Prop Position

[RV12Roger]

I am mounting my prop. Is there a preferred prop position relative to say #1 cylinder on a compression stroke at top dead center? The gear box ratio complicates this question but have not seen any reference to this.

[rvbuilder2002]

Because of the uneven prop reduction ratio, the orientation of the prop. to the firing order is always changing. That is why no specific orientation is specified.

[Catbird]

Page 121 of the Rotax Type 912 Installation Manual proves technical data for the Propeller Drive. Two gear reduction ratios are listed:

* 2.2727 (50 teeth/22 teeth)
* 2.4286 (51 teeth/21 teeth)

I can't tell which ratio is used in our 912 ULS but either way, the resulting prop rotation is constantly changing relative to the crankshaft rotation. Before going through the oil system purge procedure, I had the brilliant idea that once TDC was found on cylinder #1 to check the valve tappet clearance, a simple 180 degree prop rotation would bring me to TDC for the next cylinder in the firing order, and so on. This my deep thought process during a long hot morning shower. Then while brushing my teeth, the gearbox ratios occurred to me. Ya gotta find TDC for each cylinder becasue the prop position keeps on changing.