Piston slap poll

[TexasRebel]

piston slap or skirt slap?

they are different.

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[stanglife]

piston slap or skirt slap?

they are different.

The term piston slap is used to describe the sound of skirts contacting the cylinder wall due to the entire piston rocking slightly in the bore at certain speeds and loads.

Soooo...lets not confuse people by making up new terms.

 

[TexasRebel]

The term piston slap is used to describe the sound of skirts contacting the cylinder wall due to the entire piston rocking slightly in the bore at certain speeds and loads.

Soooo...lets not confuse people by making up new terms.

Huh, ...the piston slap I know is when the piston face contacts the head because either the pin height is wrong or the bearings are worn out.

Skirt slap is generally caused by preignition or excessive cylinder clearance.

I guess that just comes from an industry where it matters what is making the noise.:shrug:

 

[stanglife]

Huh, ...the piston slap I know is when the piston face contacts the head because either the pin height is wrong or the bearings are worn out.

Skirt slap is generally caused by preignition or excessive cylinder clearance.

I guess that just comes from an industry where it matters what is making the noise.:shrug:

I'm not sure what industry you're referencing but your terminology isn't mainstream. Just google piston slap. Then google skirt slap...it will simply bring up references to piston slap.

Preignition sound is what it sounds like...preignition.

 

[TexasRebel]

Preignition sound is what it sounds like...preignition.

Autoignition makes a sound... pinging.
Preignition, no matter the cause (autoignition or an excessively advanced spark), causes skirt slap.

Also, google simply returns a ton of youtube links to people that can't diagnose their own engine. I do not consider them authorities on the subject.

 

[montreal ponies]

Huh, ...the piston slap I know is when the piston face contacts the head because either the pin height is wrong or the bearings are worn out.

Skirt slap is generally caused by preignition or excessive cylinder clearance.

I guess that just comes from an industry where it matters what is making the noise.:shrug:

Description...

Piston slap is nothing new to piston driven internal combustion engines and compressors. It is the secondary (sideways or perpendicular) movement of a piston against the side of a cylinder bore where the primary movement of a piston is intended to be parallel (up and down) to the cylinder bore. All piston driven internal combustion engines and compressors have a certain amount of piston slap.

Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.

In the case of the famous GM piston slap engine defect, the piston design with hypereutectic (high silicon content aluminum alloy) pistons, reduced or eliminated piston skirts (to reduce reciprocating mass), and a higher ring pack to reduce unburned fuel mixture on the sides of the piston crown have made piston to cylinder bore fit much more critical. The amount of tolerance (variation or margin) in allowable clearance between the piston and cylinder bore to prevent audible piston slap has been reduced by a factor of at least 50%. Consistently hitting the narrower margin for piston to cylinder bore tolerance has not happened for GM during mass production. Thus, some engines have no audible piston slap and some have piston slap on only one or two cylinders. What might have looked really good in testing of hand built engines in the lab hasn't transferred to the production line of this corporate giant.
Make no mistake about it, while a lot of these engines don't appear to be driving rods through the blocks, the ones with louder and longer duration piston slap will wear out before the ones that are basically quiet. The perpendicular heavy impact of the piston against the cylinder wall over time will not come without a price. This is also why GM has released a recent TSB saying that opening 4 quarts of oil to add to your crankcase between a 7,500 mile recommended oil change interval (1 qt per 2K miles on an engine with 36K miles or less is "NORMAL". After 36K miles, all bets are off (there is no abnormal oil usage rate). This is why the now common offer of an engine component letter extending your warranty to 5 years or 100K miles is basically worthless. If the piston isn't laying in the oil pan in pieces, the engine will be operating "NORMAL" according to GM.

For further proof related to the damage audible piston slap can cause, you only need to look at the GMs own TSB # 01-06-01-005. GMs own illustrations will show you.

What Automotive Industry Experts say about Piston Slap

DOCUMENTATION BY INDUSTRY EXPERTS ON THE SUBJECT OF PISTON SLAP

Dr. Victor Wong/MIT (Massachusetts Institute of Technology), is one expert who states otherwise:

Piston Slap. Few technologies have received more engineering attention than the internal combustion engine. Yet engine designers continue to be troubled by a phenomenon known as "piston slap." As a piston moves up and down inside its cylinder it also shifts from side to side, bumping first one side and then the other--a behavior that wastes fuel, wears out engines and makes an annoying bang. A computer model developed by MIT researchers can disentangle the factors that lead to piston slap, helping engineers make design decisions that will reduce its intensity. Given a description of the operating conditions and design of an engine, the model can describe the pathway the piston follows inside the cylinder, the force with which it hits the wall, and even how its shape changes due to the impact. In parallel work, the researchers have validated the model using an operating experimental engine. The team was led by Dr. Victor Wong, a principal research scientist at MIT lecturer in the Department of Mechanical Engineering. The work was funded by Nissan Motor Company. reference: http://web.mit.edu/newsoffice/rd/1996/sep.html

Bob Hagin/Syndicated Columnist/ Seattle Times disagrees with GM’s rational on piston slap:

The noise called piston slap is caused by one or more pistons having too much clearance between its side skirt and the cylinder walls. In effect, the pistons become too small and wobble in the cylinder bores. It can be cause by an engine simply wearing out (not common any more), a piston seizing because of a lack of lubrication (it runs out of oil) or it's put together wrong. This is easy to check and usually it doesn't happen to all the pistons. But there could be other causes, none of which could be caused by a "wrong" oil filter. Find out what brand oil filter your shop uses and call its service reps and tell them your story.

NWclassifieds/Autos- Research It: Auto Q&A

James E. Harris, proprietor of Engine Restorations in Portland, Maine also disagrees with GM’s assertions regarding piston slap:

One way to check for piston slap: Remove three spark plugs, leaving number one in place. Crank the engine over until you feel the resistance of number one piston coming up on compression. Crank against compression until the piston is about half way up the cylinder. Now using the fan, rock the crankshaft back and forth and listen for a metallic knocking sound. If you hear a knock, you have piston slap and the only way out is to rebuild the engine.
reference:

 

[crs2879]

I've only heard the term "piston slap" defined as the rocking of the piston within the cylinder bore. Typically more noticeable when the engine is cold. Early GM 5.3 engines were notorious for it. I had a '99 GMC that knocked like it had a spun bearing every time you cranked it cold. Dealer just said is was "normal". GM built the engines with short-skirt pistons to reduce reciprocating mass. However, the varying tolerances inherent within large volume, mass produced engine manufacturing led to many customer complaints of excessive engine "knock" when cold. GM had a TSB on it that allowed for piston replacement but as I recall, it was only when visible scoring was seen on the cylinder walls.......LOL

 

[stanglife]

Autoignition makes a sound... pinging.
Preignition, no matter the cause (autoignition or an excessively advanced spark), causes skirt slap.

Also, google simply returns a ton of youtube links to people that can't diagnose their own engine. I do not consider them authorities on the subject.

Try looking at the very first links that come up instead of going out of your way to find people who don't know what they are doing. I was being polite when I said your terminology wasn't mainstream. I'm not going to argue with you.

 

[TexasRebel]

Try looking at the very first links that come up instead of going out of your way to find people who don't know what they are doing. I was being polite when I said your terminology wasn't mainstream. I'm not going to argue with you.

...and I was being polite when I was trying to explain that you don't know the terminology. In fact, based on the thread title I came in here wondering why Voodoo engines had pistons contacting the heads.

Heck one post on the first page describes gear rollover.

The links that come up googling "piston slap" are, in order:
reference.com (junk)
youtube.com (junk)
pistonslap.com(ambulance chaser, junk)
bobistheoilguy.com (forum, unreliable)

from there it's a few more forums and GM press releases... nothing authoritative. I'll hold onto the lessons I learned with the V12 Packard build, thanks.:cheers:

 

[TexasRebel]

James E. Harris, proprietor of Engine Restorations in Portland, Maine also disagrees with GM’s assertions regarding piston slap:

One way to check for piston slap: Remove three spark plugs, leaving number one in place. Crank the engine over until you feel the resistance of number one piston coming up on compression. Crank against compression until the piston is about half way up the cylinder. Now using the fan, rock the crankshaft back and forth and listen for a metallic knocking sound. If you hear a knock, you have piston slap and the only way out is to rebuild the engine.
reference:

I just saw this...

James E. Harris is ...?

Every reciprocating piston engine has skirt slap. It's no mystery either. The component of the compression forces on the connecting rod that is perpendicular to the bore is the cause. It's (one of) the same reason(s) engines with small rod:stroke ratios show more wear than engines with tall decks and high wrist pins. A larger theta means more lateral force.

By the way, that metallic clank that Mr. Harris is hearing will be caused by the clearance fit of a piston in a new bore as well as the rings hitting the tops and bottoms of their grooves. Remember, the rings move independently of the piston and have a clearance fit within the groove. The only friction they see is that of their pressure against the bore wall.

As that this is just an online forum for enthusiasts, I'm just as equal a nobody as Mr. Harris. So do your own research on the subject.

...but don't try to turn an engine over with the fan. Especially if there's a fluid coupling on it.:lol:

 

[viva_brasil]

My car has 200 miles, I hear a chatter under light throttle and low rpms that to me sounds like clutch + flywheel chatter. I haven't heard anything louder than that.

 

[iufan993]

2100-2400 RPM. Doesn't matter how cold or hot engine or ambient temperature, but always there. Track pack. Still under 500 miles.

 

[stanglife]

I just saw this...

James E. Harris is ...?

Every reciprocating piston engine has skirt slap. It's no mystery either. The component of the compression forces on the connecting rod that is perpendicular to the bore is the cause. It's (one of) the same reason(s) engines with small rod:stroke ratios show more wear than engines with tall decks and high wrist pins. A larger theta means more lateral force.

By the way, that metallic clank that Mr. Harris is hearing will be caused by the clearance fit of a piston in a new bore as well as the rings hitting the tops and bottoms of their grooves. Remember, the rings move independently of the piston and have a clearance fit within the groove. The only friction they see is that of their pressure against the bore wall.

As that this is just an online forum for enthusiasts, I'm just as equal a nobody as Mr. Harris. So do your own research on the subject.

...but don't try to turn an engine over with the fan. Especially if there's a fluid coupling on it.:lol:

You're arguing with the world. Everyone is wrong and you are right - is that what you're saying? You don't know what you're talking about.

Search your term, "skirt slap" returns results for piston slap.
https://www.google.com/#q=skirt+slap

Piston slap is the term that everyone, including the engineers who actually design the engines, use to describe what you are arguing is skirt slap...which isn't technically the term used but IS what's happening here.
Now look up offset wrist pin and piston slap and you'll learn something - the fact that NOT all engines have audible "piston slap" due to the design. Many performance engines, do.

 

[TexasRebel]

As I mentioned early on, I guess I just come from an industry where the source of the noise matters.

I never claimed it's all audible, just that at some point there is a force on the connecting rod perpendicular to the bore axis. Even in offset wrist pin applications and offset bores. The forces are just arranged so the least amount of force in the connecting rod occurs at the greatest angles.

Steam engines and fluid power engines are two examples I can think of that eliminated any considerable lateral forces on pistons, but at the cost of weight and complexity.

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