Hello everyone.

I have just commenced the annual for my 2015 Bristell NG5 purchased at the beginning of the year.  It's had 400 hours in service.

Prior to purchasing it, I had it inspected and was advised that cylinders 1 & 4 had a differential pressure of 80/65 where cylinders 2 & 3 had 80/78.

I'd now like to fix the two with the low pressure.  I had the rings changed as there was gurgling in the oil can during the pressure checks and this was the immediate advice from Rotax.   However this did not fix the problem and the pressures are still 80/65.

Before launching down the head / valve overhaul path I would appreciate any advice as to what the problem is likely to be.

There is no noticeable air leaks from the exhaust or carbys and the oil can still gurgles.   I've seen a post where someone suggested re-homing the valves with a drill but I'm not so sure about that.

When the heads were off there was noticeable marks both on the head and on the piston cap - photos attached.  The cylinders still had the honing marks on them and looked otherwise like new.

I'd like to correct this imbalance even though the engine is performing without issues whatsoever.



9503_1_Valves2.jpg (You do not have access to download this file.)
9503_1_Cylinder3.jpg (You do not have access to download this file.)
  • Re: Help Please - 912ULS Low differential pressure on Cyls 1 & 4

    by » 2 years ago

    If no air leakage at exhaust or carb during diff pressure testing, and still gurgling at oil tank, then problem is still piston blow-by.  Most likely rings not seated during overhaul.  How much oil is being consumed?  

  • Re: Help Please - 912ULS Low differential pressure on Cyls 1 & 4

    by » 2 years ago

    Thanks for your answer.  The gurgling at the oil can occurs when testing all cylinders.  A recent cross country flight (20 hours) consumed about 500ml of oil which I don't think is excessive but would happily be corrected.

    I understand the air has to go somewhere - it's certainly not at the exhaust and it was difficult to check at the carbs or airbox so I may need to check there again.  

  • Re: Help Please - 912ULS Low differential pressure on Cyls 1 & 4

    by » 2 years ago

    Hi Brian,

    I will be interested to hear what the experts advise - in the mean time:

    If your engine is running well (expected power & smooth running) / as expected (temps pressures all within range) and oil consumption acceptable, I suggest you leave the engine alone for at least another 400 hrs, monitor oil consumption and take action if it rises beyond recommended consumption..

    It may help to do some good long away legs, at a consistent 5500 rpm (high speed cruise). Climb outs not below 5200 rpm (better 5800 - max 5 mins).

    You should see engine oil temps between 100-120C on climb out (if not, either tape off a section of the oil cooler or steepen climb/load engine)

    In established cruise its okay if they hover around 90C - I wouldn't like to see anything much over 100C nor would I like to see the oil go much below 85C.

    Avoid extended ground running when/where ever possible.

    The idea is to "bed" wearing components like piston rings:wall, valve stems, seats, etc. The hope is improved cylinder pressures, reduced oil consumption.

    Too many pilots "baby" Rotax 9's - they are designed to work hard and respond accordingly.

  • Re: Help Please - 912ULS Low differential pressure on Cyls 1 & 4

    by » 2 years ago

    Unfortunately I have some experience of some these problems myself.


    1. I had rough running that got worse and worse until the plane was unflyable.


    2. The engine was sent for strip down and inspection and a faulty inlet valve was found. Prior to strip down, leak down results were good on all cylinders.


    3. The inlet valve was replaced along with the piston rings, bearings, etc. The old oil control rings were seized.


    4. Before the rebuild, the engine had started to burn a very small amount of oil.


    5. After the rebuild the engine was burning about 60ml of oil per hour and it never settled.


    6. In the end I replaced the engine.


    7. All leak down tests that I have done result in air coming out of the oil tank but the results are always good.


    8. None of my cylinders or heads have ever looked like the ones in your photos.

    It is relatively easy to remove the valves, so I would be tempted to do that from the problem cylinders and inspect them and the valve seats carefully.

    Let us know what you find.


  • Re: Help Please - 912ULS Low differential pressure on Cyls 1 & 4

    by » 2 years ago

    That cylinder looks like toast,I’d replace it. A valve job seems in order too.

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