ykp53

what is piston slap? can changing the cam cause it? how would it sound? what damage is caused by it? what are the symptoms of it? is it possible that only one cylinder would have it? (i know its not a valve lash problem)

LemansBlue68

Piston slap is due to the piston crossing the clearance in the cylinder bore. When the conn rod gets close to TDC the forces on the piston change from one side of the bore to the other. The piston crosses over as a result and strikes the opposite side of the bore. If it strikes hard enough you will hear it as a dull rapping noise.

Piston slap is most common when the engine is cold. The aluminum piston contracts at a greater rate with reduced temperatures than the cast iron block and the piston to bore clearance becomes bigger as a result. As the engine warms up, the piston expands and the clearance becomes much smaller.

Lots of things can cause piston slap, like too large of an assembly clearance, incorrect piston pin offset, forged vs. cast pistons, skirt design etc. These are more or less design/build parameters.

Other things that can cause slap are detonation (which overloads the skirts and causes them to collapse), overheating (which causes the piston to collapse because it overexpands, or scuffs the piston skirt), scuffed piston pin (prevents the piston from moving freely with respect to the conn-rod) etc. These are more related to impoper engine operation or engine damage.

If you just changed your cam, it is not likely to have caused piston slap. I would guess that you have a lifter that is collapsed or valve lash that is improperly adjusted.

A rule of thumb when trying to determine whether a particular rapping noise is piston slap or valve train related is that piston slap generally becomes less noticeable as the engine warms up from a cold start. Valve train noise will stay the same or even get worse. The exception may be a sticky lifter which will sometime go away in a few seconds after proper oil pressure is built up while piston slap tends to fade away over a few minutes. Another clue is that valve train noise won't vary so much with engine speed and load while piston slap tends to vary a lot with speed and load.

Another thing you can do is use a stethoscope and probe around the engine. Valvetrain noise will tend to be more pronounced around the valve covers while piston slap will be more apparent down lower in the block.

Enough said???

Good luck in finding your noise. Hope it's not piston noise. Valve train noise is easier to fix. :thumbs:


[Modified by LemansBlue68, 4:50 PM 4/22/2004]

ykp53

i have a sound coming from the front pasangers side of my engine that sounds like a loose lifter but 3 or 4 times louder. it is most evident at lower rpm (5-600) and seems to go away or is at least less noticble when the rpm goes above 1200 any ideas?

comp

did you just change cams???? re-check everything (adjustments etc.) do you have GOOD oil pressure at idle (test with mechanical gauge) wrist pin knock
can be trickie to find without a stethiscope, good luck :thumbs:

Aflac

Piston slap is common in LS-1 engines due to the all aluminum make up... I don't think I have ever heard of it in an older design small block...

Ganey

Said to be "impossible" to find. Probably the fuel pump pushrod.

ykp53

my oil pressure is good. what about the fuel pump, that sounds like it might be coming from that area?

Ganey

Yes, could be fuel pump or the fuel pump pushrod.

ykp53

Is there any way to test this idea? how would the feul pump rod be making noise?


[Modified by ykp53, 9:32 AM 4/23/2004]

Double Aught

I had occasional piston slap with my 78 L82 Pacecar. I sounded a little bit like rod knock, but went away when the engine reached operating temp. Just like someone said above.