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Leak-Down Test vs Compression Test

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Leak-Down Test vs Compression Test

Postby 4Strokes » Fri May 27, 2016 1:50 pm

Topic: Leak-Down Test vs. Compression Test
Author: Andy Van H
Posted: 04/20/2005 06:11:43 AM

One reason I try to do my own work on my bikes is that I learn more about engines every time. I recently built a leak-down tester for engines to check the internals (rings, valves, cylinder bore) before I tear into an engine. A leak down test tells you MUCH more than a compression test can tell you. A compression test only tells you what the compression level is. It cannot indicate what causes the problem.

A leak-down test can tell you if the rings are worn, if the cylinder wear is extreme (like at the top of the stroke)or if there is a deep scratch in the cylinder, if a valve seat needs reworking, if a valve is bent (like the intake valve on my XL600R), and other cues as well. A compression test cannot tell you any of this. Let's say a buddy does a compression test for you and it shows low compression. Everyone first suspects the rings, spends the bucks on a ring job, and the bike is only marginally better. Could have been a valve was not fully seating, and it wasn't the rings at all.

To anyone out there suspecting your engine needs internal work, spend the money to have a leak-down test done. If your dealer cannot do the test (any good dealer should be equipped) then find another dealer cause they are only guessing what to do to your bike. A good mechanic will have and use a leak-down tester before doing any work because it tells him/her what to look for when going into the engine.

I recently did a top-end repair on an older Kawasaki 305 twin street bike with only 4200 miles on it. Compression test confirmed both cylinders had low compression. But, a leak-down test indicated one exhaust valve was leaking, and one intake valve wasn't sealing. So before I did the rebuild I could tell what was needed and where to spend the money.

Reply by Rotorcraft on 02/13/2006 6:42:17 PM
I am currently fitting an old sparkplug so that I can use my compression differential tester to check my engine. I am an aircraft mechanic and to test the top end of aricraft piston engines we use this tool. Like Andy says you can use this tool to be able to further diagnose your engine far better than a comperssion tester. The tester has a fixed orfice and two gages on it along with your common air compressor fittings and cost around 60-90 bucks. On aircraft engines you put the piston at TDC and with 80 PSI on one gage and the other gage should be somewhere close to 80 PSI. If my memory is right anything higher than 68 PSI is passing. Some cylinders may show 80 in one gage and 80 on the other, that means no leakage. I've seen that quite often.

  • Intake valve leaking = hissing in the intake through carb
  • Exhause valve leaking = hissing in exhaust muffler
  • leaking rings = hissing from crankcase breather fitting
There is no guess work in using this kind of tool. It will lead you to the exact problem of your engine. If anyone needs help locating a compression differential tester e-mail me, there are several on-line venders that carry them.

Reply by Andy Van H on 03/04/2006 12:05:12 PM
Thanks Rotorcraft for the confirmation about using a leak-down tester. Who better to know the best test tactics than someone working on aircraft engines.

A leak-down tester in capable hands can even tell you if your engine has a scoured/scrathed cylinder below top dead center. With the tester hooked up, say you're maintaining a good 90% at top dead center, but as you slowly rotate the engine say 45 degrees, suddenly it drops off to about 65%. That may indicate a serious scratch or score on the cylinder wall. When the piston rings get to that point the seal leak will allow air into the crankcase and you may hear an air leak coming from the crankcase breather.

I have some professionally trained mechanic friends that rely heavily on doing a leak-down test before tearing into an engine, and sometimes even before buying a used bike or engine. A leak-down test can even be done on an engine out of the frame.

Reply by MotoRacerX36 on 03/04/2006 1:10:33 PM
My F-15 will shoot your helicopter to hell and back...take that.

Reply by Galen on 03/04/2006 2:10:36 PM
Yeah thats one of the first things we do in lab before tearing down anything that is for a customer. one thing I have to say tho, a leak down tester can be dangerous, if you are putting high pressure into an engine just sitting on a bench if it is not properly secured it can and will start moving and if not under control will "fly" violently out of control

Reply by Rotorcraft on 03/05/2006 06:10:33 AM
Can you see boobs on the beach in your F-15? When we fly the beach some girls think by flashing us it will get them a ride. We may circle around a few times then it's back to work.

Reply by cracked junior on 03/05/2006 5:17:44 PM
i got a question for you aero-mechanic peoples. how come fuel on helicopters and air planes is measured in pounds, instead of gallons or litres. when you see a air plane or something on tv or a movie, they say it holds 50,000lbs of fuel

Reply by MotoRacerX36 on 03/05/2006 8:58:26 PM
You always want to know the total weight of the aircraft, so rather than converting gallons to pounds, we just measure fuel in weight.

Reply by cracked junior on 03/06/2006 5:36:13 PM
thats a very good reason

Reply by Rotorcraft on 03/22/2006 5:27:07 PM
Most turbine engine aircraft fuel indicating systems read in pounds. It's more important to know how much weight you have versus how many gallons. On helicopters your always watching the weight, you don't want to over gross it. They rate the consumption of fuel in gallons per hour. Jet-A weighs 6.7 pounds per gallon. Our small helicopters use around 26 gallons per hour. The Bell 230 wich has two engines uses 85 gallons per hour. It's also more accurate to measure it in the weight format.

Reply by MotoRacerX36 on 03/22/2006 9:39:44 PM
Is there an echo in here?

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