piston slap

[grumpyvette]

" HEY GRUMPY?
Is there any merit to this process to determine if you have piston slap on a cold engine:

1) Start engine COLD. Let it run for 15 seconds then shut off.
2) Quickly remove all spark plugs. Squirt 2 squirts of engine oil into each spark plug hole.
3) Reinstall spark plugs.
4) Start engine. Listen for piston slap noise.
5) If noise is gone for the first 15 to 40 seconds of running, then noise returns, you have piston slap.
6) If noise has NOT gone away after restart the problem is not piston slap.

"

that advise sounds totally bogus to me, adding a bit of oil in the combustion chamber will have very little effect on the pistons bore to piston side clearance.
there has to be some minimal piston to bore wall, clearance to reduce blow-bye and some Piston clearance will result in almost no appreciable slap in a properly set up engines rotating assembly, having some bore wall to piston side clearance, is nothing new too 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 minimal clearance and some minimal amount of piston slap. the rings extend out from the piston slightly and both compression and combustion pressure locks the rinds firmly to the lower edge of the piston ring groves this tends to stabilize the pistons orientation to the bore walls,
different alloys expand at different rates and the manufacturers will provide you info on the correct side clearance and how and where to measure the pistons.
hyper-eutectic
pistons will tend to expand less between cold to full operational temps, than a forged piston so they will have a tighter bore clearance, than forged pistons.
Theres always minimal piston slap, or clearance in any cold engine until it reaches operational temps, but its usually not something you can hear, it is usually the result of cold pistons that have not reached operational temps and as metal expands as the heat increases the clearance in the bore between the piston and bore decreases, pistons that are well under operational temps have not yet expanded to the correct clearances, resulting in pistons that are too loose in the bore.
most pistons are NOT exactly round and the skirts do not expand enough to limit movement in the bores until the heat of operation expands the skirt, squirting oil will have little effect on how the skirt expands in the bore so I can,t see that test being valid

READ THESE
http://garage.grumpysperformance.com/index.php?threads/piston-to-bore-clearance.4630/#post-48955

http://garage.grumpysperformance.co...on-building-a-383-sbc-stroker.428/#post-57879

http://garage.grumpysperformance.com/index.php?threads/can-you-plan-for-quench.11298/#post-51315

http://garage.grumpysperformance.co...ling-rings-in-piston-grooves.9490/#post-34908

http://garage.grumpysperformance.co...a-cheap-piston-a-bad-piston.10481/#post-44088

http://garage.grumpysperformance.co...in-height-compression-height.5064/#post-66240

http://garage.grumpysperformance.co...nk-durring-short-blk-assembly.852/#post-39525

http://garage.grumpysperformance.com/index.php?threads/383-information-overload.11137/#post-49864

http://garage.grumpysperformance.com/index.php?threads/piston-to-bore-clearance.4630/#post-39780

http://garage.grumpysperformance.co...pull-it-down-and-investigate.5893/#post-37148

http://garage.grumpysperformance.co...ing-piston-to-bore-ring-seal.3897/#post-26602

http://www.federalmogul.com/NR/rdonlyre ... cript2.pdf

http://www.rosspistons.com/information/ ... lation.pdf

http://wicanzayu.wordpress.com/2009/04/ ... nary-tale/

http://www.briancrower.com/je/

http://www.carcraft.com/techarticles/pi ... index.html

http://www.pistonslap.com/whatisit.htm

"Around 1998, GM switched from a "Select Build" method of manufacturing and/or assembling engines to a "Net Build" method, in order to save money on manufacturing and/or assembly. In the Select Build process, pistons and cylinders are matched for size and fit. GM's new "Net Build" method of manufacturing and/or assembly, in contrast, assumes all pistons will fit equally well in all cylinders and does not allow for variations in the size of engine cylinders or pistons. The pistons of slightly varying size (all within spec) are not individually matched with the cylinders of slightly varying size (all within spec).

Excessive “piston slap†occurs because an automobile manufacturer (GM) designs and/or manufactures a defective engine in which the clearance between the piston and cylinder bore is too great. Essentially, the piston moves sideways and “slaps†or “knocks†hard against the cylinder bore and causes damage to the engine pistons and cylinders, excessive smoke emissions, excessive oil consumption, carbon buildup on piston heads, decreased mileage, and a loud and obnoxious “slapping†or “knocking†noise, all of which diminishes vehicle resale value in the trade."

 

[Grumpy]

Piston measurement and clearance

A piston must be in good condition if it is to be reinstalled in the engine. This means that it must have no cracks, scores or scratches. It must be the correct size and have the correct clearance in the cylinder.
The piston is measured with an outside micrometer or vernier calipers. The cylinder is measured with an inside micrometer and the two sets of measurements are compared to find the piston clearance.

Measuring the piston

Fig 7.7
Using a micrometer, measure the piston skirt across the thrust faces as shown in Figure 7.7. In most cases, the measurement is taken at a point which is approximately one-third of the skirt height. This is the nominal diameter of the piston and should be the greatest dimension.
Other measurements can be taken as a check and to make sure that the piston has not collapsed. A collapsed piston will have a reduced diameter at the lower end of the skirt.

Fig 7.8
The engine manufacturer’s service manual should be consulted for piston dimensions and clearances. It will also have information on how to measure the piston. Some pistons are cam-ground, some have a tapered skirt and some are slightly barrel-shaped. All pistons are not measured in the same way.
Figure 7.8 is an example of piston measurement. It shows the places on a piston where measurements would be taken. These are as follows:

BB is the nominal piston diameter
AA is the reduced diameter after cam grinding
CC is the diameter at the top of the skirt
DD is the diameter at the bottom of the skirt
EE is the reduced diameter at the lands.


·These measurements enable the piston size, cam grinding, skirt taper and land
relief to be determined.


Checking the piston in its cylinder

Fig 7.9
The fit of the piston in the cylinder can be checked with a feeler-gauge strip. The following is a typical procedure.
Place the piston in the cylinder upside down with the feeler strip, lightly oiled, placed 900 from the piston-pin holes. This is the greatest piston diameter. The feeler strip should extend the full length of the piston. Different thicknesses can be tried to determine the clearance.
A refinement of this procedure, which is more exact, requires the use of a spring scale. The force required to pull the feeler strip from between the piston and cylinder is measured (Figure 7.9). The following is an example: a strip of feeler 12 mm wide and 0.04 mm thick should be able to be withdrawn from between the piston and the cylinder wall with a pull of 20 to 40 newtons. If the feeler strip pulls out too easily, the fit is too loose, if it is too hard to pull, the fit is too tight.

Fitting piston ringsReplacement piston rings are supplied as a package kit to suit the particular engine being repaired. They can be obtained in various over sizes to suit oversize cylinder bores.
Piston rings are supplied for a particular bore diameter. Their ends should not be filed to fit them to smaller bores because they will become oval-shaped when installed in the cylinder, if the cylinder has been re-bored or made oversize by honing, it will require oversize rings; if not, standard-sized rings must be used, irrespective of the wear that might have occurred to the upper portion of the cylinder.


Fig 7.1


Checking rings in the cylinder boreNew piston rings should be checked in the cylinder to make sure that they are correct for the bore size (Figure 7.10).

A quick check can be made with the ring near the top of the bore to make sure that it has a gap. However, before a measurement is actually taken, the ring is pushed down the cylinder with the head of a piston (Figure 7.11). This makes sure that the ring is sitting squarely in the bore. Worn bores will be tapered, and so the ring should be pushed down to the part of the bore that is least worn. This will be below the lower limit of ring travel.
Check the gap between the ends of the ring with feeler gauges. A rule of thumb is that the

Fig 7.11
gap clearance should be 0.03 mm for each 10 mm of cylinder diameter, measured in the unworn part of the bore.
·The ring must not be measured at the top of the bore because its gap will close as it is moved towards the bottom of the cylinder. Without a gap, the ring will break or cause scuffing of the cylinder walls.

 

[Grumpy]

Should I Reuse My Pistons or Order New Ones? How To Tell If Pistons Are Still Good

https://www.memoparts.com/img/cms/Documents/Piston Failue.pdf

Freshening an engine has always been a game of careful disassembly and evaluating the components for wear followed by a series of judgment calls that balance the cost of new parts against further pushing the veterans. Evaluating pistons takes a practiced eye, but there are several checking points that any engine builder can use to help make the right call.

Here’s what to look for with seasoned pistons.

Short of contact damage, normal wear evaluation on a performance piston should start with a quick visual piston skirt check followed by measuring at the piston’s guide point with a micrometer. The guide point is the area on the skirt where the diameter is the largest. On some pistons, this is generally located 0.500″ above the bottom of the skirt, but you should verify this with the piston manufacturer’s individual part number.

If the engine builder recorded the original piston diameter, a simple comparison will reveal any changes. It’s possible to see pistons with partially collapsed skirts from detonation or physical contact problems that otherwise visually check out fine. A minor change in piston-to-wall clearance can be considered normal, but changes in the piston-to-wall clearance of more than 0.002″ should be considered a good excuse for a change.

Perhaps the place where wear can cost the most can be found in the top ring groove. All rings use cylinder pressure behind the top ring to increase sealing load on the ring. This demands a somewhat tight axial or vertical clearance between the ring groove and the piston. Although axial clearance recommendations will vary with specific pistons (and manufacturer), a generic clearance of 0.001 to 0.002″ is acceptable. This can be measured with a feeler gauge between the top of the ring and the groove. Worn ring lands can also exhibit more clearance toward the outboard edge of the groove creating a bell mouth effect, which will negatively affect ring seal.

One of the first things to check on a used piston is that the ring groove clearance is still within spec. Install the ring in the groove and use a feeler gauge to measure the clearance.
Beyond worn ring grooves, high output engines and especially supercharged or turbocharged engines tend to load the top ring with far more cylinder pressure. Micro-welding is a term used to describe the transfer of small amounts of aluminum from the ring land to the ring surface. This material transfer tends to reduce the axial clearance and may in fact contribute to sticking the ring in the groove. Clues that may point to lost ring seal due to mirco-welding include increased blow-by and lost power.

Check for extreme bluing on the bottom of the piston. This can be an indicator that the piston has been exposed to too much extreme heat. This piston is perfectly fine, and on a cooked example, the bluing would fill up the whole area between the struts and be much darker.
Piston rings are designed with vertical clearance so that they can freely move within the groove and are induced to move by the angle of the cross-hatch pattern honed into the cylinder wall. Micro-welding can reduce piston ring movement which also contributes to reduced sealing efficiency. When the rings are removed from the piston, evidence of micro-welding will be pitting in the lower surface of the ring groove and the lower horizontal face of the ring itself. This will be more prevalent with pistons that place the top ring closer to the piston crown as this increases the temperature the ring must face.

Checking wrist pin clearance is a great evaluator of piston condition. A pin-bore gauge can be used for extreme accuracy but sliding the pin in and out on both sides and feeling for binding is also acceptable.
If either the piston or the top ring exhibit evidence of micro-welding, the only solution is a new set of pistons and rings. Avoiding a re-occurrence of this issue involves careful initial ring break-in that allows establishing early wear patterns that remove the tallest peaks early before maximum cylinder pressure is applied.

As mentioned earlier, wrist pin and pin bore condition are also areas that should be carefully inspected. If the wrist pin appears distressed through discoloration or it is difficult to remove from either the rod or the piston, that’s a clear indication the pin not only should be replaced but also to use that as a clue pointing toward changes that will minimize that problem in the future. According to many industry experts, if the pin bore is worn more than 0.002″, the piston should be replaced.

Measure the piston at its gauge point with a micrometer to determine if it has worn significantly. In cases of extreme detonation or heat, the skirts could have collapsed which will show on the gauge.
Other potential failure points include inspecting the rings to ensure the ring end gaps, especially the top ring, has not butted. If you find the top or even second ring end is highly polished, this is a good sign that the ring end gap was too tight. This may only occur under highly loaded conditions when additional heat expands the ring. But this insufficient ring clearance will immediately bind the ring in the groove, causing excessive wear and the possibility of a broken ring land.

Over time, a piston can anneal which means that the base aluminum material has softened. Once this happens, critical machined areas such as the crown and ring grooves will begin to lose their form. A hardness tester (shown) can determine this, but severe discoloration on the underside of the piston is also an indicator.
For engines that see extended use at high engine speeds such as endurance or circle track applications, there can be a concern over loss of tensile strength due to heat cycle annealing, or a softening of the original material’s heat treatment. The only way to know for sure is to send the pistons out for a Rockwell or Brinnell hardness test, which can be expensive as it requires a dedicated testing machine.

Checking piston rings for micro-welding is another good investigative procedure. Microwelding can be a sign of damaged ring grooves, or excessive heat.
Careful inspection of the back side of the piston crown is a great indicator of piston condition. If the back side of the piston crown is discolored black, dark purple, blue or any dark color, this is an obvious warning sign that the piston crown has experienced an overheated condition and has likely gone soft. This can lead to eventual failure so swapping out these pistons would be the smart call. This also indicates that perhaps the air-fuel ratio or ignition timing needs to be more closely scrutinized. Conversely, a tan or light brown color on the piston back side is acceptable, usually caused by combustion heat oxidizing a portion of the crankcase oil.

On any engine rebuild, wire locks and spirolocks should be replaced. These are inexpensive components and not intended to be reused. Changing them is cheap insurance that a bigger, more destructive problem won’t occur.
We’ve just hit the most popular places for potential piston distress but piston technicians are always ready to answer questions beyond what we’ve covered here. It doesn’t have to be a game of chance when it comes to piston survival, especially when you can load the positive statistics on your side of the horsepower equation.

This article was sponsored by JE Pistons. For more technical help and insight, visit the JE Pistons Blog!