Custom ordering pistons - Y-Blocksforever Forums

Author Message Ted Co-Administrator


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DiLL (2/2/) Joe, would you mind sharing the specs on the forged pistons you ordered from JE? I'm in the process of trying to get my pistons ordered and would like a reference. If for competitive reasons you're unable to I understand. thanks!

55blacktie (2/2/)  
Joe, does that include forged flat-tops/w no valve reliefs and metric rings for EBU and C2 rods? Thanks.

Because there were some questions regarding piston ordering asked in a different and somewhat unrelated thread, I have started this post specific to ‘Custom Pistons’.

When ordering custom pistons, the piston rings are the first consideration and are ordered prior to ordering the pistons.  Piston rings are not available as an ‘off the shelf’ item for every possible bore size so the bore size must be determined first.  Depending upon the availability, it may be necessary to juggle the bore size in order to get the ring width sizes you are looking for.  Because of the many small bore metric sized pistons that are available, there are a number of cylinder bore size options but these may not fall in the normal 0.020, 0.030, 0.040” sizes that are typical to the American engines.  In some cases, the final bore size may fall into ranges between those numbers.  When the rings are ordered, you will need the bore size, ring thickness, and radial depth numbers for those rings so that the pistons can be machined specifically for those rings.

A word of caution about ring thickness.  As the rings get thinner, the cylinder wall finish gets more critical.

Lorena, Texas (South of Waco)

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DryLakesRacer Supercharged


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Ted is right on with custom pistons. I’ve ordered way to many and was told in the 70’s by Nick Arias who made 90% of my racing GMC 6 piston to purchase to the ring package and forget the bore as it will be dictated by the piston/ring combo and the pistons were always custom to our engine but standard to others. Pin height, crowns, skirts, etc were not rocket science to them.  I still like 1/16-1/16-3/16” for street use but have moved thinner on both our V8 and 6’s we race. Just like head gaskets have changed dictating the surface machined on the block and heads. Thanks Ted for all you share..

56 Vic, B'Ville 200 MPH Club Member, So Cal.
DiLL Supercharged


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Ted, thank you for starting this thread. I've attached a copy of the JE Pistons order form to use as a reference for questions myself and others might have. For the most part the form seems very self explanatory until I got to the camshaft section where it says "degreed in std +-"  is this in reference to cam advance and retard? My second question is how to find your valve lift at TDC. Is this something that can be calculated based off of the cam card specs alone? My last question is in regard to the piston rings themselves; Axial and radial thickness as well as distance from the piston crown. From my readings, it seems lots of people are going to thinner metric rings packs as they can aid in less weight, friction, etc. My question is, what axial and radial thickness best suit an N/A engine? What size is best for a power adder turbo, nitrous, supercharger? Will the thin metric rings hold up to boost or would a more conventionally sized 1/16, 3/16 ring pack be best when more pressure from boost? How much piston crown is needed for N/A vs boost? 


DiLL
Ted Co-Administrator


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Miles.  I have not needed the cam specs when ordering pistons.  But that ‘Degreed in STD °’ section on the order form is likely referring to the advance or retard for the camshaft.  I doubt that information is going to provide the piston manufacturer much to go on as the Ford Y is likely not in their files like some of the more main stream engines.

For flat top pistons and advertised valve lifts below 0.535”, valve pockets have not been required.  When doing the aluminum heads and going with dish pistons, I do make the dish also accommodate the valves.  This same dish would also work for a blower application depending upon the compression ratio and amount of boost.

Metric rings are available in a variety of widths with 1.5, 1.2, and 1.0mm being the more common ones to pick from for the top and second rings.  3.0mm for the oil rings is pretty much standard fare and available for most bore sizes.  For a boosted street application, I would recommend a 1.5, 1.5, 3.0 set.  For the normally aspirated street engines, the 1.2, 1.5, 3.0 and 1.2, 1.2, 3.0mm sets are my preferred choices.  If available, go with a ‘napier’ second ring as those give an additional measure of oil control.  Stay with the standard oil ring tensions for the metric rings.  Radial thicknesses (depths) are pretty much standard but will vary with the thicknesses so you will need to know what they are when ordering the pistons.

Here are a couple of pictures of the dished piston design.
  

Lorena, Texas (South of Waco)

Ted Co-Administrator


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With the rings selected, it must be determined what the top of the piston looks like whether it’s a flat top, dished, or domed.  The target compression ratio will help to determine that.

To calculate both the static and dynamic compression ratio, the following pieces of information will be required.

Bore
Stroke
Combustion chamber volume
Head gasket volume
Piston location in relation to the deck
Dome or dish volume
Connecting rod length
Camshaft intake valve closing event

Juggling some of the piston values such as the deck height and piston dome/dish volumes will allow the SCR/DCR numbers to be varied.  Once you have the compression ratio numbers in the range you are targeting for, then you’ll get a set of numbers looking like this.
  

Lorena, Texas (South of Waco)

Ted Co-Administrator


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When doing the compression ratio calculations, assume nothing.  If a particular value was not checked, it’s simply a guess.  Crankshaft strokes as well as connecting rod lengths (center to center) should be verified.  When checking the crankshaft strokes, check all four journals.  It’s not unusual for some variance there.  The same goes for the connecting rods; check all eight.  Don’t use factory combustion chamber volumes either.  They are considered ‘nominal’ on the original factory stock heads meaning that’s the best case or smallest scenario.  With any valve jobs and milling during the last half century, there’s no idea where those combustion chamber volumes are without actually measuring.

To order the pistons, you’ll need to know what the target deck height is.  For the Y, I target for 9.750” which allows for a 0.020” - 0.030” cleanup of the stock decks.  If the block has had the decks previously milled, then take that into consideration.  Once the target deck height of the block is known, then the wrist pin location (aka compression height) can be calculated.

That calculation is: Deck Height – (stroke divided by two) – connecting rod length = wrist pin CH
Or Deck Height – ((stroke divided by two) + connecting rod length) = wrist pin CH

For the compression ratio example in the prior post, the compression height of the piston looks like this.

9.750 - ((3.3/2) + 6.320) = 1.780”

Lorena, Texas (South of Waco)

DiLL Supercharged


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Ted  thank you for sharing, I'll be sure to thoroughly verify all the journals and the c/c on the rods. I was finally able to find a good link for calculating static/dynamic compression ratios and was able to repeat the numbers you had posted. This brings me to another question in regard to piston domes/chamber volume and compression. It seems to be commonly stated that for a pump gas engine the dynamic compression ratio is best kept around 8:1-8.5:1. Given the stats you provided while using a 60cc chamber, I calculated a 9.8:1 static and a 7.99:1 dynamic compression. Is it best practice to mill the heads for increased compression or would you recommend adding a dome to the pistons and leaving the chamber as is? When adding boost to an engine, does dynamic compression stay the same or does it too need to be adjusted/lowered to support the added pressure? I hope I'm asking this in a way that it makes sense. My calculations led me to the following:

301" 60cc chamber, 10cc gasket, zero deck ht, flat top piston, 10 psi

static 9.8:1
Dynamic 7.99:1
Effective: 11.75





DiLL
Ted Co-Administrator


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If going with a custom piston, then the preferred choice for altering the compression ratio is to work that change into the piston with a deck height and/or dome/dish configuration.  Cylinder head modifications can prove to be extensive depending upon if trying to open up the combustion chambers to reduce the compression ratio or milling to increase the compression ratio.

For a normally aspirated engine, 8.2:1 DCR is the recommended maximum but that 8.2:1 number does not leave any room or tolerance for premium grade fuel that is less than advertised.  For that reason, it's typically best to target for 8.0:1 DCR to leave just enough latitude for gasoline that may be slightly less than 91 octane with an engine tuned for 91-93 octane fuel.

When adding boost to an engine, the compression ratio is being artificially increased so the DCR number must be decreased accordingly.  My rule of thumb is to reduce a full point of compression for each half an atmosphere of boost.  A full atmosphere of boost is 14.7 psi so half of that would be 7.4 psi so if going with 7-8 psi of boost, then lower the DCR a full compression point.  Instead of using the 8.0-8.2 DCR value as used for a normally aspirated engine, target that DCR number for 7.0-7.2 DCR for 8 psi of boost.  All these numbers are for readily available pump premium gasoline (91-93 octane) so if using additives or racing fuel, then a higher compression ratio would be sustainable.

Lorena, Texas (South of Waco)

Ted Co-Administrator


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If going with 10 psi boost, then that calculates to be a 68% increase in atmospheric pressure.  The compression ratio would need to decrease approximately 1.36:1 compression points in order to compensate.  For the changed values you posted above, the new calculation would look something like this.

Lorena, Texas (South of Waco)

55blacktie

Repeated cycles of searing combustion heat alternating with cool incoming air and fuel, extreme load reversal, thrust forces that slam the pistons into the cylinder wallsits quite amazing that aluminum pistons can survive in a performance engine at all. That they do is a tribute to todays aftermarket material science and advanced manufacturing techniques. Technology has advanced to the point that what just a few years ago were considered custom race-only parts are now available at relatively affordable prices for even everyday performance use. Big-time companies like Federal Mogul/Speed-Pro are introducing new-generation lightweight mass-produced parts, while on the extreme high-end, new-tech niche manufacturerssuch as HTC Products and CP Pistonsare working with advanced design processes and exotic materials to merge rocket science with race science.

Developments have focused on better materials, shaving weight, improved manufacturing precision, computer-aided design, and CNC-machining processes. Better materials and weight savings go hand in hand; stronger materials can be made lighter without sacrificing durability. And tighter manufacturing tolerances ultimately translate into better, closer-fitting parts that can be installed with reduced clearances for better sealing. Computer-aided design combined with finite element analysis allows testing the part before it actually leaves the drawing board, ensuring it is both as light and as strong as possible. With CNC milling, custom pistons are easier than ever to make, with lead time often in days instead of weeks and months. And what used to be custom orders are now in many cases in-stock items.

Pistons

The first step up from the common cast piston is the hypereutectic casting, which is strengthened with additional silicon content added to the aluminum brew. As with a conventional cast piston, a properly designed hypereutectic piston permits relatively tight piston-to-wall clearances, making for less noise under cold-start conditions.

All cast pistonsbe they standard or hypereutecticare made by pouring melted aluminum into a mold that shapes the slug into a piston. In contrast, forged pistons are formed using a giant press that takes a block of metal and pounds it into shape under thousands of tons of pressure. The tooling needed to do this is much more expensive than the tooling used to make a casting, and it wears out quicker. This makes forged pistons more costly than castings.

However, forgings have inherent advantages in terms of density, ultimate strength, and durability. Forging eliminates metal porosity, improves ductility, and generally allows the piston to run cooler than a casting. Within reason, forgings can be lightened without adversely affecting structural integrity. However, forged pistons expand and contract more under changing temperatures, so they traditionally require greater piston-to-wall clearance than cast pistons. In recent years, CNC-machining processes, diamond tooling, and careful attention to piston skirt profiling has given piston makers the ability to finely adjust skirt contact areas for more even loading. Barrel-type profiles now accommodate greater expansion at operating temperature. One result is that todays short-skirted pistons have better contact areas than the old long-skirt designs, and wear is reduced even as piston-to-wall clearances are tightened up.

Forged pistons are generally made from one of several different aluminum alloys, with each offering different benefits depending on the application. The two most popular alloys are and . Speed-Pro typically uses VMS-75, which is fairly close to both contain about 11 percent silicon, which helps with ring groove and skirt durability. These are the best choice for applications expected to have decent longevity, such as street vehicles and entry-level bracket racing and oval track combos. Although has better high-temperature characteristics, it contains virtually no silicon. This material expands and contracts more, so greater bore clearances are needed to prevent scuffing. Pistons using are best suited to nitrous, blowers, or higher end race applications where frequent inspection and replacement are not a problem.

If you are looking for more details, kindly visit Custom Piston Ring.

A recent innovation is Ultralloy, a secret patented ceramic-aluminum alloy presently available from HTC Products, a premier manufacturer and distributor for most brands and types of cranks, rods, pistons, and rings. The silicon particles in Ultralloy have unprecedented uniformity in terms of their size, shape, and dispersion in the aluminum matrix. The new alloys strength is on par with titanium (and costs less) and parts can be made much lighter.

One of the most important advantages of forged pistons is what happens at the point of piston failure. Under extreme conditionslike detonationforgings tend to go plastic and fail gradually. You generally have time to replace them before the entire engine is toast. Hypereutectics, although relatively strong in terms of ultimate tensile strength, have less ductility and are prone to fracture when their limits are exceeded. However, a lightweight hypereutectic piston specifically designed for high-performance use can withstand considerable cylinder pressures if the tune is right. When considering which style of piston is right for your application, you should consider how much abuse the piston will see, your budget, and the need to remain competitive in your form of racing. Sustained heat is the biggest piston-longevity limiter.

Rings

Piston rings perform a number of important functions. They seal the gap between the piston and cylinder wall to prevent combustion gases from blowing by into the crankcase. They stabilize the piston as it travels up and down in the bore. They help cool the piston by transferring heat into the engine block. And they scrape oil off the cylinder walls. Thats a tall order, and in recent years the theory on how to make rings best carry out these tasks has undergone revision.

Old-school thinking followed a brute force approach: Make everything as rigid as possible to force the rings into contact with the walls. Today, the trend in current production and racing engines is towards a more flexible ring package that better conforms to the cylinder wall. Back in the musclecar days, most production engines used a 5/64-5/64-3/16-inch package. The 1/16-1/16-3/16-inch packages were for all-out racing. These days, Detroit automakers and many racers are gravitating toward even thinner metric rings. Standard-tension oil rings have been replaced by low-tension rings. Many of the new ring packages feature reduced radial wall thickness. Besides decreasing friction, this makes for a more stable packageassuming the piston rings, piston profile, and cylinder wall finish take advantage of these improvements. In the custom piston world, most build-to-order pistons can be ordered for reduced radial thickness rings; otherwise, spacer stock can be used to convert conventional pistons.

Ring groove design is far more important than it may appear at first glance. Properly designed ring grooves have a small degree of vertical uplift, which compensates for uneven temperature growth as the piston reaches operating temperature. Ring groove smoothness is likewise extremely important; any waviness or roughness causes poor ring seal and can lead to microweldinga destructive situation where, under extreme pressure, the rings momentarily attach themselves to high spots on the ring groove. There also should be a small radius where the vertical and horizontal portion of the ring grooves meet. Pistons without this radius are more prone to groove distortion and ring land breakage.

Thinking on piston ring gaps has also changed. In the old days, second ring gap specs were tighter than those for top rings because they didnt see as much heat. But this didnt account for inter-ring gas-pressure buildup between the top and second rings. If the pressure between these rings equals or exceeds the pressure above the top ring, it can cause the top ring to lift off the bottom of the piston ring groove and lose contact with the sealing surfaces. It also inhibits the rings ability to transfer heat from the piston. To keep inter-ring pressure from becoming a problem, the current trend is to create an easy escape path for the built-up pressure by gapping the second ring larger than the top ring. Another benefit is that because gas pressure is now directed downward towards the sump, any oil that has collected in the ring pack areas will go with it.

Of course, normal ring wear causes the gaps to open up, allowing more combustion gases to escape. At least one ring manufacturerTotal Sealoffers gapless rings. Traditionally, these gapless rings went in the second groove along with a conventional top ring, but ring technology refinements plus the new thinking on ring sealing has led Total Seal to revise this installation scheme and introduce a new line of gapless top rings that achieve significantly less blow-by under real-world running conditions.

The ultimate in ring seal is drilling the pistons for gas ports. Compression rings normally need about 0.002-0.004-inch (vertical) ring-to-groove side clearance to allow cylinder pressure to get behind the ring and force it to seal against the groove and cylinder wall. Gas ports apply combustion pressure directly to the back of the ring, allowing the virtual elimination of side clearance. Since the ring is restrained by the groove itself, theres less opportunity for high-rpm ring flutter. Very thin, narrow, and lightweight 0.043-inchthick rings are needed to reap gas-portings full benefits. Gas ports work best with short piston-compression heights (under 1.200 inches) on engines running 7,000 rpm or higher. The major drawback is that all this positive pressure greatly shortens ring life, so its not recommended for street use.

No matter the specific thickness and configuration, most high-performance and racing engines now use moly-faced rings in the top groove. Plasma-sprayed moly over a ductile-iron base material is the preferred choice, but steel is becoming more popular because its at least as strong and easier to machine.

Chrome-plated rings still have a place in off-road or dirt-track applications. Just as high-end pistons are now machined to close tolerances, many racers now custom-prep (remachine, if you will) piston rings to higher tolerances to reap the full benefits of the new high-tech pistons. For example, precision ring grooves allow a reduced back clearance if ring thickness tolerances are likewise more tightly controlled. Custom-prepped precision-gappable rings are also offered by several aftermarket ring makers such as Total Seal.

Matching rings and piston design only reaffirms what weve been stressing for years: When it comes to engines, theres no magic bullet or individual component. Everything has to be considered as part of a total package. Lightweight, close-tolerance pistons demand higher-quality rings. But to work they require a higher level of cylinder wall preparation. And reducing friction by running low-tension oil rings may mandate trick oil pans with windage trays, crank scrappers, adjustment of bottom-end bearing clearances, lightweight synthetic oils, and even positive crankcase evacuation pumps. Nevertheless, custom piston makers assert that the right high-end piston and ring combination can be worth up to 30 hp on a 1.5hp/ci engine if the rest of the combination is optimized to take full advantage of them.

For more information, please visit motorcycle piston ring.