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Discussion Starter #1 (Edited)
Tonight I did a leakdown test on my motor. I did it with the motor warm/hot. I rebuilt it to fairly common and mostly stock specs: motronic pistons, hasting rings, SPICA by Wes, 10548 intake cam. Head re-done with guides and seals on all valves. It makes nice power, showed fairly even cold compression (173, 185, 177, 185) 1-4. Approx 2000mi since the rebuild. I'm still doing a more accurate test but I though it may be using oil.

The leakdown showed just about zero on cyls 4,3,2 but significant leakage on #1, approx 20% with leakage coming from the oil cap. I removed the valve cover to visually verify I had the cams facing out and then repeated the test, moving the motor slightly back and forth while verifying that the cams never touched the buckets (I could rotate the buckets easily with my finger). What I noticed is that #1 was very sensitive to small rotation of the motor and I could coax 8% leakage from it. but more often than not I got 20%. I repeated the test for 4,3,2 this way and got nearly zero leakage for them every time, so the test seemed reliable.

All I can come up with is that I broke a ring installing #1? But then why can I coax better results from it? I'm tempted to say there's some carbon junking up a valve on #1 but then why would the leakage be VERY clearly felt from the oil fill cap?
 

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Two simple test
Take the car for a run build up some speed then take your foot off the gas and let the car pull the engine for a few seconds, then floor it, if you get a big cloud of blue black smoke you have a worn valve stem

Put a small amount of gear oil in the cylinder and repeat your leak down test if it solves the broblem you have a ring to cylinder wall issue
Did you stagger the ring gaps when you installed the pistons?

If it's leaking past the rings it goes into the sump which breaths through the valve cover cap when removed or the vent pipe with cap fitted.
Could be damaged rings or a marked cylinder wall and just rocking the piston finds a spot where it leaks less
Mark up your cams at tdc number 1 cyl rotate the engine so the pistons are all halfway down the bore, split the cam chain and rotate the cams back to a point where valves are fully closed and test again, if it still leaks it's likely rings if it's good it's more likely a scratch in the liner wall, you can also rock the cam to see if you can make a difference if it does this indicates valves and a worn valve guide worn guide could allow the valve to seat differently
Put the cams back to the marked position and bring number one back to TDC and refit the chain it's important to rotate the crank oposite way so if you moved cylinders to half way by rotating crank clockwise put it back to TDC by rotating anti clock wise that way it all re aligns and no risk of pistons and valves meeting
 

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Discussion Starter #3
That is really great advice I appreciate that thank you. I forgot to mention that I first performed tge test totally cold and only on cylinder one I saw the same approximate 20% leak then squirted a few shots of oil into the cylinder and got more or less perfect results. I then went out and got the car up to operating temperature and redid the test with the results I posted above.
 

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Compression and leak-down readings don't tell you anything about the oil rings. They tell you about the compression rings. These are typical/fine compression numbers.

If you're burning a lot of oil, odds are it's not the valve guides or seals. It's hard to burn a lot through that path unless you have an extremely worn head. Oil rings are the typical culprit. Extent of smoke is not always a reliable indicator either. I rebuilt a friend's Spica 2000 in his GTV last year, we did all kinds of on-the-road tests first, coasting downhill, flooring the gas afterward, etc. The car smoked but not a lot. It was burning a quart every few hundred miles. It was rings.

Andrew
 

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Since you are concerned about the leak-down results on #1, drive the car more, perform a leak-down every 1000 miles, and monitor oil consumption. The other option is drive it and don't worry about it.

As Andrew points out, your compression numbers are good. The leak-down on #1 is perhaps bothersome but what are you going to do, pull the engine apart based on one test? I wouldn't.
 

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Discussion Starter #6
I should have mentioned the head was re-done with seals on all valves.

Thanks Andrew that is helpful. Your friends car sounds like mine. I've built a few tight 911 motors with low consumption so I'd be disappointed if I messed up a ring here. I specifically used hastings in lieu of the rings the motronic pistons came with for this reason. I will put some miles on it this week/end to do a better measure of oil consumption. I filled it precisely to MAX and will see how many miles before the MIN.
 

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Yes, Hastings is the way to go. One-piece cast oil rings, I generally toss them.
Agreed, don't do anything based on one compression test. See how it is over time.
You broke in the rings per Hastings's instructions?
What oil you using, and what rate of consumption if you know yet?
Andrew
 

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Discussion Starter #8
Yes I used the Hastings varied driving load and RPM, avoiding redline but going at it reasonably hard. That said I was also shaking down the rest of the car after a ten year rebuild so take that for what its worth.

Rotella 15w40 dino. No good numbers on consumption yet.
 

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Sounds right. Interested to hear how it goes.
Does Rotella have zinc? Is that a diesel oil?
Andrew
 

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Discussion Starter #10
Yes its high Zinc diesel oil, I picked up the habit from aircooled 911s but its not ideal, I'll shift to VR1 next change probably.
 

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Keeping an aircooled 911 alive and healthy is the best experience for doing so with any other engine!
I haven't read up much on the whys and wherefors of using diesel oil in a gas engine. I haven't done, not sure of the reasons. If the oil makers don't recommend it, I wouldn't do it. So much particle stuff in diesels, the oil deals with it differently from in gas engines. I use a ZDDP additive each oil change.
Andrew
 

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Discussion Starter #12
It's funny I was thinking okay no big deal I'll pull one cylinder and replace the rings... wait... its not a flat six... that won't work :)

I think Rotella got to be in fashion with air cooled guys when all the Zinc went out of other oil but there are now several forumations specifically advertised for Flat Tappets including VR1 20w50 dino and Mobil 1 20w50 Synthetic. I used later on my E30 bmw and will probably shift the 964 over to it once I'm confident its broken in from its top end rebuild.
 

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Tappets/zinc are only one consideration. Like I say, diesels make their oil full of particles immediately and it stays that way til you change it. Diesel oil becomes black on the first use. I would think if it were recommended for spark engines they wouldn't make two different oils.

Andrew
 

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I spoke to a Vavoline tech advisor last week, concerning the 15W-40 diesel oil for gasoline engines. He told me the diesel oils are still rated for gas engines but why would I want to do that? I told him because I can 45 5 gallon pails for free! Then he said it was ok to use but VR-1 would be better. He told me that diesel oil actually has more zinc then the VR-1 because it has a detergent, detergents reduce the lubricity of the oil hence the addition of more zinc. Diesel oils have the detergent to scrub the contaminants from combustion.
 

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I have been using Rotella T4 15W40 plus Rislone ZDDP additive for years with no observable problems. It is cam wear that is the issue and I run high lift racing cams and high pressure racing valve springs and I frequently take the motor to 7000 rpm and I have no observable cam wear.
 

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That you get a variable leak down when you slightly rock the engine is almost certainly indicating compression ring issues. This is exactly the technique we use when diagnosing the condition of an aircraft engine.

The modern guidance on AC engines is to monitor the affected cylinder, but no rush to change. Chances are the imperfect ring is making good compression when the engine is running and the ring is bathed in oil.

Closely monitor your oil consumption. Consider a borescope. If the cylinder starts to show low compression no matter if you “rock” it, or squirt a little light oil (Marvel Mystery) into the cylinder, then its time to go in.

It’s not unheard of to find a piston pin retaining clip has popped out and scored the cylinder.
 

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Im impressed that the other three are so tight! Great job on that -- even on a high perf motor getting <3 % leakdown is darn good! How much time did you spend lapping the valves?
 

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Discussion Starter #20
That is very helpful thanks Don. Should I consider any type of piston/cylinder cleaning that can be accomplished in-situ? As I have been dialing the motor in I have no doubt had it running pig rich, too lean etc. I have run a bottle of seafoam thru once but have not attempted any de-carboning such as adding water to the spark plug hole etc.
 
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