I just recently over tightened my prop on my SK .90 engine and locked the engine up. I backed the nut off and helped some (still had alittle drag but the engine seemed to idle and run fine), but in mid flight the engine stopped. After getting my plane back to the pits I turned the prop and it felt like sand in the bearings. After doing a search for torque specs for prop nuts on various engine sizes, I came up with nothing.
I purchased a 1/4 inch drive torque wrench that has settings from 20 in/lbs to 200 in/lbs.
I was hoping to find information with prop nut torque specs according to engine size.
I searched my manuals to all my engines and even the manufacturers don't list prop nut torque specs for their engines.


Any help someone could give me would be greatly appreciated!

You have some other problem wrong with your bearings. It was not the prop nut torque.

The torque specs would be in the neighborhood of 22 Newton-Meters or 200 inch-pounds or 17 ft lbs

I think you should contact the manufacturer of the propeller.
17 foot/pounds with a 3/8 X 24 nut would crush a zinger wood prop to splinters.
It would probably deform a master plastic and would probably be fine with a carbon.
But W8ye is right, you either have a bearing problem or deformed the prop flange to the point that it hits the case.

I suspect what's happened is that the overtightening has driven the prop driver back onto the crankcase making it rub. I don't know the setup on the SK engines but if it uses a tapered collet then it could have split the prop driver (as happens with the cast prop driver on ST engines) or if it uses a D drive and thrust shim (like most OS) then it may have squeezed the prop driver into the shim making it touch the crankcase. If the bearing still feels rough without the prop or driver then it's possible some shavings got into the bearing.

It makes me cringe when I read guys saying to tighten the prop nut as tight as possible even when an engine only has the normal 1/4x28 thread because even using the highest grade steel the recommended torque is only a little over 11 ft-pounds. With the lowest grade (and some engines use unhardened crank threads) the torque can be as low as 3 ft-pounds.

Here's a calculator I found to get torques for different thread sizes and steels.
http://www.futek.com/boltcalc.aspx

MillerSS,


If tightening the prop-nut causes your engine to lock up, there are several possibilities:

1. The rear bearing is not fully seated in the crankcase (tightening the nut pulls the crankshaft forward, exerting excessive force on both bearings - pulling the inner races toward each other - excessive pre-load).

2. The crankshaft is not fully seated in the rear bearing (same effect as 1).

3. Something is missing, or broken in the front, allowing the prop-driver to press directly against the crankcase, or against the front bearing's outer race.


When everything is correctly assembled, tightening the prop nut clutches the front bearing's inner race, between the crankshaft's larger journal and the prop-drive items (prop-driver, or drive cone). This locates the crankshaft axially.

The large journal of the crankshaft just sits in the rear bearing, which takes the radial loads without much axial force applied to it.


If '1', or '2' are true, your engine's bearings could have sustained damage (Brinelling the races, flats on the balls...) from firmly tightening the prop nut...



If it feels like sand, you need new bearings... Is your engine still under warranty?

As a young mechanic 5 decades ago the older mechanics suggested that I try not to wring it off.

Bill

What he said. I have tightened some to a point of cracking the prop

Snug it up with a 6' adjustable. But on my 4 strokes, I use a nylon locknut instead of a jam nut. Even then I tighten it fairly snug with a 6" crescent. That's from an OS 15 to an OS 1.20 Surpass.

Hello all,
Just an update on my engine... I called Kangke Industrial USA about my SK .90 and talked to their tech. He told me pretty much what all you guys are telling me that you difinitely don't want to over tighten the prop nut. He also recommended running wood props compared to plastic props. He told me that they would repair my engine under warranty.


Thanks to all who replied to my post! It was all very helpful!

Miller,


They will, by all likelihood, just send you a replacement engine.