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Discussion Starter · #1 ·
I have just pulled the number 1 piston out of my car due to a failed leak down test. I was hoping to find a broken ring but no such luck (I can't believe I just said that). During the teardown I found no evidence of leakage on any gaskets. I did notice that the wear on the top ring is inconsistant around the ring. One part of the ring shows about 50% wear and on other places shows about 10% or less. Is this normal wear and part of the seating process? Cylinder walls look perfect and I have 1600 miles on the engine.

Thanks,
Mike
 

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Discussion Starter · #2 ·
ttt
 

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Well, I'll take a shot at this because nobody else is.

First, is this a standard-bore block? The wear pattern you described is not normal. If it is a stock rebuild with no cylinder work, I would measure the diameter across the bore from a variety of angles (i.e., parallel to the crank, perpendicular to the crank, 45* to those, etc.) to establish that the bore is actually round.

If the block has been bored recently, it's almost inconceivable that the cylinder could be out of round...but not impossible. If it was bored/honed, was a torque plate used? The 5.0 liter block is actually relatively flexible, and mounting the heads introduces some flex. To counteract this, many machine shops use a 'torque plate' during machining, which is a big chunk of steel or aluminum that bolts to the block using head bolts, but has oversize holes through it to allow machine access to the cylinders. Theoretically, the cylinders could go out-of-round when the torque plate is removed, then go back into round when the heads are installed.

Completely different thought: what kind of rings are being used? Is it possible that you got one bad ring in the box? How do the others look, or did you only pull the one set?

Hope this helps.

AJ
 

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Discussion Starter · #5 ·
AJ,
The block was bored .030 and honed. The second compression ring shows even wear around 20% of the ring. Which, leads me to believe it is not out of round. Not sure what tooling was used when the block work was done. Might take the piston up to the machine shop and see what they think. I might also pull another piston to see what kind of wear it shows. All the other cylinders held 70 to 72 psi when 75 psi was added. This one showed 35 until I moved the crank a little then it shot up to 66 psi.
 

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The compression ring is the ring that takes all the grief in the engine. If it has ever gotten hot, it can lose some of it's temper. Piston ring grooves can also get carbon deposits that cause the rings to bind and not float.

It's not commonly known, but rings actually rotate around the piston. When they stop rotating, they take a set and will wear more on the top and bottom sides of the cylinder because these are the areas of thrust.

Thrust - if you are using forged pistons with racing clearances on the street, the piston will have a tendancy to "rock" in the bore. This moves the rings off center and causes more edge wear. The reason forged racing pistons are given so much more clearance is because they are expected to grow so much under full power. Running forged pistons in a street car with racing clearances is a recipe for cylinder leakage and eventual failure of the pistons.

I am a large advocate of using closely fit well-made hypereutectic pistons in both street engines and race engines that are under 500hp. They (Hyperuetectic pistons) have proven themselves in the NASCAR Truck Series as well as all of the LS1 engines used by the ASA. They seal better, are more quiet and are much longer lived than their forged counterparts.
Certain applications such as those that are turbocharged or supercharged with higher levels of boost require a forged piston that can take higher temperatures as well as a higher incidence of detonation. In a normally aspirated car that has the potential to develop more than 500hp, I would still use Hypereutectics if the enginer were street driven and the higher HP levels were only seen for short periods of time such as a few 1/4 mile blasts or a few dyno pulls.

Make sure that you only do leakdown tests with the cylinder at TDC. Anywhere in between will give you a false reading. Since you said "This one showed 35 until I moved the crank a little then it shot up to 66 psi." - this would lead me to believe two things;
1) Piston was not at TDC when tested.
2) Piston is rocking in it's bore due to excessive clearance.

Hope this helps shed some light on your issues.
 

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Discussion Starter · #7 ·
Randy,
I understand that the test has to be done at TDC and I did it that way. I don't understand why though. Shouldn't the ring seal the same anywhere along the stroke?

Mike
 

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Originally posted by Mike DeGuire:
Randy,
I understand that the test has to be done at TDC and I did it that way. I don't understand why though. Shouldn't the ring seal the same anywhere along the stroke?

Mike
Hi Mike - the reasons it should be done at TDC are:

1) That is where the piston needs to seal the most/best because of the ignition process - therefore it is the area of the cylinder and seal that needs to be judged.

2) If done at BDC - the piston's skirts are usually protruding into the crankcase and the piston will rock in it's bore with the compressed air doing it's best to escape.

3) If done in the center of the bore, the air pressure is trying to push the piston to the bottom of the bore and as it does the volume of the compressed air chamber changes.

4) Because that's the way you're `sposed to do it!


Not trying to be a smart-alec, well okay so I am. Blame Cobra Cory - he started it! :D

Happy new years!
 
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