A Question about Cylinder Health…

[Piper J3]

I’m beginning my annual condition inspection and started with differential pressure (leak-down) test for my early SN 912ULS with 1030 TT. Plane was last flown in October before weather closed in. So, engine has sat for almost two months. Today I did compression test and all four cylinders are 79/80 with engine stone cold (Rotax says 5.5 – 6 bar ≈ 80–87 psi). I use Rotax 0.040” orifice tester at 80 psig to have some comparison to Lyc and Continental engines. No leakage at intake or exhaust valves with minimal blow-by on piston rings. All four cylinders sound similar for ring leakage at the oil tank vent. So, everything looks good. Oil consumption is 1 liter / 75 hrs. running Costco (top tier) 93E10 Mogas and Mobil 1 Racing 4T 10W-40 synthetic oil. Engine is run at 5500 rpm in cruise.

My question is… I bought this airplane with 48TT and have operated it for ten years. I’m trying to remember how the engine compression felt (burping) when new compared to now with 1000+ hrs. I think the compression now is slightly less than new and yet the diff press test looks excellent. Does anyone else with mid-time engine have similar observation?

 

[BoydBirchler]

Ran a 912 ULS on my wife's Rans S7 for 1200 Hours, using 87PSI in the compressions were never lower than 80/87. If you use 80PSI in you would likely see 79 or 80 over 80 with the 40 thousandths orifice. Last I heard it is still running strong.

The Rotax being water cooled the tolerances are very tight compared to a Lycoming or Continental air cooled engine..

 

[Bob Y]

I’ll just add that Lockwood told me the Rotax is so tightly toleranced that it will pass a compression test with no rings.

 

[Joe Mckamey]

Can you guys recommend the best differential pressure tester for our 912iS?

 

[Amadeus]

Can you guys recommend the best differential pressure tester for our 912iS?

Lockwood has the best available for purchase.

 

[docjjrmd]

Can you guys recommend the best differential pressure tester for our 912iS?

I like the Aircraft Tool Supply . I highly suggest getting the rigid 12mm extension spark plug port extension. Plug Extension . If budget is a real concern Harbor Freight makes a passable tool thats accurate enough but doesn't have the pressure relief valves. See HF Leak down gauge. Note HF calls it a leak down gauge.

 

[docjjrmd]

Note if doing differential pressure checking, Line Maintenance Manual section 12-00-00 pages 8-9 reference base to be 6 bar or 87 psi. This is different than the typical Lyco/Conti 80 psi starting pressure which many are used to seeing. As mentioned before, the cylinder tolerances are much tighter in Rotax and the cylinder may hold pressure at 80 psi even with ring damage. LMM calls the no-pass threshold to be 25% of 87 or 4.5 bar out of 6 bar (65 psi). Many A&Ps without Rotax experience would be nervous seeing 67or 68 over 80. Usually due to tolerances, if seeing significant leak down take a close look at valves before cylinders in the 912x particular if engine is equipped with the old style non-sodium valves. FWIW.

 

[j4cbo]

Note if doing differential pressure checking, Line Maintenance Manual section 12-00-00 pages 8-9 reference base to be 6 bar or 87 psi. This is different than the typical Lyco/Conti 80 psi starting pressure which many are used to seeing.

Here's what I see in section 12-20-00 page 9 of the latest revision of the maintenance manual:

5. Now put constant pressure, between 5.5-6 bar (80–87 psi) on the line and take readings at pressure gauge (C).

So 80 psi or 87 psi are both acceptable.