Leak down test for lightweight pistons in Cmdo motor

I'm attempting to get an accurate leakdown test on a Commando motor with total seal rings and lightweight pistons after 20,000 miles. I bought this "differential cylinder pressure tester" from an aircraft parts supplier as per Ken Canaga's recommendation. Its an ETC E-2a with a .040" orifice (smallest orifice I could get). Warm up the motor, plug in the air and HANG ON TO THE WRENCH. I was expecting to calculate the leakage percentage by comparing the two gauges. But I'm seeing no (hardly any) measurable leakage.

I tried cranking up the PSI to around 100 at TDC and the gauges were still balanced. Maneuvering the crank at mid stroke was too risky with a breaker bar and the rear break.

[video]http://youtu.be/3S50Lvo9c-0[/video]

Your input pressure is much too low. It should be 100 PSI for an aircraft gauge set. At the pressure you are using, the orfice between the gauges is allowing them to equalize.. Put 100 PSI on the input side and you should then see the other gauge indicating somewhat less. Lock the rear brake and put the bike in 4th gear to address the crank wanting to rotate.

Note that other than for aircraft engines and aircraft-certified gauge sets, there is no standard for use/input pressure/readings of leak down gauges. IOW, a certain percentage for one set of gauges does not correlate to that of a different set of gauges.

We used to do a lot of leak down tests. They are great for troubleshooting and, coupled with a compression test, can tell you a lot about an engine. BUT, a one time test is not all that helpful unless there is a serious issue. The best use of leak down is to do periodic tests with the same gauge set/procedures and record/monitor the results. It's similar to oil sample testing - a one time test is not very helpful; regular periodic testing allows you to identify trends as they develop.

I've always used 80 psi, because that's what the instructions that came with my tester said to use. My A&E friend and fellow racer told me that was the standard for measuring aircraft engines. My tester also has the standard .040" orifice for smaller bores like our Nortons. I've mostly used it for troubleshooting. A large number always means something is not right, and listening for the noise of escaping air through the ports or crankcase breather can help identify the likely culprit. If the leakage is really low, using 100 psi might make it easier to get a reading on the gauge, and at least you don't have to divide by 80 to get the value in percentage. I'll have to try a direct comparison of 80 psi and 100 psi testing on the same engine, just to see how much difference there is. I've used several methods to keep the engine from rotating under pressure, mostly some method of putting a wrench socket and breaker bar on the crankshaft and blocking it with a bar set crosswise through frame tubes.

Ken

mike996 said:

Your input pressure is much too low. It should be 100 PSI for an aircraft gauge set.

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Yeah I did crank up the PSI after contacting Ken and telling him that it seemed too good to be true.- that was before I made the video.
It worked fine at TDC and I got the same difference - both gauges indicated nearly the same pressure - close to being balanced. Going away from TDC is a problem with 80 or 100 PSI. With the higher pressure I can only manage to hold the crank part way through the stroke with a long breaker bar and a cheater extension and its more torque than I want to put on the crank nut. The rear brake helps a lot but you can't control turning the crank very well - Its just too risky and I don't see any point because increasing the PSI isn't making any real difference between the gauges or leaking.

Well, each division on the dial is 2% at 100 psi, so you would appear to have at most 1% leakdown (depending on how well you can read the scales, accuracy of the gauges, etc.). If I got that reading on my bike, I'd be quite happy with it.

Ken

Amend to dangerous torque developed just off TDC and it would be dangerous to ride a cycle with super touchy rear brake strong enough to hold 80 lb. By using alu plate lock blocks to trap clutch against primary boss one should be able to test a bit off TDC. Testing off TDC is supposed to be done by moving crank either side of TDC so don't know if above invalidates the test. Still good news its holding pressure so well with the mileage. Dyno's are similar sliding scale tools.

I've seen mention here a couple times of testing with piston just past TDC, and I'm curious about the reason. Is it to get the rings a bit below the top wear ridge? Or is there some more obvious reason I'm missing?

Ken

I know I know, ugh do I ever. There is not as much bore wear factor at TDC where everything slows down and stops as just a degree or 3 before or after - so best practice for actual runing leaking requires moving off TDC crank balancing act - than can twist off a crank nut over come rear brake with a buddy standing on it tied down or pensively strain a rear spoke a bar put through. I found pressure balancing crank at TDC is best way to find real life "functional" TDC and double check cruder dial degree method which can easy be a degree or so off. There is so little info gained by leak down testing but where leaking most and so dangerous to damaging I've sworn off doing it again until better way to hold the torque. Maybe a bike size jig to grab tire and let it slip a limited bit off TDC then read and listen.

Jim, you used the tester correctly and with 80# in and 80# out it would indicate near perfect ring seal and no leaking valves and considering your using Total Seal rings I'd be happy with that. If you were using standard gapped rings and getting that high a reading then I'd be looking in the cylinders and seeing if there may be excess oil above the rings causing a better seal and inaccurate indication of ring seal. With an aircraft engine, we have the long arm of the prop to hang on to so can rock the piston either side of TDC easily, however if you're not careful it can still get away from you and if you're not quick, it can be painful.

marksterrtt said:

Jim, you used the tester correctly and with 80# in and 80# out it would indicate near perfect ring seal ...

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Thanks Marksterrtt - there's a reason I'm doing this and I'll explain below.

lcrken said:

I've seen mention here a couple times of testing with piston just past TDC... Is it to get the rings a bit below the top wear ridge? Ken

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I'm trying to establish a standard and a way to compare cylinder sealing that would be relevant from one motor to another. On my last high mileage motor I only saw a ring ridge near the top. Most of the measurable wear was within the first inch. With the lightweight pistons I'm not seeing any wear yet after 20,000 miles. So I'm happy with my piston design and the service they offer in reducing stress and vibration etc. But high RPM racebikes can put wear further down the bore. Its usually on the front side. I mention that in the video and proceeded to check the power stroke at mid point for leakage. I’m not seeing any wear there yet but I’m not stressing things nearly as much as a full fledged race bike even though I thrash it on occasion and rev it to its limits. So I’ll just keep putting more miles on it and check it annually etc. If gapless rings show less leakage than regular rings then that should be an upgrade to seriously consider. If racers experience more wear then I want to know more about it and offer ways to stop it such as the cylinder bore carbide treatment or switching to bean oil or devising ways to reduce friction etc. Having a tool to accurately measure leakage is important and I hope it leads to finding ways to make these motors run better and last longer. 50 PSI is a practical test when using a breaker bar to rotate the crank. A .040” orifice is standard and so far seems to be accurate even at 50 lbs. It may be that a smaller orifice is needed (say .020”) at 50 PSI but this would require using a non standard orifice and this could cause confusion. But if regular rings show leakage when tested at 50 PSI with a .040" orifice then it shouldn’t be necessary to use 80 or 100 PSI. So as usual – more tests need to me made. Any volunteers?

Just too much torque applied on either side of TDC to tolerate full pressure testing w/o making up a gizmo to hold the twist via the primary system or rear wheel clamping. Miight have to rethink this compression or leak down testing for our antiques. Maybe just time the leak downs to get sense of wear at various bore locations w/o tearing down or standardize on lower pressure testing.