the problem. I will not forget though!

I usually realize I have left some small but vital and hard to find part in a package the day after I have thrown out the package.

jess

12900 on a MA 10x6 and Powermaster YS 20/20. Might get more if I tune the carb better. I initally forgot what needle was what. So after smacking my fingers 4 times I left it that.

102°F OAT.

Okay, slight correction. I hit my fingers three times and burned my thumb one time.

Here is a video. At the 3 minute mark it's good for a laugh.

http://media.putfile.com/CoMo-40-Eng...stored-and-Run

Musta been the heat that caused it...
102! Sheesh...we were lucky to get into the mid 50's here today.