Cogging/Chatter When Turning Engine Backwards...

[Piper J3]

I’m helping a friend install a new gearbox on RV-12 Rotax 912ULS. We are removing the crankshaft gear, and in the process of setting the crankshaft at TDC #1 to insert locking pin, we turned the engine backwards about one turn total. I know Rotax advises not to turn engine backwards, but I think this is only when oil hoses are open for possibility of air ingress.

When engine is rotated backwards we feel a cogging or chatter. Forward direct is perfectly smooth. I think the Sprag clutch is back-driving the starter motor when engine is turned backwards and that is the cogging that we sense. Starter motor works well and easily spins the engine.

My question… has anyone experienced this when turning engine backwards? My RV-12 is laid up for the winter, but I’m pretty sure the engine turns smoothly in both directions.

Photo is crankshaft with gear removed...

 

[SPRV12]

You are feeling the sprag clutch grabbing.

 

[Piper J3]

You are feeling the sprag clutch grabbing.

And that's normal and OK?

 

[CTCole]

And that's normal and OK?

It's the eccentric rollers of the sprag clutch grabbing. Make sure you purge the engine (not burping) before starting the engine. You could have pumped air into the galleries or even the lifters (remember, the engine turns over 2.43 times for each turn of the prop). You can line up for the crank locking pin without rotating the prop backwards more than a wiggle.

 

[rvbuilder2002]

but I think this is only when oil hoses are open for possibility of air ingress.

The recommendation applies regardless of whether hoses are attached. Since it’s a dry sump engine, there is a potential for back sucking air into the oiling system.
What you’re feeling is the engagement of the starter through the Sprague clutch.

 

[Piper J3]

The recommendation applies regardless of whether hoses are attached. Since it’s a dry sump engine, there is a potential for back sucking air into the oiling system.
What you’re feeling is the engagement of the starter through the Sprague clutch.

Yes, I surmised that the starter motor was being driven by the Sprag clutch when turning engine backwards. In my original post - I was wondering has anyone experienced this cogging/chatter when turning engine backwards? My RV-12 is laid up for the winter, but I’m pretty sure the engine turns smoothly in both directions…

 

[rvbuilder2002]

Yes, I surmised that the starter motor was being driven by the Sprag clutch when turning engine backwards. In my original post - I was wondering has anyone experienced this cogging/chatter when turning engine backwards? My RV-12 is laid up for the winter, but I’m pretty sure the engine turns smoothly in both directions…

Most people probably follow the recommendation to not ever turn it backwards, so they wouldn’t know…