How-To: Leak Down Test, DIY Build
Why perform a leak down test?
A leak down test is performed after low and/or varying results (more than 10%) from a compression test, or to get an idea of the measurable amount of leakage a motorcycle engine may have. A leak down test may be your only option of assessing engine condition if your motorcycle has an automatic de-compression valve or mechanism. Most modern 4 stroke motocross bikes have this feature to ease starting.
What does a leak down test do?
A leak down test will pressurize the engine through the spark plug hole while at top dead center (TDC). When at TDC, all intake and exhaust valves will be closed, and the piston will be at the top of its stroke. The only way air pressure can escape is through the valves, piston rings, and head gasket.
So what is considered acceptable leakage?
- Leakage under 5% indicates engine in excellent condition
- Leakage up to 15% indicates engine in good condition
- Leakage above 15% indicates that engine components are worn
How-To: DIY Leak down tester
The easiest and cheapest leak down tester may already be in your toolbox. You can take the hose from your compression tester and simply remove the Schrader valve from one end. The disadvantage with this method is that you can’t assign a value or percentage of leak in the system since no gauge is present.
I’ve decided to make my own leak down tester since some units can run well over $100. Below is a picture and video of how to make one. Everything can be purchased locally and will take you around 30min to build.
The key to the leak down tester is to make a restriction between the pressure regulator and pressure gauge. I used 5min epoxy and plugged the brass nipple about ¼-1/2” deep. After the epoxy dries, use a .040” (#60 drill bit) drill bit and drill through the epoxy.
You can also use the pressure gauge from your compression tester on a quick connect to save a bit of money. You should have a compression tester anyway if you have moved onto leak down testing.
How-To perform a motorcycle leak down test?
The process to perform this test is really easy. The engine should be COLD during testing.
- Bring engine to top dead center (TDC). Refer to your service manual for this procedure.
- Put motorcycle in high gear and have a helper apply rear brake.
- Remove spark plug(s). Remove Schrader valve from compression tester hose and insert into plug hole.
- Hook up leak down tester to 120psi air supply. Regulate leak down tester pressure to 100psi.
- Connect compression hose to leak tester. Record pressure on leak down tester.
- Use a mechanics stethoscope to listen for leaks in air intake, exhaust, and crankcase. Look under radiator cap for air bubbles indicating a bad head gasket.
NOTE: Do not have any hand tools attached to engine as it may rotate under pressure.
Results
Leakage will be 100psi minus recorded pressure. For example, let’s say we get 90psi, 100psi -90psi equals 10psi, or 10% leakage.
im trying to test an old virago motor. how would i perform this test with the engine out of the bike and not hooked to anything? do i need to rig the starter?
thanks
cale
Cale, roll the motor over to TDC on the cylinder you want to test. You may need to use lower pressure if 100PSI causes the engine to rotate. Rotate the motor over with a wrench.
Matt, your leak down tester is a pretty cool tool. One question though, why did you drill the hole in the restrictor with a #60 or 0.040 drill and not some other size?
Charlie
Charlie,
The orifice size is a FAA spec I found here: https://en.wikipedia.org/wiki/Leak-down_tester
Matt, thanks for the info on the restrictor hole size. Got the tester made and it works great. However, one of the hardest things to do was locate 2 gauges that read the same. I had a nice high quality 2-1/2″ glycerin filled gauge with a stainless steel case. I tried all of the other gauges I had and none read a pressure even close to the glycerin filled one so I bit the bullet and purchased another glycerin filled one like the first one. Readings are now the same!
Charlie,
I now use only one gauge on my setup. Set up pressure prior to hooking up to engine and just measure difference afterwards. Also, you may find that higher pressures will rotate the motor, so you might have to use a lower initial pressure.
Thanks Matt, that sounds like a good idea, I’ll give it a try and let you know how it works out.
cool tool. I tried this but when hooked up to motorcycle the gauge reads the same 10 percent weather the regulator is 25 or 100. But it always goes from 0 back to the set number if i take my finger on and off. Do i some how have the orafice wrong?
10% is the leakage you have. No matter what starting pressure you start with, leakage will be 10%. That is the whole point of the formula, to you don’t have to use 100psi and have the motor roll over on you.
Yes, the results with finger on/off is normal.
Hi Matthew, enjoy your videos. I left a message over on one of your YouTube videos with the same question: My .040 drill bit is inexplicably missing from my set. Can I use a .041 bit or will that cause a problem?
I think that will work just fine
Awesome…thanks! I figured it would. Can’t imagine that 1 thousandth would make a difference? Bit runout is probably more than that? I used a drill bit the same inside diameter as the fitting from each end of the fitting to get the (roughly) 60 degree approach. And used my calipers to make sure the epoxy was .250 from end to end not including the approaches. It looks great and no leaks in the assembly. The problem now is that these Harbor Freight air gauges SUCK! Big surprise right? Surprisingly the small one on the valve is pretty accurate, but the bigger one is off by close to 5psi. I need 2 (or at least 1 if I do what you did and use the compressor gauge as the first gauge) good gauges…any recommendations?
I went to home depot and bought a 0-30psi gauge in the plumbing section. You can use low pressure to test leak down, just make sure you do the math right.
Read more here: https://howtomotorcyclerepair.com/motorcycleleakdowntest/
Also, I notice that the specs say “0.250 in (6.4 mm) long, 60-degree approach angle” Is that .250” depth critical to get exactly right? And what do they mean by 60-degree approach angle”? Is the hole to be drilled at an angle?
Ok, I figured out the approach angle LOL Now the question is, is the .250” length INCLUDING the approach angle on both ends or EXCLUDING the approach angle?
Hi Matthew, I’m having major problems with this leak down tester. I went to great pains to make sure the orifice is correct, but when introducing a small leak to the system, the 2nd gauge of course moves as expected, but the first gauge is moving too! ?? Granted it’s one of those Harbor Freight gauge/valve combos, but still? It drops about 10-21 psi and doesn’t drop lower even if I make the leak so bad it drops the 2nd gauge to zero. Strangely enough if I take the 1st gauge out and just use my compressor pressure gauge (similar to what I believe you do), the compressor gauge does NOT drop when a leak is introduced. Crappy Harbor Freight stuff or did I do something wrong? I really need 2 gauges because my huge 80 gallon compressor is in the basement and it’s a pain to run back and forth. Any ideas?