Twister .61 prop driver to crankcase interference

[Grasshopper]

On taxi back to the pit after landing, the engine quit. After retrieving the plane, the engine appeared to be locked up. Later, it turned roughly with what seemed like a bad bearing. Even later, the roughness went away. I noticed a gray residue at the prop driver (#20-6101) to crankcase (#10-6102) interface. Perhaps the gray residue was from the two aluminum parts rubbing together, and the lockup/roughness was from heat expansion of the crankcase.

This engine was broke-in exactly as specified in the "Owner's Guide" for 5 tanks of fuel on a test stand, then had 5 more airborne tanks as specified. The problem didn't appear until the last tank. What is the solution to this problem? :?:

[MECOA]

There should be about .030" between the back to the prop driver and the front of the crankcase. Check to be sure the prop driver isn't split. This would allow the collet to slip further into the prop driver.

If you remove the prop driver check to see it the collet sticks out the back of the driver about .030" when just set into it, not pressed into it.

Where did you buy the engine?

[Grasshopper]

Using a feeler gauge, I find .004" clearance between the prop driver and the crankcase! Viewed with a jewler's loop while the prop driver is in place on the engine, there are no cracks/splits in the aluminum prop driver.

How does one remove the prop driver? It is tightly in place on the collet, and certainly can't be removed by hand.

The engine was purchased directly from the factory - K&B Invoice #13705 dated 10/19/2005.

[MECOA]

Send it back and we'll fix it for you. I don't know if it's the case driver or collet.

[Grasshopper]

I have made a puller and removed the prop driver. With very little pressure it popped free from the collet. Now that I can inspect the front of the crankcase and the back of the prop driver, the two are slightly galled giving evidence of interference. The prop driver is not cracked.

I measured the amount the collet is "proud" of the back side of the driver as you suggested, and found 0.013" using a digital readout. I'm sure that measurement decreases as the collet closes while tightening the prop.

Thank you for the offer to fix the problem! I'd really like to minimize the time I'm without the engine - I have only one plane currently. Couldn't you just mail me a replacement prop driver and collet instead?

I am VERY pleased with the performance of the engine. It's power was impressive even while tuned rich during the airborne break-in tanks, AND it is so light for it's displacement! :D

[MECOA]

Please call at 626-359-6972 MF 9-4 PST
We can not just send out parts as we must be sure where the problem it and get the old parts in return.