VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.


Go Back   VAF Forums > RV Firewall Forward Section > Traditional Aircraft Engines
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 01-04-2013, 04:30 PM
swisseagle's Avatar
swisseagle swisseagle is offline
 
Join Date: Feb 2005
Location: 20km outside of Zurich, Switzerland
Posts: 518
Question How to measure cylinder compression, which tool do we need?

Hello

I'm still building, but my buddy's around me are flying since a while. We want to buy the service tools that we need to keep our RV's in good condition.

One is cylinder compression ... ok, but there are many supplyers and different styles like simple ones, differential pressure tester with or without orifice ...

1. ACS, simple one.http://www.aircraftspruce.com/pages/...omptestkit.php

2. ATS, differential pressure tester http://www.aircraft-tool.com/shop/de...?PRODUCT_ID=2E

3. ATS, differential pressure tester with master orifice http://www.aircraft-tool.com/shop/de...PRODUCT_ID=2EM

Which one should we buy and how does they work?

Do I run the engine by hand to get the compression up on the manometer, or do I use the starter for this?
Do I need compressed air to get the tester running?

We run Lycoming 320 and 360.

Any help to make us smarter are very welcome
__________________
Dominik

RV-7A, TMX-IO-320, FM-150, WWA 200RV
Flying since 28. April 2016

Last edited by swisseagle : 01-04-2013 at 04:34 PM.
Reply With Quote
  #2  
Old 01-04-2013, 05:10 PM
Mark Albery's Avatar
Mark Albery Mark Albery is offline
 
Join Date: Jul 2007
Location: Warwickshire UK
Posts: 721
Default

You want the differential pressure tester with the .040" orifice. Although AC43 says use the .050 orifice on piston diameters more than 5", the O360 is only slightly larger and everybody I know sticks with the .040 (you'll get a slightly 'better' reading with the .050").

There are plenty of instructions on-line, but basically you warm up the engine, remove 1 plug per cylinder, position each cylinder in turn at TDC on the compression stroke, pressurise the upper chamber of the tester to 80psi then read the resulting pressure in the downstream chamber.

If the leakage area is less than the equivalent of the orifice then it will be choked at the point of leakage and the two gauges will both read 80. As the leakage area increases due to ring wear, valve leakage etc, then the downstream chamber will read a lower pressure. 70-80psi is fairly normal, below 60 may indicate a problem, but there are many caveats, so don't pull a cylinder just on the basis of one low reading.
Reply With Quote
  #3  
Old 01-04-2013, 05:15 PM
Mike S's Avatar
Mike S Mike S is offline
Senior Curmudgeon
 
Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 16,167
Default

Most folks use the second one you listed, or one similar.


To use this is a two person job-----one holds the prop so the piston is at TDC, and the other works the gauge. I suppose it could be done solo, but there is a bit of a hazard doing so, the prop can whack you pretty good if it slips loose.

Instructions should be included with the unit.

This unit does not really check the compression like most folks think of, instead it checks how well the cyl assembly holds pressure. You feed it a measured pressure of 80psi, and it gives you the pressure that the cyl maintains.

That is why you see folks quote a figure of 77/80, or 75/80 etc for an engine with good compression.

You can also find videos on the internet as to how to check this. U tube.
__________________
Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
Reply With Quote
  #4  
Old 01-04-2013, 05:58 PM
John Clark's Avatar
John Clark John Clark is offline
 
Join Date: May 2005
Location: Santa Barbara, CA
Posts: 1,324
Default

Quote:
Originally Posted by Mike S View Post
To use this is a two person job-----one holds the prop so the piston is at TDC, and the other works the gauge. I suppose it could be done solo, but there is a bit of a hazard doing so, the prop can whack you pretty good if it slips loose.
Just to add to Mike's comment, it can do more than "whack you pretty good". Many years ago there was a young A&P killed at SBA while doing a compression test solo. The prop got loose and fractured his skull. The lesson here is to, as always, treat the prop as if it is "hot" even with the plugs out and never stand in the prop arc.

John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

Last edited by John Clark : 01-05-2013 at 01:12 AM. Reason: spelling
Reply With Quote
  #5  
Old 01-04-2013, 10:44 PM
az_gila's Avatar
az_gila az_gila is offline
 
Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
Exclamation

Quote:
Originally Posted by Mark Albery View Post
You want the differential pressure tester with the .040" orifice. Although AC43 says use the .050 orifice on piston diameters more than 5", the O360 is only slightly larger and everybody I know sticks with the .040 (you'll get a slightly 'better' reading with the .050").

......
They stick with the 0.040 orifice because that is what Lycoming calls for, and the manufacturers instructions trump AC-43....

The error is a bit more than "slightly better".

70/80 with the correct 0.040 orifice will read 73.5/80 with the larger orifice.

http://home.comcast.net/~r123rs/Docu...ompression.pdf
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 01-04-2013 at 10:49 PM.
Reply With Quote
  #6  
Old 01-05-2013, 09:11 AM
Greg Arehart's Avatar
Greg Arehart Greg Arehart is offline
 
Join Date: Jul 2007
Location: Delta, CO/Atlin, BC
Posts: 2,439
Default

And if you get an anomalous low reading on one cylinder, rock the prop a bit and try again. May be that the valve or rings didn't seat quite perfectly (kind of like gear lash) when you stopped moving the prop. If I get a low reading, I'll move the prop slightly back and forth and often then get a good one.

Greg
__________________
Greg Arehart
RV-9B (Big tires) Tipup @AJZ or CYSQ
N 7965A
Reply With Quote
  #7  
Old 01-06-2013, 03:17 PM
swisseagle's Avatar
swisseagle swisseagle is offline
 
Join Date: Feb 2005
Location: 20km outside of Zurich, Switzerland
Posts: 518
Default Thank you!

Whow, all these response!

Thats all what I need to know, I will go with the No. 2.

Thanks alot and best regards,
__________________
Dominik

RV-7A, TMX-IO-320, FM-150, WWA 200RV
Flying since 28. April 2016
Reply With Quote
  #8  
Old 01-06-2013, 05:21 PM
DanBaier's Avatar
DanBaier DanBaier is offline
 
Join Date: Mar 2009
Location: Rochester NY
Posts: 703
Default

As to moving the prop around to try and "bump up" the reading ... it doesn't take super-human strength to hold the prop when it's under pressure right at TDC. But, if you get it very far off, I don't think I know anyone (or two, or four for that matter) who could keep the prop from getting away.

I'd suggest bringing up the pressure slowly to be sure you have it under control and being ready to quickly dump the pressure on the cylinder if necessary.

Dan
__________________
RV7A (N7101) - Flying 10/2008
CFI- SE/ME/Inst
A&P
KC2ZEL
Reply With Quote
  #9  
Old 01-06-2013, 07:50 PM
aerhed aerhed is offline
 
Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
Default

Dan's advice is good. I still sneak up on the pressure on some engines. Start with like 30 psi. Go to TDC and hold. Then turn on the air. Then you can carefully move the prop around to see just how much force there is a few degrees off center. It's tremendous. Go back to TDC, then dial up to 80 psi.
__________________
Actual repeat offender.
Reply With Quote
  #10  
Old 01-07-2013, 06:02 AM
PCHunt PCHunt is offline
 
Join Date: Feb 2007
Location: San Diego, CA
Posts: 1,819
Default Suggestion

For SwissEagle: If you have someone nearby who has done a compression check before, it would be my suggestion to get local help for the first check. After that, you will likely be comfortable doing it yourself.

Nothing better than someone to show you all the little in's and out's of the process.
__________________
Pete Hunt, [San Diego] VAF #1069
RV-6, RV-6A, T-6G
ATP, CFII, A&P

2021 Donation+, Gladly Sent
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:31 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.