Compression

[gyrene]

I am in the very final stages of buying a very well equipped a 1986 M20K 252 with about 1650 hours. The compressions are 65, 68, 64, 56, 58, and 60. The owners says it runs perfectly. I realize that that TBO is very close but exactly how close? What are these numbers telling me? Closing tomorrow so any input would be appreciated!!

[MooneyBob]

Isn't everything below 60 illegal. I don't know about the Continental engines but I am pretty sure that's the case for Lycoming.

[orionflt]

you are dealing with a continental engine so those numbers could be ok if the test was performed with a master orifice and IAW continentals procedures. If that was just a standard compression test then you 2 cylinders that are out of limits and the rest are low meaning you need a top end now. 

if you haven't had a reputable A&P do a pre purchase inspection then I recommend you stop the proceedings and have that done to protect your interest. I would assume that is the case because a good A&P would have talked to you about what was found and it's implications on future maintenance cost. 

 

Brian

[gyrene]

The inspection was done by Don Maxwell Aviation Services, Longview<Texas about 60 hours ago. No mention in the report of anything other than the compressions and the other maintenance work performed. No mention of the use of a master orifice or the procedures followed. Maybe I should call Mr. Maxwell...

[jetdriven]

The compression reading can be as low as ~45 with the master orifice. Another thing to consider, how much has it flown recently and how long it's been sitting since the last flight.

Busch explains it better than I do. http://www.avweb.com/news/savvyaviator/188758-1.html

[orionflt]

Very good idea, If don has been maintaining the plane he will be a good source of info. but as I said if you did not do a pre purchase inspection I Highly recommend that you put the sale off for a couple of days and send it to Don or another reputable Mooney service center for a pre purchase/annual inspection before closing the deal.  

 

Brian

[Marauder]

If the plane has a data reporting engine analyzer, I would ask for some recent flight data.

[Andy95W]

Per Continental's instructions, your cylinders are okay. At lower compressions, the next step is to borescope the cylinders and specifically observe the exhaust valve. If that inspection passes, the cylinder is still okay per Continental's service instructions.

[DonMuncy]

My information is that Continental says that if the compressions are low like this, you should go fly the plane for a half hour or so and recheck. If the low ones are up within range, (even if others are now low), it is Ok. If still low, borescope.

My K model has at various times had compressions this low, and is still dong fine.

If everything else on the plane is good, I would not walk away from it. If the compressions come back within range, I would discount the price a little and go for it.

[WilliamR]

Keep in mind, as others have stated, this is a continental.  Tolerances for low compression are more than a Lycoming.  Table 1 of the Service Bulletin SB03-3 has a nice decision tree for testing the cylinders and next action.  

 

Bore scope, of course is the sure fire way of knowing the health of a cylinder. 

 

W

[mooneyflyfast]

Even with a Lycoming below 60 does not necessarily require pulling the cylinder. I believe Lycoming says you should consider removing the cylinder if compression is less than 60. The exception is if leakage is past an exhaust valve. If that is the case it has to be repaired. A cylinder with say 55 over 80 will produce just as much power as one with 60 or 70 and compression may be back up to snuff the next time you check it.

[cujet]

http://www.aviationpros.com/article/10388599/cylinder-differential-compression-testing-subject-to-interpretation

 

Note: There are 2 common aircraft differential pressure orifice sizes. (this is not a master orifice, which is a calibration and testing tool)

 

1) 0.040 inch orifice, 0.250 inch length, 60 degree approach angle for cylinders with bores smaller than 5 inches

2) 0.060 inch orifice, 0.250 inch length, 60 degree approach angle for cylinders with bores larger than 5 inches. 

 

This means the IO-360 6 cylinder Continental should probably use the smaller orifice when testing. As a larger one will give too high a reading.