Low compression on one cylinder only
in 912 / 914 Technical Questions
10 years ago
hi there all, hoping I might get some help with an issue I have.....
I have a compression problem on my 912 uls. #4 cylinder was trending lower over a 6 month period, eventually to less than 60/80 psi, with the differential leak down test showing air rushing past the piston into the oil tank during the test, and no air escaping through either the inlet or exhaust manifolds. All other cylinders 75/80, engine approx 410 hours since new, approximately 6 years old fitted to a flight design CTSW.
I suspected piston rings, and upon disassembly found that the ring end gaps had near aligned at the top of the piston, which rang true with my initial thoughts. As I had new rings, I fitted them with the ring gaps in accordance with the heavy maintenance manual, only to find after the first engine run that the compression was still low, with the same differential compression test symptoms. Upon 2nd disassembly, to my surprise I discovered the ring gaps had realigned themselves towards the top of the piston. I then refitted and aligned the rings as per the heavy maintenance manual for a second time (oil scraper and 2nd ring 30 degrees offset at the top, 1st ring gap directly at the bottom), ran the engine again and same low compression symptoms upon differential test....I can only assume that the rings have once again moved to re align at the top of the piston again. I haven't yet disassembled to check this though.
With the head removed both valves exhibited zero leakage even with the simple petrol leak test. The barrel looks to be in good condition, no scoring (I didn't measure it, but cross hatching still evident). Old Ring end gaps measured at 0.023", new ring end gaps measured at 0.022".
Any ideas from the group? I'm at a loss right now....any help would be appreciated.
Cheers, J
I have a compression problem on my 912 uls. #4 cylinder was trending lower over a 6 month period, eventually to less than 60/80 psi, with the differential leak down test showing air rushing past the piston into the oil tank during the test, and no air escaping through either the inlet or exhaust manifolds. All other cylinders 75/80, engine approx 410 hours since new, approximately 6 years old fitted to a flight design CTSW.
I suspected piston rings, and upon disassembly found that the ring end gaps had near aligned at the top of the piston, which rang true with my initial thoughts. As I had new rings, I fitted them with the ring gaps in accordance with the heavy maintenance manual, only to find after the first engine run that the compression was still low, with the same differential compression test symptoms. Upon 2nd disassembly, to my surprise I discovered the ring gaps had realigned themselves towards the top of the piston. I then refitted and aligned the rings as per the heavy maintenance manual for a second time (oil scraper and 2nd ring 30 degrees offset at the top, 1st ring gap directly at the bottom), ran the engine again and same low compression symptoms upon differential test....I can only assume that the rings have once again moved to re align at the top of the piston again. I haven't yet disassembled to check this though.
With the head removed both valves exhibited zero leakage even with the simple petrol leak test. The barrel looks to be in good condition, no scoring (I didn't measure it, but cross hatching still evident). Old Ring end gaps measured at 0.023", new ring end gaps measured at 0.022".
Any ideas from the group? I'm at a loss right now....any help would be appreciated.
Cheers, J
