I have recently overhauled my engine for a PA24-250 and we installed new lord mounts during the overhaul process. The airplane has a three blade McCauley prop installed some time ago before I purchased the airplane. When they installed the prop it appears they did not replace the lord mounts as required by STC. The part number for the old mounts is J3804-20. The part number for the mounts to complete the STC is J3804-41 (now according to McCauley the part number is -42). The shop that installed the overhauled engine assumed the STC was complete and installed new mounts using the part number J3804-20. Since flying the airplane we have noticed some shaking around 2300RPM and lower until manifold pressure is below 20 inches. 2350 seems to be ok, but not the best. I called McCauley and they told me that I would need 4 new mounts at a cost of $419 each (great I can buy one a year). Is it my only worry that the airplane has some minor shaking going on or is this a much larger issue that needs to be remedied immediately? We did install the dual exhaust system for the airplane, but I don't think that has anything to do with the issue.

Jack,

When I bought my airplane last year I noticed that anything below 2400 RPM I had a vibration. I have the three blade Hartzell. At 2500 RPM and above it ran as smooth as silk but anything below that with high manifold pressure I had a vibration. I replaced all my engine mounts during my last annual and also installed new mags. The vibration was still there. I was thinking it was a prop balance issue but it wasn't. It turns out that I had a few bad spark plugs. The champion plugs that I had only had 200 hours on them. However, I resistance tested them and three were way out of limits. I changed the plugs over to tempest UREM40E's and all the vibration went away! I can now run at 23/23 with no vibration. Might not be your problem but it is quick and easy to check before you spend big money on mounts.

MR

Thanks Michael I will see if I can check the plugs. They are brand new, however the first couple of flights with the overhauled engine were normal or smooth, so it could be the plugs.

Do you which lord mounts you have for your prop?

JH

I will have to check the logs. I think they were the standard Lord mounts required for a 250 with the 0-540.

Do you have a good engine monitor? You can get a lot of information from that. It may rule out a number of problems.

MR

I too just changed over to new unison plugs today and the smoothness was remarkable. I had the prop dynamically balanced a couple months back and did not notice much changed, but the new plugs did what I had expected from balance job! It feels like I put on a three bladed prop. Ray B

I have a JPI 700. What should I be looking for on the JPI that may tell me I have a plug issue?

Thanks
JH

Hi Michael,

Just so it is clear to me, just how does one do a resistance test on a spark plug? Am I using a standard multimeter and if so what #'s am I looking for? Or?

Thanks, Don

Two points of view out there.

Champion says 60,000 ohms is okay and Aircraft Spark Plug Service does not test for resistance so they do not feel it's important either.

Tempest indicates to replace when resistance between the terminal in the barrel and the center electrode is at or above 5000 ohms.