"Piston Slap" is a term I have heard, at various times, for years but I really don't know exactly what it means or what it sounds like. Can anybody out there enlighten me on this?
basically its a loose piston...too much clearence in the piston bore..it will give like a slapping sound or a knocking sound but different than a knock from ignition
some newer gm motors are having this problem right now..my 5.4 L z-71 has this in the morning when shes cold..when it gets to operating temp it goes away..but it does damage so if it is something you built or are building that is doing it i would suggest you fix it..it will eventually damage the piston and the piston bore
OK, I'll pipe in hear since no one else will, Piston slap is what you do to some knuckle head that ask's all those dumb qwestions while your trying to get some work done! No Ok, when the pistion and bore on the block are wore out, you have to mutch space between the skirt of the pistion and the wall of the cillender, so when you are running the motor, you will hear the pistion slapping the cill wall, and there ya are.......Roach.
sometimes forged pistons will slap when the motor is cold. we uset to fit them looser in the cylinder than cast pistons. they warm up and they dont slap but the skirts are strong enough to take that on forged pistons. if cast pistons slap the skirts arent long for this world!
Thanks for the info. This is sort of what I thought it might be but I knew you guys would know for sure. The car in question is an old MG that occasionally makes a knocking noise that doesn't sound like a main bearing to me. Someone suggested it might be piston slap but could not tell me exactly what that was. Thanks to all, Rusty.
My 2006 SS silverado with 16k has a serious issue when I get on the go pedal. Chevy said thats a normal engine sound then told me it was because I am using full synthetic oil. It sounds like an alum can being rolled in your hands
yeah Gm is not owning up to alot of internal engine problems i think almost all of their V8's from 99 to 06 have a problem with excessive piston clearence issues..they just pass it off as "normal" for their new engines..i call Bullshit..if it aint tight it aint right..hell i got a buddy that works for Chrysler and he got 2 weeks off because they fucked up on an engine that goes in most all their cars..it was from excessive piston to cly.cleaence seems the engine would run for 30 minuets almost enought time to get them off the line than it would sieze up..lol fuckin yellow pages engineers..they all (ford Chevy Chrysler) keep trying to re-invent the fukin wheel..when are they gonna learn there is a reason certain parts have to FIT right or they are junk!
Actually the best way to get a loose piston is to fit it too tight in the first place. same goes for bearings. All this comes from the tighter is better notion when actual right is better. An old MG? Well they wre never really quiet in the frst place. I was a BMC mechanic for several years. Quiet was not a term I would ever have used on their motors. After hearing the diesels run it is hard to imagine why anyone would worry about a little noise in a gasoline engine. Stinken Klinken Rauchen Dingen
Don't ignore other possibilities, wrist pins, they make a noise that keeps on going when you rev it a bit, main knock, you hear it at idle but it goes away(till it's bad),,fuel pump push rod, constant I think, any more? Hey man, if yer workin' on a british car you'd best get used to the sounds they make when they run good and learn to listen for new sounds!
You said MG, used to have one a long time ago. Mine was a57 1500. Had a noise that everyone said was either a bad piston or a bent rod. Naturally for a small fee they would rebuild it and the problem would be fixed. At the time not many people worked on foriegn stuff. Being poor I decided to do it myself. The problem turned out to be the big nut on the camshaft. It holds the gear to the cam. It somehow got loose and allowed the gear to rock back and forth on the cam. Only made the noise at idle, anything above and the noise went away.
Such loose clearances on most NA applications shouldn't be necessary these days if using a quality, modern design forged piston. Mine were spec'd at .002" and with a teflon coating added to the skirts, this was able to be reduced to .0015"
We were able to find out which cylinder had the bad wrist pin or piston slap by unplugging the spark plugs one at a time. No spark no noise.
alot of the newer cars that have piston slap have it due to the meterials used in hypereutectic pistons and the meterials in the wrist pins. the pistons heat and expand more due to the amounts of silicone in them so they are making the holes for the wrist pins bigger so when the motor gets to operating temp it stops due to the pins getting tight. you can get piston slap from bad wrist pins, oversized bores and a number of other things.
If you want to hear piston slap first hand blip the throttle on a high mileage Honda from cold thats the noise kind of like a cam rattle but twice as fast and you can play a tune with it. MG motors rarely slap due to the long pistons and number of rings, I've had B motors that would run nicel and quiet to with enough bore wear that they needed a 60 thou rebore . Some motors make a nasty piston slap type noise when a Rod bearing is worn, the Rod bearing will get worse as the motor gets hot where as Piston slap will disappear. Hope this helps.
Koppak, the noise does appear when the engine is hot and I put it under strain. Idles ok and actually gets down the road ok but the valve train noise is pretty loud(normal, I guess)and that little knock starts under load. This is an XPAG engine.
I've never had an XPAG motor apart but tha does sound like a possible rod bearing to me then, probably best to check before any more damage is done. Has that motor got vacuum advance if so pull the pipe and try it will probably be quieter with it disconnected
Koppak, there is no vacuum advance on the XPAG. I suppose it could be a rod bearing but I didn't really want to entertain that thought. I will take all the good advice that has been given and try to narrow it down. It may mean hauling it out and doing a rebuild this winter but parts are very expensive for the darned thing.
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