Compression Test on RV-12iS how to.

[jackking123]

Friend with RV-12iS with to LSRI course and they did not show him leak down compression test?
He indicated something about a dynamic compression test.
Is that correct. Is a compression test required? I am confused.
I want to help him do a compression test for his second condition inspection.
He did not do one on his first condition inspection.
The spark plug threads are 12mm I assume.

 

[whity1231$]

Yes, 12mm. Rotax now recommended using a higher pressure than traditional engines. I use 90psi and do it on a cold engine and it’s always 89/90. The engines have such chose tolerance’s. If something a little off, then I would do it on a warm the engine . You’re supposed to use a little of the heat paste on the plug thread’s when you torque them back in. Rotax has videos on their forum site if your friend is interested , that are very good.

 

[CTCole]

Rotax Line Maintenance Manual has a very abbreviated description. Here's a YouTube video on the procedure:

Note this is for the iS engine with 0 degree dogs in the clutch.


1. Mags off, remove top plugs; confirm tester orifice is 0.040 inch.
2. Cap off oil tank so you can hear gurgle
3. Thumb lightly plug hole on cylinder #1, feel for max compression as you rotate prop quickly in direction of rotation (clockwise as seen looking forward). Then use bamboo skewer to feel for TDC.
4. Screw adapter into plug hole.
5. Make sure tester gauge adjustment screw valve is closed; plug into air compressor line, then connect tester gauge to adapter
6. Screw in adjustment valve to bring pressure up to 20 psi – prop may move if piston is not TDC
7. Rotate prop by hand (if air pressure turned the prop clockwise, you turn it back counterclockwise (less than ½ turn) to neutral state, taking up the play as the dogs move back & forth– you can feel the compression as you rotate the prop. .
8. Bring pressure up to 30 psi; band for feeling the “center of the lump” will be narrower; wiggle prop back & forth to find center of lump. Less lash with 0̊ dogs than with 30̊ dogs. If you don’t get the TDC right on an engine with 30̊ dogs, it can snap one way or the other and rip the prop out of your hand and spin.
8. Bring pressure up to 87 psi. Stay clear of prop! If engine is TDC, prop will not turn. You’ll hear a gurgling sound from oil tank. Record pressure differential. Close screw valve.
9. Rotate prop clockwise to bring cylinder #4 to TDC, first feeling for air pressure with thumb and then checking with skewer.
10. Attach adapter and gauges. Bring pressure up to 20 psi, watching for prop to turn. Move prop to neutral position.
11. Check for TDC at 30 psi.
12. Bring pressure up to full 87 psi, record differential. Close screw valve.
13. Continue with cylinders 2 & 3 (firing order is 1-4-2-3; check in that order).

When at full pressure, rocking the prop back & forth very slightly can shift rings & improve seating.

 

[DHeal]

Chris does a nice job of describing the process. Here's an added refinement: When the cylinder is under pressure you can listen for any excessive air leaking past the valves by listening for air venting through the subject cylinder's air intake system (weak intake valve?) and/or exhaust system (weak exhaust valve?). To aid you in listening, a length of plastic hose can be used as a primitive stethoscope. Watch out for that pressure loaded prop!!

On non-Rotax engines you could also listen for air venting through the oil filler port (weak rings?) but on the Rotax that doesn't seem to reveal much due to all that gurgling. Watch out for that pressure loaded prop!!

Some useful info on the Savvy site:

 

[bbaggerman]

Rotax now recommended using a higher pressure than traditional engines. I use 90psi and do it on a cold engine and it’s always 89/90.

The Rotax Line Maintenance Manual Rev 2 Ed. 04 (June 01 2025), which is the latest I've been able to find, still shows using 80 - 87 psi.



Where is this recommendation from Rotax to use a higher pressure? What is the value of this higher pressure recommendation?

 

[docjjrmd]

The Rotax Line Maintenance Manual Rev 2 Ed. 04 (June 01 2025), which is the latest I've been able to find, still shows using 80 - 87 psi.

View attachment 111891

Where is this recommendation from Rotax to use a higher pressure? What is the value of this higher pressure recommendation?

The paragraph immediately below the table referenced above states 6 bar or 87 psi as the base and the fail threshold is 25% of 6 bar. I believe the factory recommended test pressure is 87 psi or 6 bar :

"The maximum permissible pressure drop is 25 %, e.g. from 6 to 4.5 bar (87 psi to 65 psi)(D)." Line Maintenance 12-20-00 pg. 9

I must advise great caution (IMHO don't do it) in moving prop with cylinder under pressure, even 20 or 30 psi, because the 30 degree dogs will certainly snap with a very ugly result if limb, body, or head is in strike range of the prop. Bleed off the pressure with relief valve on gauge set or undo the hose from cylinder. The 12 mm plug extension is well worth the cost and efficiency.

 

[whity1231$]

Rotax Line Maintenance Manual has a very abbreviated description. Here's a YouTube video on the procedure:

Note this is for the iS engine with 0 degree dogs in the clutch.


1. Mags off, remove top plugs; confirm tester orifice is 0.040 inch.
2. Cap off oil tank so you can hear gurgle
3. Thumb lightly plug hole on cylinder #1, feel for max compression as you rotate prop quickly in direction of rotation (clockwise as seen looking forward). Then use bamboo skewer to feel for TDC.
4. Screw adapter into plug hole.
5. Make sure tester gauge adjustment screw valve is closed; plug into air compressor line, then connect tester gauge to adapter
6. Screw in adjustment valve to bring pressure up to 20 psi – prop may move if piston is not TDC
7. Rotate prop by hand (if air pressure turned the prop clockwise, you turn it back counterclockwise (less than ½ turn) to neutral state, taking up the play as the dogs move back & forth– you can feel the compression as you rotate the prop. .
8. Bring pressure up to 30 psi; band for feeling the “center of the lump” will be narrower; wiggle prop back & forth to find center of lump. Less lash with 0̊ dogs than with 30̊ dogs. If you don’t get the TDC right on an engine with 30̊ dogs, it can snap one way or the other and rip the prop out of your hand and spin.
8. Bring pressure up to 87 psi. Stay clear of prop! If engine is TDC, prop will not turn. You’ll hear a gurgling sound from oil tank. Record pressure differential. Close screw valve.
9. Rotate prop clockwise to bring cylinder #4 to TDC, first feeling for air pressure with thumb and then checking with skewer.
10. Attach adapter and gauges. Bring pressure up to 20 psi, watching for prop to turn. Move prop to neutral position.
11. Check for TDC at 30 psi.
12. Bring pressure up to full 87 psi, record differential. Close screw valve.
13. Continue with cylinders 2 & 3 (firing order is 1-4-2-3; check in that order).

When at full pressure, rocking the prop back & forth very slightly can shift rings & improve seating.

I like that guy’s video also. Makes it simple and safer

 

[docjjrmd]

I like that guy’s video also. Makes it simple and safer

That guy is Morris Smith and teaches Rotax courses at Southern Mississippi Legacy Aviation. Lucedale, MS. His father Ronnie Smith, the fellow in the red polo looking over his shoulder, was one of the first Rotax dealers in US. Lot's of experience with the Rotax line.

 

[bbaggerman]

The paragraph immediately below the table referenced above states 6 bar or 87 psi as the base and the fail threshold is 25% of 6 bar. I believe the factory recommended test pressure is 87 psi or 6 bar :

"The maximum permissible pressure drop is 25 %, e.g. from 6 to 4.5 bar (87 psi to 65 psi)(D)." Line Maintenance 12-20-00 pg. 9

"e.g." is short for the Latin exempli gratia, meaning "for example". And so 6 bar (or 87 psi) is an example, not a requirement. 80 psi, as described in Step 5 of the instructions, is acceptable.

 

[docjjrmd]

"e.g." is short for the Latin exempli gratia, meaning "for example". And so 6 bar (or 87 psi) is an example, not a requirement. 80 psi, as described in Step 5 of the instructions, is acceptable.

Won't argue the semantics, never said 80 wasn't allowed, just trying to answer the threads question where the higher test pressure came from. Rotax instructors teach the 87 psi pressure for 9xx series engines currently.

 

[bbaggerman]

Won't argue the semantics, never said 80 wasn't allowed, just trying to answer the threads question where the higher test pressure came from. Rotax instructors teach the 87 psi pressure for 9xx series engines currently.

Dean Vogel at Lockwood was my Rotax instructor when I completed my RFSC Service and Maintenance Specialty certifications. Looking at my notes now, Dean teaches both 80 and 87 psi are OK.