Leak Down Testing

Leak down testing is something everyone should do on a regular basis. This is the most important diagnostic test you can perform on the health of your engine. This technique was developed by NASCAR engine builders back in the 60s and was a closely guarded secret until someone realized they could make a lot of money selling testers. NASCAR engine builders refer to this sometimes as a blow down test, and or if “Seeing if you have a motor”.

This is how it works. You place a piston at top dead center on the compression stroke, like you would when adjusting valves. This doesn’t have to be real precise like when timing cams. I usually place a long phillips screwdriver in the spark plug hole and turn the crank until the screwdriver is all the way at the top. You can also use the timing mark on the stock advancer. With the spark plug removed you screw the tester adapter into the hole. This is a short piece of tube with 10mm threads on one end and an air compressor fitting on the other. The leak down tester applies 100 PSI of pressure into the cylinder. If the pressure drops to 95, then you have a 5 percent leakdown. If the pressure drops to 90, you have 10 percent leak down.

What is good?? I like to see all of my new engines at 2-3 percent. Most good race motors and street bikes should leak no less than 4-5percent. Most engine builders rebuild at 6-7 percent. Some people think up to 10 percent is OK. That’s ludicrous. If you have a engine leaking over 7 percent, YOU DON’T HAVE NO MOTOR, at least not one that’s going to win a race! Look at it this way. If only 1/3 of the heat energy is going to push down on the piston, and your loosing 10 percent of that, how much is left?

A example between two identical ZX7s, one leaking at 2-3, another leaking at 10, the difference in Horsepower will be about 14.

Leaks can occur around valves, head gaskets, and piston rings (most likely). YOU SHOULD NEVER HAVE ANY LEAKAGE PAST YOUR VALVES, 0 percent. YOU SHOULD NEVER HAVE ANY LEAKAGE PAST YOUR HEAD GASKET, 0 percent.

All leakage should occur past the rings. A perfect mechanical seal is never possible. Even total seal gapless rings leak at 1-1.5 percent. If you have leakage past your intake valves, you will hear it coming out of the intake ports or carbs or airbox. Leaks past exhaust valves will be heard coming out of the exhaust canister. Leaks past the rings will be heard coming out of the side of the motor where the advancer cover is removed or out of crankcase vent. Leaks past head gasket will show as bubbles in the coolant with radiator cap removed, or in severe cases a plume of coolant will shoot up.

Places to purchase the testers www.tavia.com, www.powerhouseproducts.com. The tester powerhouse sells is identical to the one that Tavia builds. These are very high quality testers for about 75 dollars. You will also need to purchase the 10mm hose adaptor. Now you need a source of compressed air. Any type of air compressor works fine. I also use a modified scuba tank. I cut a regulator off an old assembly and put an air compressor fitting on and now have a 3000psi 80 cubic foot source of air regulated down to about 125 psi to use for clean dry air for leak testing and filling tires at the track and for impact wrench at track.

Any leakage past valves or head gasket means immediate engine repairs. Leakage past 7 percent and you can stop bothering to try jetting or any other monkey motion, your wasting your time. It just doesn’t respond.

The best way I have found to get good ring seal is to flex hone a cylinder with a tool commonly known as a dingleberry or bottle brush hone.

After boring a cylinder you must use a RIGID hone for final sizing. Everyone does this myself included. But lets look at how that works. A boring tool is like a plow digging through dirt. You can imagine how the furrows dug up looks in the metal of your cylinder wall. So this must be finished down with a rigid hone to final size. In other words you bore about .003 to .004 to small then remove the rest with the rigid stone. Some companys even advertise they use a SUNNEN hone. What does this mean to you. These Sunnens do a great job no doubt, but the surface left on your cylinder wall will look like the great Teton mountains in Colorado. You have the valleys for oil retention, but very sharp peaks your rings must abrade down (ie ring seating). Why put your rings though this hell? NO rigid hone can hone to size without folding, tearing or smearing the final surface. Obviously something you can only see with a microscope. Now the flex hone tool, will machine those peaks off the top. Have you ever driven up a steep mountain grade, then get to the top of what’s known as a plateau? Perfectly flat for miles. You don’t even know your on top of a Mtn. until you start down the other side. THAT’S THE SURFACE YOU WANT ON YOUR CYLINDER WALLS! I can’t stress this enough. Think about it. You have the valley for oil retention and the smooth plateaus for your piston rings to ride on. This is perfect ring seal, which can be had instantly, that’s right instantly. The flex hone tools unique construction gives a plateaued surface free of torn folded and smeared metal. So I bore almost to size, then size out with my sunnen, then flex hone my bores. Very few people I know of flex hone after using a 20,000 machine. Why run a 40 dollar tool through a hole just perfectly machined by a 20,000machine? Most people use the flex hone for deglazing and reringing and have no clue as to what a great tool this is. I flex hone all my bores, after rigid honing to size and get a 2-3 percent leakdown everytime. And get this, if the flex hone perfects the wall texture, YOUR RINGS DON’T HAVE TO DO IT, AND THE RING SEAL LASTS ALMOST FOREVER!! This is a great secret. The flex hone is patented by a company in California, called BRUSH RESEARCH MANUFACTURING. About 4 different sizes covers all motorcycles. I use a 76mm 320 grit which works the range of 70-76mm. Perfect for our sevens and nines!

So again imagine a mountainous surface on your cylinder walls, one with plateaus, and one with peaks like the Tetons in Colorado. I bet Josh knows what those Teton Mtns look like. Which surface do you want on your cylinder walls??? No RIGID hone (MACHINE) can give you this wall texture free of torn, cut and folded metal. YES you must use the rigid hone, no doubt, then FLEX HONE IT. That’s the secret!

I check my leakdown at each valve adjustment. By the way I use the Snap-On tester model #MT 324. A much more expensive tester, but then so is all of their tools.

The key to horse power lies in ring seal, valve seal. If you have that then cams pipes cam timing and jetting changes become so much sharper and effective.

For more information on the Flex Hone tool, go to www.brushresearch.com. The whole idea behind this is, after Boring the cylinders,(THE PLOW),this surface has to be final sized by the Rigid hone no doubt. This rigid hone puts tremendous pressures against this rough surface, further folding tearing and smearing the metal under this high pressure. The Flex Hone uses very light springy action, which cuts the surface to a much smoother plateaued surface by its very design….

One other thing, if you go to their website click on to Flex Hone technical booklets and there you can order these books for FREE! and see for yourself. I also flexhone my valve guides, a small amount of oil getting past valve guide seals will always be retained in the cross hatch providing constant lubrication to the valve sliding up and down in its guide. I even flex hone my main crank bearings and con rod bearings. Imagine having a crosshatch for oil retention on these surfaces also. I also flex hone the cam journals for the same reasons. This sh!t works. I had a CBR900RR come in to my shop for a valve adjustment. The owner had read about tight cam bearing clearances in UK magazines. We checked his and they were to tight, according to the published Honda specs. We flex honed these out to proper specs and the bike picked up 5 HP.

Ken Waters

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