Prop Bumping During Differential Pressure Test

[avrojockey]

Random question?

Occasionally, working the prop forward/back causes the compression number to increase as much as 8-10 psi, usually only 1-2 psi though. Sometimes there's and audible click coming from the jug associated with it.

What is happening the causes the increase? Are rings seating better against the cylinder, against the groove?

 

[rocketbob]

Random question?

Occasionally, working the prop forward/back causes the compression number to increase as much as 8-10 psi, usually only 1-2 psi though. Sometimes there's and audible click coming from the jug associated with it.

What is happening the causes the increase? Are rings seating better against the cylinder, against the groove?

Valve guides wear oblong on opposite ends due to side loading by the rocker arms. Valve seats and valve faces being harder dont wear as much so they hang up slightly before being fully seated due to the imperfect geometry. The rocking of the valves at TDC gets them to seat properly, with air pressure in the cylinder assisting, thus the click.

 

[Full Throttle]

Valve guides wear oblong on opposite ends due to side loading by the rocker arms. Valve seats and valve faces being harder dont wear as much so they hang up slightly before being fully seated due to the imperfect geometry. The rocking of the valves at TDC gets them to seat properly, with air pressure in the cylinder assisting, thus the click.

I don't think the valves move do to rocking the Prop at TDC. The cam lobe is at it's lowest point way before and after TDC. Maybe your right about the air pressure. Mostly likely, the rings seat against the ring land and the cylinder wall when rocking the pistons and pressurized air is injected into the combustion chamber.
Most pistons have a offset piston pin that allows the piston to rock on the wrist pin when the crankshaft or prop is moved back and forth.

 

[Capt]

It actually increases momentarily
the compression, pumping it up a little. It’s a common practice especially for Conty engines which generally have a weaker result.

 

[Taltruda]

Some people say not to rock the prop, but according to Lycoming SI 1191a, you should rock the prop to seat the rings. I heard some engines make that click and some don’t, I believe it may be the piston cocking but not sure..

https://www.lycoming.com/sites/default/files/attachments/Cylinder%2520Compression.pdf

“To assure that the piston rings are seated, the propeller is moved slightly back and forth with a rocking motion while air pressure is applied; thus providing a more accurate reading. Meanwhile, a second person adjusts the air supply pressure to 80 psi, indicated on the supply pressure gage of the differential compression device. Then, observation of the engine cylinder pressure gage will give an indication of the condition of the parts in the com-
bustion chamber of the cylinder.”