Bent push rod - now aircraft vibration?!

[Tidders]

We experienced a 'bent' push rod (see previous post below) which have since been replaced and the cylinder was also sent away and refurbished and reinstalled.

Yesterday we took the aircraft for a planned one hour flight to get the engine parts moving after maintenance work completed.

However, we have now detected that there is a vibration (not evident before push rod incident) that can be felt through the controls when climbing out and in the cruise. It appears to be a very smooth vibration in the same cadence as the engine. It is not like a rough running engine vibration in fact the cylinder temps were all good across all cylinders throughout the flight. We had a second flight attempt as it appeared to clear itself on the ground but the vibration was evident on both flights.

Sounds odd but it is a very smooth, fast and uniform vibration that can be felt through controls and rudder pedals,

We had a chat on the ground and did not arrive at any conclusion.

Anyone else had a similar experience and can throw up any possible causes?

Many thanks

Previous post:

We are based in the UK and have a 1959 Comanche 250 which is fitted with an EDM700 displaying exhaust gas/cylinder head temps for each cylinder. On a recent trip around Southern France and Spain we noticed that on cold start the display for #2 cylinder did not show up on the EDM. This appeared to clear itself after a couple of minutes and all power checks were good and when flying the cylinder showed temps in the normal range. It happened to 2-3 times on the trip but not when the engine was started when warm/hot.

Anyway we returned to the UK and have done a number of flights since then with no problems indicated.

Couple of weeks ago and shortly after departure and in the cruise the display for #2 disappeared from the EDM and the engine started running very rough with a lot of vibration. Returned to the airfield and the local maintenance outfit had a look and found a bent push rod on #2 cylinder (and also the the outer tube was bent). This was clearly the cause of the fault in flight and I guess the valve was sticking.

Has anyone experienced a similar fault and if so any advice out there on a possible cause would be much appreciated?

Note: For the last couple of years we have only carried out an oil change on the aircraft's Annual (fly about 70-100 hours per year) - previously we used to do it every 50 hours, could this be a contributory factor to the bent push rod?

Thanks in advance.

[BobPicker]

Sorry to hear you had an issue. What caused your bent rod? Most likely as stuck valve. After rework you described, I would be checking every cylinder to ensure none of them are sticking also. Are you sure that the previous problem did not effect some other internal engine component such as a connecting rod or the cam shaft? When a push rod bends there is a tremendous load placed on the entire mechanism and other things can break or be damaged besides the push rod and valve train.

In order to get the vibration you are explaining something else has been affected. I would not fly it, but start digging cylinder by cylinder looking for bent or damaged components. The only way you may have inspect some parts may be a inspection scope to get a video of the internals.

I am not an A&P, just a very experienced owner that does a lot of work on my Comanche under supervision of an A&P/IA.

Good luck,

Bob

[GMTW]

Hi;

I have a hanger mate with a 180 that needed a new cylinder. I believe after a lot of research he found that the superior and lycoming cylinders are not compatible due to different weights of piston pins or plugs and unless that is dealt with you end up with a vibration that after 100hrs or so ends badly.

[MV Case Jr]

Does the vibration frequency follow the engine RPM ?

T-6 Driver

[JIMICS2452]

How old are the motor mounts? Recommended replacement is every 5 years. They get stiff with age and can transmit normal engine vibrations to the airframe.

[Tidders]

Thank you for the replies.

The vibration was not there before we had the bent push rod - but appeared after replacement together with refurbished cylinder by our maintenance outfit. It id a very smooth and fast vibration that appears to be in step with engine RPM.

Comment from our maintenance outfit today was that they found the primer unlocked (we are not sure that is correct?) and they are saying that this could be the cause of the vibration. If this were the case I would have thought that we would have experienced a 'lumpy' engine not a 'smooth' vibration?

When doing the startup checks on the morning of the flight we did experince a big mag drop (at least 350) on one of the mags, but this eventually appeared to clear itself.

We want to understand the cause of the vibration in case it is another component that has also been bent - but it is a mystery.

Thanks again.

[William Hughes]

If the replacement cylinder came with a different piston, then you might have mismatched piston weights. Opposite side pistons are supposed to be within a certain tolerance on how much they weigh. New pistons are lighter. Might have something to do with it?

[Tidders]

Thank you for the suggestion William - we will ask the questions of our maintenance outfit.

Regards

[Clarence Beintema]

Mismatched piston weights also happens when one cylinder is hone to 0.010" oversize. When this is done, the opposite cylinder has to be done as well to keep weights the same.

Clarence