Normal leakdown on fresh unrun motor?

[bpu699]

Just did a leakdown as assembly is complete. I know the rings aren’t seated, as they are new.

Got 2 cylinders at 10%, couple at 6%, couple at 4%. All leakage via the rings into the crankcase...

I assume that’s normal as the rings aren’t broken in? Motor obviously cold...

All new rings, heads redone....

I assume this improve quite a bit once running and broken in?

[Neil Harvey]

A couple of things to check before you run the engine and hopefully before it's back into the chassis.

What were the dimensions of the cylinders especially at the top. Ovality (lack of) is very important here. As is the piston OD and the resulting clearances?

Ring tension, did you happen to measure this in case your were supplied low tension rings?

Were the cylinders reconditioned and what was the final surface finish? Now is the time to be sure about everything, so if this tendency continues after some running, you can eliminate these factors.

For those that have not considered this, a cylinder leak down can be done without the heads installed. Lock the engine with that piston at TDC, or slightly below and do the same test. It requires a plate with an O ring to be made and bolted down to the cylinder top. We do this each time especially on the newer water engines where installing the pistons with very thin oil rails is somewhat blind.

There is a company that sells german tooling and a similar plate I think.

Now is the time to be sure and eliminate any doubts.

[bpu699]

Ovality was within normal...

Oddly, before the rebuild 2 cylinders were down on leakdown... one via the exhaust valve, one via a stuck ring. The others had a 5% leakdown or less, on a 60,000 mile motor...

The two cylinders with the best readings prior to rebuild, now have the lowest. All the pistons and cylinders went back to their original locations. The only thing that changed is new rings... so ovality shouldn’t play a role?

Rings were from our host...

Cylinders were Nickasil scrubbed with scotchbrite.

Googled this, as there really aren’t a lot of posts on leakdown done before a motor is run. Couple threads on pelican with folks getting 20% leakdown before rings were seated. The Supra forums seem to suggest the same, with folks getting 20% leakdown before running the motor for the first time...lots of posts suggesting readings before seating the rings are pointless...

Was pretty surprised to see a 4% leakdown on cylinder #1, as that was previously my worst cylinder with the stuck ring...

Just to be clear, all the cylinders have a leakdown of less than 10%. Pretty amazing on a cold motor with unseated rings...

[lvporschepilot]

Also note that different leakdown testers can post different results. An older Snapon Mt324 is the most sensitive tester I've ever seen, and any number under 15% is great with that particular tester. Same cylinder tests at 6% with a different brand 100psi tester.

Let's not all beat ourselves up over leakdown testing being under 5% etc too. The FAA leakdown standard is very specific, anything under 25% is airworthy using a tester with a .040 aperture and 80psi. The lower the psi tester, the higher the leakage shown on the guage is my experience. So a 15psi tester showing 20% leak is not the same as a 100psi tester showing 20% leak, they will show totally different results. It's all a bit fuzzy using such low psi as well considering peak power is generally over thousand psi in any given cylinder, so how is just 80-100psi going to tell us much. It's better than nothing, just keep an eye on peak power and oil burning.

But I digress;yes, you can expect a difference once the rings have seated and everything is up to temperature.

[bpu699]

Originally Posted by lvporschepilot

Also note that different leakdown testers can post different results. An older Snapon Mt324 is the most sensitive tester I've ever seen, and any number under 15% is great with that particular tester. Same cylinder tests at 6% with a different brand 100psi tester.

Let's not all beat ourselves up over leakdown testing being under 5% etc too. The FAA leakdown standard is very specific, anything under 25% is airworthy using a tester with a .040 aperture and 80psi. The lower the psi tester, the higher the leakage shown on the guage is my experience. So a 15psi tester showing 20% leak is not the same as a 100psi tester showing 20% leak, they will show totally different results. It's all a bit fuzzy using such low psi as well considering peak power is generally over thousand psi in any given cylinder, so how is just 80-100psi going to tell us much. It's better than nothing, just keep an eye on peak power and oil burning.

But I digress;yes, you can expect a difference once the rings have seated and everything is up to temperature.

Yep, the question is really about leakage before rings seat...

I did the test to make sure there wasn’t something absolutely wacky, like one cylinder with 50% leakage or something...

Was using a higher end tester...

At low psi, there was very little leakage, and it went up as psi went up...

For example, at a 40 psi input, cylinder would be 39 psi... at 60 psi, maybe 58 psi...

At 80 psi, perhaps 75 psi..

At 100 psi, 90-96 psi... (4-10% leakdown)...

Oddly, the two cylinder with the 10% leakage had the best leakdown before the rebuild...

[dannobee]

Oil used during assembly could affect it too. If the low leakdown cylinders got a liberal dose of oil, that could help explain the uneven numbers. Add some oil to all of the cylinders as best you can and see if the numbers even out.

[bpu699]

Originally Posted by dannobee

Oil used during assembly could affect it too. If the low leakdown cylinders got a liberal dose of oil, that could help explain the uneven numbers. Add some oil to all of the cylinders as best you can and see if the numbers even out.

Followed pelicans suggestions, lightly oiled the cylinders, then wiped clean...

Pouring in oil would increase risk of stuck rings, no? I usually do this to see where air is being lost, here, I know its via the rings...

Found more posts about this on pelican. Some folks have had leak down losses of 30-40% after new rings (wow?!), were told not to worry until rings are seated. Others, are stating they had less then 3% loss across the board with fresh rings, unrun...

Will look at it again next weekend, with fresh eyes...

Sometimes I think not checking these things until after 500 miles or so is the way to go...

[VFR750]

The leak down tester sold by harbor freight is a low pressure design with a huge orifice. >>0.04”. I found the readings to be sporadic and unrepeatable.

I converted mine to the 0.04” orfice and a high pressure gage downstream.

Uses 100psi and seems to be easier to use and interpret.

[bpu699]

Fixed the issue...

The last 2 cylinders I checked with the leakdown tester had the lowest readings. Went back, rechecked. Turns out the rubber o-ring on the leakdown tester was leaking! New o ring, pressure went up 5 psi on last 2 cylinders...

Odd, but makes sense. First cylinder had highest reading, then tapered down with each successive cylinder...

I think the o ring got a bit chewed up...

[mikedsilva]

Originally Posted by bpu699

Fixed the issue...

The last 2 cylinders I checked with the leakdown tester had the lowest readings. Went back, rechecked. Turns out the rubber o-ring on the leakdown tester was leaking! New o ring, pressure went up 5 psi on last 2 cylinders...

Odd, but makes sense. First cylinder had highest reading, then tapered down with each successive cylinder...

I think the o ring got a bit chewed up...

Someone once advised me that the spark plug seat can get mangled from all the years of plug changes by the mechanically ham-fisted!

Glad to see yours is good.