Piston Slap

[Jacksonlll]

Need your thoughts on piston slap. I have a friend who has it, I think.
How do I recognize it? How do you fix it? What does it hurt? The bore I assume.

 

[Mikeinnj]

What causes it:
Piston slap is caused by improper fit of the piston in the cylinder bore. Typically the piston is too small or the bore is too large, making the piston wobble back and forth in the cylinder. When the engine is running this slap can become very loud and annoying. It makes a regular car engine sound very much like a diesel engine.
The noise that piston slap produces usually clears up as the engine heats up. The pistons expand and fit in the bores more snugly.

Outcome:
The results of piston slap usually lead to increased wear of the piston, cylinder wall, increased oil consumption, failure to pass exhaust emissions, and eventually catastrophic engine failure.

 

[John]

In the old days when $$$ were few, machine shops used to knurl the piston skirts and/or use skirt expander inserts to reduce the piston slap. Now days, just bore and fit new pistons.

 

[Jim Brierley]

John, It's still a good fix but pulling the head and checking cyl. wear will tell you if this is a practical solution. Also ring lands should be good.

 

[CWPASADENA]

If in fact the Pistons are a little loose, the correct repair requires disassembly of the engine

If the engine runs OK and has decent compression, I would try running heavier oil, 10/40 or 20/50.

If it is a bit quieter, run it, it will not hurt anything

A lot of A's have run for years with Pistons a little loose

My opinion
Chris W

 

[Kohnke Rebabbitting]

Piston slap comes from Rods out of alignment. Twist, Bend, and Offset. Wrist pin clearance, wrist pins not at a right angle to the bore. Cylinders not at a right angle to the crank.

Loose pistons by them selves seldom cause problems.

Herm.

 

[mike657894]

no noise on low rpm and gets noisey if you rev it. I have 15 thousands of wear in the center of the bore. problaby can run forever like that with the super thick rings of the model A. I was thinking the other day I should have left my engine in. thinking of doing inserts while its out but the babbit looks un phased or redone. So i will probalby bore and new pistons and rings and some light valve work.

I would say leave it till it breaks unless you're bored and got atleast and extra 500-1000 around for parts and all new gaskets and vavle guides and valves and timeing gears. and any thing else you find worn out.


other than the bore being larger it doesnt seem gulged up there is a heavy ridge where the rings stop at the bottom. there was no ridge at the top and the rings dont go all the way to the top. I was thinking it may have been reamed at some time. there was evidence of heavy cleaning on the tops of my pistons may have been re ringed or ball honed. maybe multiple times. the pistons look different but i assume stock with some wild henry ideas. large side cuts and a skirt expansion cut on one side. and holes under the wrist pins. casting marked ford ex. at the center of the pistons they matched stock spec.

 

[Railcarmover]

The problem with rebuilding engines is you cant do it halfway, once you're in your all in.The other problem is you can never factor what it will cost till you measure it out. I've a friend who has a low compression knocker in his '28 coupe, he's lived with it for years.As a result, he rarely drives the car and never goes on tours with the club.. he fears the teardown, repair cost and the hassle involved so much it cheats him out of enjoying his car.

 

[700rpm]

Piston slap comes from Rods out of alignment. Twist, Bend, and Offset. Wrist pin clearance, wrist pins not at a right angle to the bore. Cylinders not at a right angle to the crank.

Loose pistons by them selves seldom cause problems.

Herm.

Herm, can piston slap be positively identified with an engine stethoscope?

 

[Kohnke Rebabbitting]

Herm, can piston slap be positively identified with an engine stethoscope?

It's always a Guess, Mr. 700.

One of the things with piston slap, that is where it starts when the rods are put in.

Most rod builders do not check the alignment of their rods. Who ever puts their motor together, does not check, or even knows that they should be checked.

Rod alignment is just as important an a proper reground crank, in my view, even more.

We have T, and A rods shipped in from modern shops all the time for Rod alignment checks, their all out, just as the ones we do here are.

There is NO rod machine that will cut a rod straight, that is why they make Rod aligners, and Rod presses rod alignment machines, and Rod presses.

 

[Kohnke Rebabbitting]

Pictures, Alignment machine, Rod Press, Showing Piston using a bent Rod.

 

[700rpm]

Pictures, Alignment machine, Rod Press, Showing Piston using a bent Rod.

Thanks Herm. I had my rods checked for true on a KRW setup that a local rebuilder owns, and they were good. I got them from Bill Barlow, who you may know; he is one of the best, so I wasn't too concerned, but I checked 'em anyway, just to see how it was done.

Thanks for your response.