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Leak down test noob

 
Old 07-09-2009, 04:58 PM
  #16  
WallyP

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Piston slap affects neither compression test nor leakdown check, so won't be detected by either. Piston slap is caused by excessive clearance between the piston skirt and the cylinder, and the tiny amount of extra clearance that causes slap won't affect the ring seal enough to show.

You will have serious leakage into the intake manifold near BDC while the intake is still off the valve seat. As soon as the lifter comes off of the closing ramp on the cam, the valve closes and you should no longer have leakage. It will be pretty obvious.

Holding an engine against air pressure can get really exciting really quickly, especially if you are at ninety degrees rotation on that crank throw. Twelve square inches of piston area times 100 pounds per square inches equals 1200 pounds on the piston. That times 3.11 inches equals 3792 lb/in of torque, or 315 lb/ft. Not easy to hold with one hand. It's easier to hold it with a prop, but you still have to be careful.

Rocking a 928 engine back and forth is not a good idea...

"Clear" is what every prop pilot yells before he hits the starter. No joke, but rather admiration.
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Old 07-09-2009, 05:05 PM
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Originally Posted by WallyP View Post

Rocking a 928 engine back and forth is not a good idea...
~15deg around TDC is a problem? I would think a properly tensioned belt system in the 928 can tolerate this. I know I've done much worse to the 928 engine and have suffered no problems. Now as for stopping a camshaft with a small metal ball under the lobe, I can say from experience that it is fatal to cams!
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Old 07-09-2009, 05:22 PM
  #18  
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Porsche seemed to think that it could be a problem.

From the Factory Workshop Manual, "Note: Never turn engine anticlockwise, since this could destroy the camshaft drive belt tensioner."
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Old 07-09-2009, 05:24 PM
  #19  
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The rocking backwards issue is especially critical if you happen to have a dry tensioner, when there's no damping or restriction to internal flow when the tensioner is compressing. The tensioner in my car has a little check-valve at the bottom of the piston that allows the piston to extend quickly yet compress slowly. The tensioner is on the slack side of the belt, where the belt is fed back on to the drive gear on the crank. Reversing the direction of pull by turninig the crank the wrong way puts all the tension of the drive onto the tensioner, so a dry tensioner is likely to collapse some more than a filled one. Meanwhile, all the normal slack plus any new slack resulting from the tensioner compressing will stack up on the crank gear, possibly cauing it to lift up and bridge a tooth. Then another and another until the belt jumps time at the crank or the end of a cam is broken off. Or both. Bottom line is that it's NEVER a good idea to turn the crank backwards. It's even worse when the tensioner is dry. Porkensioner won't save you from this either, BTW.
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Old 07-09-2009, 05:25 PM
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Originally Posted by WallyP View Post


"Clear" is what every prop pilot yells before he hits the starter. No joke, but rather admirationn.

I have lots of that for the Cole Brothers.
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Old 07-10-2009, 02:25 AM
  #21  
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Originally Posted by dr bob View Post
The rocking backwards issue is especially critical if you happen to have a dry tensioner, when there's no damping or restriction to internal flow when the tensioner is compressing .
Bummer. Ok, new rule, don't move the crankshaft backwards.
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Old 07-10-2009, 12:14 PM
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Originally Posted by WallyP View Post
<<...>>


"Clear" is what every prop pilot yells before he hits the starter.

<<...>>

This is a great habit to develop when working with others around machinery, including cars. Got the hood open and friends in the garage? Pop the question (it should be a question with a response even with the airplane...) and potentially save someones fingers.
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Old 07-11-2009, 06:42 PM
  #23  
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I don't know what sound you might be chasing down, but there is an oil check valve in the cylinder head which causes a rattling sound when it fails. I don't know if it's already been suggested or if it isn't appropriate, but it can cause engine noise so you might want to look into it.
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