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Remove rocker arms for leak down test?

I purchased a Lonacre 73010 Leak Down tester. Longacre leak down instructions state to remove the rocker arms prior to doing the leak down test. I assume this is to ensure the valves are closed. This is not something I'm lookin forward to doing with the engine in the car... Ideally, I'd like not to mess with that stuf till I get the engine out.

If I am running a euro SC stock cam do I still have to remove the rockers? At BDC, prior to the compression stroke, both valves are closed, correct? Could I simply loosen the valve tappets all the way instead? Sounds a lot easier...

Just to check, from TDC, BDC prior to compression is 1.5 rev (540 deg) or so?

Thanks!

-Michael
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Old 07-18-2005, 04:34 PM
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That's nuts. Unless your valves are misadjusted the rockers are loose/not pressing on the valves when you do the leakdown.
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Old 07-18-2005, 04:52 PM
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I've not done a leakdown test but I always thought they were done at TDC not BDC. Of course you'd have to be at exactly TDC or the piston will push down, turning the crankshaft. At BDC you may still have a valve open but you can determine that by wiggling the rocker arms. You are correct that you could back off the adjusters provided that they back off enough to close the valve all the way.

-Andy
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Old 07-18-2005, 05:13 PM
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If you do get a poor leakdown reading justt remove the valve cover and wiggle the rocker to verify clearance. Don't even mess with backing off the adjustmet screws.
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Old 07-18-2005, 06:05 PM
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Yiup, instructions specifically state BDC and also warn of injury to engine or self if not at BDC.

I suspected removing the rocker arms was not applicable here. Though I had not considered just wiggling the arms to see if they are on the valves.

Thanks for the comments!

-Michael
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Old 07-19-2005, 05:24 PM
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Some elaboration on the pithy "nuts" might be in order, as it surprised me also to hear what Longacre had to say.

TDC works fine to see if the valves are sealing well (alas, I've had more personal experience with that than I would have liked) or perhaps head leaks (I suppose, though I have no experience with that) or burned pistons or their equivalent (sigh). If your rings are bad, it should tell you that too. Where I saw worse values for some cylinders on my SC when it seemed down on power, these mapped to the broken rings I found when I tore it down. These have all been TDC readings. Maybe I have been lucky, but I just turn the crank to the spark plug firing TDC in the usual way (the Z1 mark) and add air. The valve springs on the other cylinders usually conspire to keep the crank from turning when pushing at pretty close to, if not right at, a sort of zero crank angle where the torque on the crank is very low.

TDC readings may not give you an indication of overall cylinder wear. I think the wisdom is that wear is least at TDC and BDC because things are moving slowest there. If a person were worried about this, he could take readings at various places in the compression stroke after the intake closes right up to TDC. You'd have to fix the crank at about the position you wanted (putting in 5th gear with brakes locked might do). But I can't imagine taking the time to diasble all the valves just for that, far less for wanting to measure BDC (where I would suppose wear might be the least, as cylinder pressures and temperatures must be lowest there). Better to spend the money on a chassis dyno run if you are getting into that kind of detail on overall engine condition.

Longacre must be thinking of pushrod V-8s, where disabling the rockers can be a snap. Why BDC there I can't guess.

Walt Fricke (carries his leakdown tester in the tool box to the track)
Old 07-19-2005, 07:10 PM
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"Longacre must be thinking of pushrod V-8s, where disabling the rockers can be a snap. Why BDC there I can't guess."

I think Walt (welcome Walt, btw) has the most plausible explanation. However, positioning the test cylinder at BDC is a puzzle for me too. Perhaps they want to check for cracks in the cylinder wall. Yet the bottom of a cylinder is almost a non-wear area. If the cylinder were completely closed, I'd guess placing the piston in the middle of the stroke where cylinder wear is greatest might provide a more real-world indication of compression sealing.

For pcar engines and most others, TDC is the suggested position.

Sherwood
Old 07-19-2005, 08:30 PM
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Maybe Longacre suggests BDC because there can be no movement of the crank there, and hence no chance someone gets a body part mangled when the crank moves if the user got it wrong trying to use TDC or some other angle.

Walt
Old 07-20-2005, 09:44 AM
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From LongAcre's website instructions:

1. Warm the motor up to normal operating temperature.

2. Remove all spark plugs. Put transmission in neutral. Block wheels.

3. Remove the rocker arms of the cylinder to be checked.

Important note:_ TURN THE CRANKSHAFT SO THAT THE PISTON IS AT BOTTOM DEAD CENTER. If you do not, the motor will suddenly turn over when you connect the air, possibly causing serious injury to you or someone else working on the motor.


4. ......

Sounds like a no-liability procedure for the DIYer. No chance for injuries due to car lurching in gear or limbs smashed by rotating engine during the air pressure test.

I suppose if your rocker arms were that easy to remove, that'd be the safest way to do it.

Sherwood
Old 07-20-2005, 10:42 AM
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Thanks for the discussion guys! I believe that Longacre specifically said BDC to prevent the crank from turning when you plug in the air. But, now this issue about what is better, BDC or TDC is interesting.

Wear is highest in the center of the stroke... Seems logical as that is the piston's/ring's highest velocity point. But, what about the pressure at TDC just after ignition. Seems that might cause a lot of wear at the top of the cyl. I just saw a set of cylinders on eBay with a wear spot right at the top that might be evidence of that. Just speculation really.

So where to do the leakdown test? I'd say BDC prior to compression stroke might be easiest. Though TDC is only a few degrees further . Are there any kind of technical differences? I cannot think of any off hand...

Comments?

-Michael
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Old 07-21-2005, 11:02 AM
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a. If you can position a piston at BDC with both valves closed, go ahead and try it.

b. If you can't, back off the rocker adjusters to close the valves and try it.

c. If you can't, remove the rocker arms and try it.

d. If you can't (I'll give you 3 stars if you complete c), just follow the suggestions in this thread for our engines and ignore what Longacre's generic instructions suggest.

BTW, you could probably back off the rocker adjusters all the way and position the piston further down the cylinder where you suspect there's greater cyl. wear. How far depends on the cam profile. IMO, I don't think it's necessary. If the rings are weak, that fact should show up at TDC. If there's a crack in the cylinder that will be revealed by a leak test, then you have a rare situation that hasn't been documented before (to my knowledge).

Sherwood
Old 07-21-2005, 11:17 AM
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Hi Gang

It sure looks like Longacre is avoiding liability. I am a tech on GT3 cup cars in the Grand Am Rolex series. We do leakdown at TDC. I would not mess with rockers as they are a ***** to get out while the engine is in the car and you may cause a rocker shaft leak at re-assy. I'd recommend TDC then holding the crank pulley bolt with a breaker bar when you add pressure, to avoid engine rotation. Leakage by the valve(s) should be heard from the exhaust or intake, depending on which valve is a problem. Piston leak-by can be heard from the oil resevoir (remove cap during test).
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Old 07-23-2005, 05:59 AM
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One more tip I leaarned on another thread. Dont take the sparke plugs out until the cylinder is at TDC. Less chance of crud falling in and lodging when you are turning the engine over by hand, causing a leak.
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Old 07-25-2005, 06:55 PM
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Quote:
Originally posted by Walt Fricke
Maybe Longacre suggests BDC because there can be no movement of the crank there, and hence no chance someone gets a body part mangled when the crank moves if the user got it wrong trying to use TDC or some other angle.

Walt
That's what I was thinking... Sounds like more of a liability issue. Imagine some yahoo puts the car in 1st... Puts to much pressure while on a cheapo jack, car rolls off and crushes said Yahoo...

Jailarity and litigousness ensue...
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Old 07-25-2005, 08:23 PM
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