Clicking on new Rebuild, Please Help

[lr172]

I recently rebuilt my O-320 and hung it on the RV-6. I was working on fitting the cowl and while turning the prop, I heard a metallic click. Upon further investigation, I found an odd and disturbing situation.

When rotating the engine in the standard direction, a click can be heard when either prop blade reaches the 1:30 position (one prop tip is indexed to TDC on #1) . In other words it happens in 180* repetition in exactly the same spot. I can back up a few degrees and repeat it, but if I only back up slightly, it won't repeat. It is very consistent. I can also swing the blade in the opposite direction and will get the click with either blade at the 10:30 position.

The click sounds like tension is built and is released at the point the click is heard. You can feel it in the prop blade. It is very light, but can be felt.

It is difficult to locate exactly where the sound is coming from but my best guess is in the case behind the #4 cylinder.

When I assembled the oil pump, there was a spot that was just a bit tighter than the rest of the rotation, but it certainly didn't seem significant and assumed the gears needed to be run in to mate properly. My first step will be to pull the accessory case to eliminate this as the cause.

I am baffled by this. I just can't visualize what could be causing this. I am hoping some of the more experienced members will have some ideas.

Thanks in advance for your assistance here.

Larry

EDIT: Failed to mention that the engine is bare of accessories. No mags, fuel pump, etc.

[AJ85WA]

What magnetos are you running?

If its like mine Slick mags,the left impulse coupled mag would do exactly what your describing, winding up the spring and then firing. (Pretty sure it also does it exactly ever 180degrees)

Just a thought

[PCHunt]

The impulse mag "click" is not subtle in any way. It is a very noticeable click at exactly the same position in the prop rotation every time.

Maybe get someone else to listen?

[William]

Originally Posted by lr172

It is difficult to locate exactly where the sound is coming from but my best guess is in the case behind the #4 cylinder.

Larry

I agree with the others that have replied. The left mag is typically where the impulse mag is installed. This is the mag located by cylinder #4.

Best regards
Bill

[lr172]

Sorry guys I wasn't clear enough in my original post. The engine is bare of accessories. No mags, fuel pump, govenor, etc. For clarity of the problem, the click is much quieter and less powerfull than that created by the impulse mag.

[David-aviator]

Originally Posted by lr172

Sorry guys I wasn't clear enough in my original post. The engine is bare of accessories. No mags, fuel pump, govenor, etc. For clarity of the problem, the click is much quieter and less powerfull than that created by the impulse mag.

Could it be a valve hanging up and slamming into its face as push rod tension is relaxed?

[rocketbob]

The click could be the movement of the rings as one cylinder comes back up on compression. Since there's no oil it may be more noticeable. Normally you don't hear it on a Lycoming but on some radials like the 1340 you hear it especially when doing a compression check.

[mahlon_r]

I think what you are hearing is cam whip. What is happening is the pressure from one of the valve springs is putting a side load against a cam lobe by pushing on the valve train when that valve train has an open valve. This in turn causes the cam to rotate slightly against other components in the gear train and valve train. It is very subtle and sounds almost like a wet, light, tick or click you could make with your tongue. If that is what it is, it is normal and not preventable.
It is also more noticeable when accessories are not installed, as it is easier to hear the lash being taken up.
You normally can't feel any resistance when it happens but you can feel a very slight bump in the prop from when the gear backlash is taken up.
Once the backlash is taken up it will be quiet, if you continue in the direction of rotation, until you back up enough or go in the opposite direction enough, to unload the gear and valve train.
Good Luck,
Mahlon

[Mel]

My first thought was the fuel pump pushrod. But that would not repeat. Once it slid down without the pump in place, it would stay there.

[lr172]

Originally Posted by mahlon_r

I think what you are hearing is cam whip. What is happening is the pressure from one of the valve springs is putting a side load against a cam lobe by pushing on the valve train when that valve train has an open valve. This in turn causes the cam to rotate slightly against other components in the gear train and valve train. It is very subtle and sounds almost like a wet, light, tick or click you could make with your tongue. If that is what it is, it is normal and not preventable.
It is also more noticeable when accessories are not installed, as it is easier to hear the lash being taken up.
You normally can't feel any resistance when it happens but you can feel a very slight bump in the prop from when the gear backlash is taken up.
Once the backlash is taken up it will be quiet, if you continue in the direction of rotation, until you back up enough or go in the opposite direction enough, to unload the gear and valve train.
Good Luck,
Mahlon

This makes a lot of sense. When I hung the engine and installed the prop, I was still waiting for my new pushrods, so they were not installed. I never noticed this clicking at that time and didn't hear it as I was turning the prop to do the final valve lash checks yesterday. It was only after installing all of the pushrods that I noticed the clicking. This would also explain why I get the click at different points in different directions.

I had thought the backlash might be involved based upon the noise, but couldn't not figure out what would cause it to reverse or hang up.

Thanks so much for your knowledge and experience here. I'll sleep better tonight.

Larry