Leak down test results.

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ryrypizza
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Leak down test results.

Post by ryrypizza » Sat Sep 10, 2016 2:10 pm

So I just did a leak down test twice, with the same result each time.

I am hearing and feeling air come from the exhaust side push rod tube. I can hear no air coming from the breather, exhaust or carb intake (carb off)

So what does that mean? Does that still suggest a ring issue
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Re: Leak down test results.

Post by rivers » Sat Sep 10, 2016 3:54 pm

Yes. Hearing air exiting the pushrod tube seals is an indication when you're pressurizing the cylinder air is getting past the rings and pressurizing the crankcase.
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Re: Leak down test results.

Post by Michal » Sun Sep 11, 2016 3:53 pm

Breather is open at particular crankshaft angle only, unlike all other engines. I suggest to remove oil filler plug next time. By the way, cylinder is no air tight, piston rings are not made of rubber. The best and in the end simplest test is to take head and cylinder off for inspection. Russian valves and guides usually need care sooner than anything could be tested from outside.
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Re: Leak down test results.

Post by LRP » Sun Sep 11, 2016 7:57 pm

Maybe a hole in piston, seems to happen with some Power Arc settings.
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Re: Leak down test results.

Post by rivers » Sun Sep 11, 2016 8:12 pm

As Michael mentioned: the rings aren't air tight so some leak will occur. For checking approx ring or valve condition with a leakdown tester is the reason the tester includes the gauges. Condition is determined by "the rate of the leakdown" not if it does or doesn't leak cause it will.
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Re: Leak down test results.

Post by ryrypizza » Sun Sep 11, 2016 8:35 pm

rivers wrote:As Michael mentioned: the rings aren't air tight so some leak will occur. For checking approx ring or valve condition with a leakdown tester is the reason the tester includes the gauges. Condition is determined by "the rate of the leakdown" not if it does or doesn't leak cause it will.
What exactly do you mean by rate. I took everything a part and there are no noticeable signs of worn rings or scratches on the cylinder walls. There is some discoloration on the exhaust side push rod tube exit though.

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Re: Leak down test results.

Post by gobium » Sun Sep 11, 2016 8:52 pm

What is that hole in middle of cylinder"

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Re: Leak down test results.

Post by ryrypizza » Sun Sep 11, 2016 8:59 pm

Uh oh, if Van is asking what that hole is then I must be in trouble.

I assumed it was normal. I'm out of town out the moment so I'll have to check when I get back.

This is the original piston
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Re: Leak down test results.

Post by rivers » Sun Sep 11, 2016 9:22 pm

ryrypizza wrote: What exactly do you mean by rate.
Rate was a poor choice of words. amount(%) would have been better. Essentially a LD tester has two gauges. First inline is the regulator gauge. 2nd inline is the cyl gauge. Some cyl gauges are calibrated in % some identical to the regulator gauge in PSI. Mines the later so ya gotta do the math. Once everything is set and the tester is connected to the cyl the 2nd gauge reads the psi in the cyl and you figure the percentage of what the regulator gauge is set at. I personally don't get excited if at or under 20%. I consider good enough for a street motor. A little concern develops at 30 % or more.At 40% I dig deeper. There's some set up details I left out but essentially you're comparing one gauge to the other. Sorry again, %(psi difference) not rate.
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Re: Leak down test results.

Post by ryrypizza » Sun Sep 11, 2016 9:33 pm

rivers wrote:
ryrypizza wrote: What exactly do you mean by rate.
Rate was a poor choice of words. amount(%) would have been better. Essentially a LD tester has two gauges. First inline is the regulator gauge. 2nd inline is the cyl gauge. Some cyl gauges are calibrated in % some identical to the regulator gauge in PSI. Mines the later so ya gotta do the math. Once everything is set and the tester is connected to the cyl the 2nd gauge reads the psi in the cyl and you figure the percentage of what the regulator gauge is set at. I personally don't get excited if at or under 20%. I consider good enough for a street motor. A little concern develops at 30 % or more.At 40% I dig deeper. There's some set up details I left out but essentially you're comparing one gauge to the other. Sorry again, %(psi difference) not rate.
The percentages are just comparing between cylinders though right. Don't I know there's a problem if there's a discrepancy when doing a compression test? Which there was in my previous post
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Re: Leak down test results.

Post by rivers » Sun Sep 11, 2016 9:54 pm

ryrypizza wrote:
The percentages are just comparing between cylinders though right. Don't I know there's a problem if there's a discrepancy when doing a compression test? Which there was in my previous post
No. You can't do a leakdown test on more than one cyl at a time. Piston has to be at TDC in the cyl your testing. The comparison is between the regulator gauge of your tester(pressure your compressor is pumping into the cyl. Set and locked via the reg gauge the 2nd gauge on the tester is showing in % or psi what the cyl is holding in relation to the regulator gauge. Reread the instruction sheet that came with your tester. It'll make more sense than I do.
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Re: Leak down test results.

Post by Michal » Mon Sep 12, 2016 4:18 am

Pictures show no visible damage so far. That little piston scratches are normal, even twice as much would be still OK. That brownish colour is just burned oil. Air heads get very hot, there is very little oil flow as there is no oil channel and few oil types can stand 200C temperature. It's not a problem, Ural head is designed to run that hot.

I suggest to remove at least upper piston ring - carefully, they are easy to break - mark top side orientation - they use to be conical. Then insert the ring inside the cylinder at least an inch bellow the top and check the gap using gap gauges. If the gap is over 1.5mm rings are worn. New ring gap should be in range 0.4 to 0.8mm. If it's less, file the end of the ring a little. Too small gap can break the ring when it gets hot and the gap disappears due to thermal expansion.

Also remove valves and check for small black dots (little pots). If the are several dots, use lapping paste usual way.
Valve stem should wobble just little in the guide.
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Re: Leak down test results.

Post by WEGUNTER » Mon Sep 12, 2016 4:23 am

Just curious. What is the hole in the center of the piston??? I went onto the myural web site and looked at various piston heads and none of the pictures show any with a hole in the center. Not sure what I am looking at but the hole in the previous pictures looks to be machine made or intentional.

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Re: Leak down test results.

Post by VWK75S » Mon Sep 12, 2016 5:24 am

Michal wrote: Russian guides usually need care
+1 :thumbsup:
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Re: Leak down test results.

Post by dneprlover » Mon Sep 12, 2016 8:28 am

. Failure of the breather will usually cause high crankcase pressure and seal failures.
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