Piston Slap: is it a problem?

[Ninkumpoop]

I got my engine rebuilt from from a shop with forged internals, iron sleeves, lower compression, the whole 9 yards. I get it back and now there's an audible piston slap. If I run the car somewhat hard, or at least go into boost for 10 seconds or so, the noise goes away, then come back after 20 seconds or so after the pistons cool down again (at idle).

So my question is this:
is piston slap bad? I know it's not good, but since it goes away when I start driving, is it still really bad?

 

[JReichle1]

I am thinking they might have honed the block too much. Were the pistons oversized? Piston slap should not be noticeable on a newly rebuilt engine...

 

[Ninkumpoop]

I am thinking they might have honed the block too much. Were the pistons oversized? Piston slap should not be noticeable on a newly rebuilt engine...

I believe they are oversized but not sure by how much. So...how bad is piston slap? Like I stated before, the piston slap stops after 30 seconds of cruising or 10 seconds of hard boosting.

 

[MSP2k3]

all the research i have read about piston slap says that it does not have any long term effects on reliability.....

but, the only piston slap i have heard goes away once the car is warm, even at idle.

It is a pretty bad case if it is slapping even when the engine is warm.

And like it was said before... piston slap really shouldn't be noticeable on a newly built motor

 

[MSMS3]

OP: Ask your builder how they set the pistion skirt to cylinder wall clearance. With short skirt pistons and the ability to bore/hone sleeved blocks to very tight specs with a torque plate in place (hope they had one), the standard clearance may be as small as .001 inches. I have not looked up the actual value, but this is now common and I would not be surprised that the standard clearance is that tight.

Some builders, especially if they know the car is to be raced, will set up the clearance at .002 or even .003 inches with high performance forged short skirt pistons. Setting up the engine "loose," not only on the cylinder wall spec, but on bearing clearances, can make the engine produce less internal friction and give you a few extra ponies at the expense of noise when the engine starts cold, and possibly engine life.

Did you talk to your builder about your goals and objectives? This is important. It's hard to say whether an extra thousandths or two on piston to wall clearance will reduce the life of your engine. My guess is that it will, somewhat, as a trade off for less internal friction. How much, who knows?

IMHO, you should avoid putting any significant load on the engine at startup until the slap has stopped after the pistons expand and the noise stops. Also, you might want to have your oil tested at your next oil change to see if any excessive metal contamination is showing up in the oil, not only for aluminum from the piston skirt, but also the alloys that are in the main and rod bearing surfaces.

If there is a high percentage of aluminum or metal alloys on that oil test, you might want to have a discussion with your builder about this and about engine life.

Whatever you do, do not run heavier bottom number weight oil to try to get rid of the slap. The passageways for the oil to drain out of the cylinder head back into the block are very tiny. You could end up with oil pooling up in the cylinder head and starving the bottom end of oil. That has been the cause of the demise of many modern 5w- oil spec engines when running heavier oil.

If you have not already been running full synthetic oil, now would be the time to step up to that as an extra measure of protection. 5w-30, or 5w-40 in warmer climates.

Talk to your builder.

 

[Ninkumpoop]

Thanks for all your input, guys. I'll have my oil checked after my next oil change and talk with the shop if there's too much metal deposit.

 

[Jarod7920]

Yes talk to your builder about the clearance. If i remeber correctly my PWC is set at .005, and with that i get ZERO piston slap even on cold start up. My builder told me that this was right on the verge of it, and it may or may not pistons slap.

 

[meicalnissyen]

sounds like a 10 over bore and stock pistons

 

[Ninkumpoop]

sounds like a 10 over bore and stock pistons

On paper it says sleeved and bored out to factory specs. with Arias pistons. I'm thinking they don't wanna admit they made a mistake. I'm just wondering if the iron sleeves will have any problems.

 

[creedns]

im bored .040 over and i get zero slap

 

[justanothermp5]

yea...piston slap is not good, im bored .020 over, and put the engine together myself
zero slap, sooo...i would get that checked out

 

[cymx5]

Perhaps I have higher expectations than some. I would not accept a rebuilt engine that sounded like that.

 

[Jarod7920]

It has nothing to do with the actual over bore. Its the clearance between piston and wall. So long as its set right and done right per application you may or may not get piston slap.

 

[cymx5]

"The clearance between the piston and the wall" (i.e. internal diameter....ie bore). duh. It has everything to do with the actual bore. Unless they forgot to put the rings on your pistons.

 

[justanothermp5]

lol if they forgot to put rings on...i would get my money back and run far away from that place

 

[MSMS3]

To echo another poster. Over bore diameter has nothing, I repeat, absolutely nothing to do with piston slap. In an over bore, the pistons are over bore too.

It's the skirt clearance in thousandths of an inch that is the issue. That is measured with both an outside micrometer (for the pistons at the maximum skirt diameter) and inside micrometer (for the cylinder wall) to determine the actual bore of the cylinder wall and the actual maximum diameter of the piston skirt and verified, usually with a feeler gauge during assembly to make sure it's within the spec the builder is seeking depending on the application and depending on how the engine is to be used.

Dedicated racing engines tend to be set up looser to have less internal friction at the expense of engine life. Racing engines also tend to run heavier weight oil (at least in some applications, especially where overhead cams are not involved -- not an option on our engines due to very tiny oil return passages to the block from the cylinder head).

This can also be affected by whether his builder either purchased or made a torque plate and attached it to the block before making the final boring or honing passes on the cast iron sleeves. You want to apply torque to the head bolt threads in the same manner as if the head were attached, as otherwise, you can cause perfectly round cylinders to become distorted when you torque the head to the block when assembling the engine. Those changes can result in shrinking the minimal cylinder diameter and making it bigger in other places (think elliptical rather than round, but in very tiny ways, perhaps only "tenths" -- .0001 intervals).

Because of production line variations, even on custom, expensive forged pistons, a really good race engine builder will actually hand fit each piston to each cylinder and number them accordingly during install and do the final honing of the block to be specific for the particular piston going in that hole, so that every cylinder has exactly the same skirt clearance the builder intends.

That's why I originally asked OP to go back to his builder and find out how they fitted the pistons. I have not checked the spec sheets for skirt clearance value for our engine, but my guess is that it is only about .001" wall to skirt. His builder probably set it up looser to try to pick up a few ponies. Either that or his builder made a mistake. He needs to know just how "loose" they are.

 

[shane02pro5]

By nature a forged piston has much more audible slap... I have a noticable difference.

 

[MSMS3]

Oh, I forgot to mention that the aluminum alloy used in forged pistons makes a difference.

One alloy, 4032, has high silicon content. It is more rigid, has a long life cycle, can be machined to very tight tolerances, expands very little when going from cold to hot, and is typically set up with very tight skirt to cylinder wall clearances. It is, by nature, very quiet in operation. It is more prone to detonation damage, however than the other alloy I mention next.

The second alloy, 2618, is typically reserved for racing applications. It has lower or almost non-existent silicon content. It is softer, more maleable, and expands and contracts with heat up and cool down much more than 4032. But it is much more resistant to detonation and has a greater ability to survive in high boost and high horsepower applications. But because of its expansion properties when heated, it must be set up "loose" with more cylinder to skirt clearance when cold. It will be more likely to "slap" when cold, but if the clearances are set up properly, the noise will go away when the engine is fully warmed up. This is a hard core piston that is not typically used for daily driver, street applications. This is NASCAR, NHRA and road racing type engine building.

These pistons do have a shorter useful life but work better in higher power applications.

If OP has 2618 alloy forged pistons, that may be the reason for his "slap." If he has the more typical 4032 alloy and has slap, he needs to know how the builder set up the engine, and may need to discuss with his builder whether the objective of a loose clearance was a sound one for a daily driver. Or worse, the builder screwed up and provided too much clearance.

In either event, OP should not apply load to the engine until the "slap" stops after warmup is completed. The fact that most modern engines now use short skirt pistions, allowing for more "wobble" in the cylinder to begin with, makes the risk of "slap" all the more likely.

 

[Ninkumpoop]

From what I just looked up for Mazdaspeed3 pistons from Arias, they're 4032. This is so irritating...

 

[cymx5]

MSMS3, awesome input. Thank you for sharing your knowledge.