Why a compression gauge doesn’t tell the whole story about rings by MrViper700

Many guys think that a compression check tells them if they need to replace rings in their sled. A compression test simply tells you the engines ability to build pressure rapidly but what it doesn’t tell you is how long you can hang onto that pressure. A compression test is good to see if the engine is in good operable shape as if you were buying a sled used, a good test would be if all 3 cylinders or 2 cylinders if a twin engine, 4 if a 4cyl engine would all be within a small percentage of each other. This means all the cylinders are not blown up or severely damaged with pinched rings from prior engine damage, etc.

with a compression test your reading will vary by certain amounts depending upon which brand of tester used and more importantly the hose length, and the thread adaptor used to screw into the spark plug hole. A long hose and short thread adaptor adds volume to the head and thus your reading will be lower than with a short hose and long thread adaptor. Don’t concentrate on the number as long as all the cylinders are within reasonably close reading wise to each other.

A leak down test is what actually determines your rings ability to seal and tells the truth about their condition as per miles on them, etc.
It’s all about how long you can keep the seal. When you have rings that are worn from miles put on the sled you will not "feel "a drastic change in performance. it happens over time slowly and you are used to it from the miles you keep putting on the sled but may notice the RPM’s are not what they used to be on top end, doesn’t seem to pull the same clutch weight it did when new, the sled doesn’t get as good of gas mileage, it doesn’t hit the same top speed numbers as before, etc. These are little hints as to the rings life span. The real life span is about 2500-3000 miles with good oil, they may "run just fine" but they won’t be perfect beyond those miles. Worn rings contribute to contamination of the crankcase fresh fuel charge. With worn rings it allows exhaust gasses to leak past the rings and contaminate the fresh fuel charge below in the crankcase, this makes the charge less powerful and richer as the exhaust gasses still contain fuel molecules yet to be completely burned. Tell tale signs of worn rings will be black on the piston sides, below the second ring. This is the blow by gasses seeping below as described, you will also notice a blackish/brown film on the bottom and sides of the cylinder sleeves/castings, and this is from worn rings also. I will post some pics to this thread to show examples of blow by.

I have posted a couple pics to show you what blow by looks like, you can see the bottom of the cylinder re-inhaling the exhaust blow by back up into the transfer ports contaminating the fresh mixture in the crank case. On the 2 pistons notice how the blow by has gotten far below the wrist pin, helping to pass blow by into the crankcase. These engines both showed a good 120-125psi of compression, yet they failed the leak down to a tune of about 10-14%, they each have around 5000 miles. The difference in the wear comes from the use of the sled and the oil used; you can see which one used yamalube!

A 2 stroke engine relies heavily upon piston and ring seal as it’s the camshaft, valves, and heart of the engine. The 2 stroke also beats up the piston twice as much as a 4 stroke does. The loss of stability from worn rings leads to piston walking back and forth in the bore, this leads to premature skirt wear and eventually major engine failure will result. The piston begins to rock back and forth in the bore and will break off the intake skirt, sending large shrapnel thru the engine and will almost always break the crankcase bottom, from becoming wedged between the crank and case.

Leak down Test

A leak down test is quite simple to do and all you need is a kit like the pic posted and an air compressor. I simply bring the cylinder I want to check up to TDC, insert a wooden dowel, old broom handle or whatever thru the primary clutch across the belly pan, this way when you apply the air pressure to the cylinder it won’t want to "spin" the engine down and lose all your pressure out the exhaust port.
Insert the thread adaptor, hook up the gauge unit and apply 100lbs of air pressure to the cylinder, shut off the regulator and wait. I wait 20-30mins and check what the pressure drop is. There are lots of different opinions as to what’s acceptable and what’s not, I personally don’t like to see more then 5% loss, that’s up to you what percentage you use.
What your checking is the rings ability to keep a seal, this is the real indicator on how good of condition they are because even a engine with worn rings can show "ok" compression numbers on a compression gauge simply from the fast movement of pulling it over, it can build pressure fast but if it can’t hang onto the pressure its leaking it down past the rings into the crankcase.

Question #1

Have you guys ever done a crankcase leak down test on a Yamaha? This is a test where you close off all crankcase openings (intake, exhaust, exhaust valve, spark plug, breather, oil, etc.) and then put 5-10 psi of pressure in the crankcase. You check later to see if it held or not. This test is very helpful in finding leaks in the cases that might not be visible.

Skidoo used to put this test right in the service manual for many engines including the rotary valve ones. The rotary shaft engines tended to be such good engines that the seals on the rotary shaft would fail before the engine wore out (coolant in the oil, and oil in the coolant). This was one way to pinpoint this problem.

Just wondering if this is an outdated test or still a valid one.

Answer #1

That would still be a very valid test of the crank seals, crankcase halves sealing. You’d need to make 3 solid reed blocks, 3 exhaust port block offs, and have the sparkplugs in and test it off the vacuum pulse port on rear of case. Used to do this on the cr500 dirt bike engines and then spray soapy water on them to help detect hidden leaks.

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