Cylinder compressions

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LToxvard
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Joined: Wed Jun 08, 2016 11:22 pm

Cylinder compressions

Post by LToxvard »

Just sent the 170 in for the ADS-B upgrade and was able to get the annual done at the same time. All compression were in the high seventies, but jug # 2 at a 71. At every annual they vary a little #2 two has been as high as 78 and as low as 72, along with the rest of the cylinders #1 was a 72 last year, but this year is a 78. The recommendation is fly it a little and retest, any comments?
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n2582d
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Re: Cylinder compressions

Post by n2582d »

Gary
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ghostflyer
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Joined: Mon Apr 21, 2008 3:06 am

Re: Cylinder compressions

Post by ghostflyer »

Well before cylinder compression checks , I fly it like I stole it and lean it also . Making sure all temperatures have stabilized during flight and on the ground before shutting it down . I wouldn’t be slashing my wrists if your compressions are down to even 65 psi delta. When doing your compression checks and when on the top dead center of the compression stroke listen for air flow escaping. If it comes out of the exhaust it’s the exhaust valve having a sealing issue and if it’s coming out of the intake [carby] it’s a intake valve having a issue . Also listen to the where the oil filler pipe is and if air can be heard escaping ,it can a ring issue . So if coming out of the oil filler pipe , remove the delta diff gauge and pour a few drops of oil into the cylinder and turn the motor slowly by hand and then refit the delta diff gauge and check if compressions have changed . Check if air is still,coming out of the oil filler pipe . If not it could be be rings issue. If air is coming out any of the valves remove the rocker cover and give each valve stem a hit with a block of wood . To me compressions should be noted from 100hrly to 100hrly and notice a trend . I have had compressions on “MY” engine come in at 50 psi . I then give it a floggen ground run and next even when red hot bounce up to 78 psi delta.
SAFETY note. Please ensure mags are off and grounded . Fuel is off . I always leave the top spark plugs removed during compression checks and have some one holding the prop correctly . That prop on a runway can make someone’s day very bad. So be careful. Keep on lookers away , you might know what’s about to happen but lookers are a pain and often haven’t a clue what’s happening or could happen . Well I am off to have lunch after all that .

This was written for entertainment value only , also check/comply with the appropriate maintenance manual and your friendly A/P and I/A and naturally with the FAA directions / legislation. What a bloody mouthful with lunch .
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gfeher
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Re: Cylinder compressions

Post by gfeher »

Everything Gary and Ghostflyer said. I'm not sure what you are worried about with those numbers. Compression checks are notoriously imperfect and the number for a bad cylinder is much lower now (and varies). Plus you don't go by a low reading in a single test. See SB 03-3 that Gary identified (now incorporated in Continental's universal maintenance manual M-0). Mike Busch has a pretty good article on compression tests that discusses SB 03-3. https://www.avweb.com/news/savvyaviator/188758-1.html
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
LToxvard
Posts: 20
Joined: Wed Jun 08, 2016 11:22 pm

Re: Cylinder compressions

Post by LToxvard »

Good stuff, thanks!
hilltop170
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Re: Cylinder compressions

Post by hilltop170 »

One thing I never see written about compression testing but is in the FAA recommended AC 43-13 procedures I have on file is to apply a steady 20psi pressure on the cylinder as the piston is brought up to TDC on the compression stroke. This ensures the rings are seated in the bottom of the groove and providing maximum sealing. If there are two competent people doing the test, the second person can gently increase the pressure to 80psi as the prop is rotated to TDC but this greatly increases the risk of injury and the rotational force if the prop is not fully restrained and allowed to spin on its own.

Then park the prop at TDC and increase pressure to 80psi to check compression staying well away from the prop arc. The prop should stay in position (unless deliberately moved) if it is truly at TDC.

THIS IS A VERY DANGEROUS OPERATION AND SHOULD ONLY BE ATTEMPTED BY SKILLED AND KNOWLEDGEABLE INDIVIDUALS!!!!!!!! And nobody else close to the prop! You have to be extremely careful during the entire operation as even though 20psi is very manageable for one person to handle bringing the piston up to TDC, if rotated past TDC, the prop wants to continue to rotate, or if not quite at TDC will want to kick back, both ways with enough force to cause serious injury.

Done correctly, you should get the best compression reading possible. I have seen mechanics jerking the prop back and forth trying to find the best "sweet spot" for the reading (usually not at TDC), but that is not the way the engine operates. The piston comes smoothly up to TDC then retreats back down on the power stroke, no jerking back and forth.

If the compression is low, all the previously stated reasons and remedies apply.

The other important thing is to use the correct compression tester for the engine being tested, as in, correct restrictor orifice. For our engines, a .040"dia orifice is correct.
Compression test procedure.1
Compression test procedure.1
Compression test procedure.2
Compression test procedure.2
Richard Pulley
2014-2016 TIC170A Past President
1951 170A, N1715D, s/n 20158, O-300D
Owned from 1973 to 1984.
Bought again in 2006 after 22 years.
It's not for sale!
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