Billet Pistons
(Article begins on the Tech, Tests & Installs page HERE)

The enthusiast community measures engine power in terms of horsepower and torque. But cylinder pressure read as BMEP and IMEP are more accurate indicators. These abbreviations refer to Brake Mean Effective Pressure and Indicated Mean Effective Pressure, which are more precise measurements since gas pressure in the cylinder varies from a maximum at the beginning of the expansion stroke to a minimum near its end. Also, these classifications can very accurately compare the power of engines of different displacements since it is the pressure per piston area that is being examined.

Recognizing that cylinder pressure is the true dynamic that creates horsepower compels one to consider the piston and the forces applied to it. In high performance and racing applications the piston needs to withstand and then transfer the cylinder pressure to the crankshaft while it also maintains its shape, provides a long service life, partakes in sealing the bore, and compliments and not detracts from the effectiveness of the cylinder head and combustion chamber. That’s a long list of tasks!

Over the years, the performance engine-building community has gone from using an original-equipment-style cast piston to a stronger forged design that also allows more freedom in manufacturing and a reduction in the reciprocating mass. High quality, advanced aluminum materials are employed and forged pistons are now available with outboard or with the narrower inboard pin towers. Additionally, they provide excellent load paths and rigid structures and attract an array of competent coatings to protect them. However, the forged piston has one major shortcoming: infinite design potential. If, for example, a piston designer or engine builder or race team wishes to change the piston structure or adopt different load paths or experiment with different struts and buttresses, only a billet piston will accommodate these requirements.

NASCAR teams were quick to recognize the billet’s potential, taking advantage of its versatility. They continue to develop and test new designs constantly. Among piston producers, Diamond Pistons was one of the first production houses to embrace the new technology, initially making billets available to NHRA Pro Stock teams. Nonetheless, bringing a cost-effective billet piston program to the general market turned out to be a formidable challenge—often cost prohibitive and, therefore, unsustainable. In fact, the program only became viable when they established a special department with dedicated engineering staff and equipment. Once they had established the technique, however, race engine builders no longer needed to work within the confines of a forging. Many engine builders openly admit they felt they were on a short leash with any forged piston. Not being able to have it fully meet their requirements they compromised as best they could. Race teams not only openly embrace the freedom that billet pistons offer but also the ability to re-examine the cylinder head, combustion chamber and valve angle for further power gains. The possibility for another great leap in power is one of the most exciting aspects of the billet piston.

With regard to material strength there is no appreciable difference between the forging and the billet piston. As mentioned earlier, most of the NASCAR teams and the NHRA Pro Stock elite are already using billet and have proved its durability. Some NASCAR teams switched to billet pistons because they detected slight variations from forging to forging. Pro Stock and other high-rank drag racing teams longed for the opportunity to experiment with piston designs not possible with existing forgings. In addition, for highly competitive race teams, having access to a billet piston program provides them with much prized exclusivity—they prefer their secrets to remain safe.

With a forging program, the piston maker’s position is greatly compromised. When a new forging is needed he is compelled to invest in new tooling, often costing in excess of $10,000; obliged to wait months before receiving the forgings; and often required to purchase the first 500 slugs from the new tooling. Obviously, the piston-maker has to amortize the costs and as a result everyone benefits from the great idea, and exclusivity is minimal. However, a billet program eliminates the need for special tooling, associated delays, and minimum-order quantities. Now the great idea remains the property of the one who conceives it. Usually machined from a solid piece of 2618 billet aluminum, the piston has an expansion rate slightly greater than its forged counterpart. Most users set the piston-to-wall clearance between .0065in to 0.008in. With regard to weight, the billet version is typically one to two percent lighter than a comparable forged piston for the same application.

Of course billets easily accommodate reduced skirt areas, which minimize friction and weight. They also permit the optional use of buttons instead of spiral locks. Buttons make it much easier and quicker to change pistons should the need arise. In addition buttons prevent the expander in the oil control ring from distorting around the half-moon openings in the back of the groove on the piston where the pin bores intercept the oil control ring groove. As already stated billet pistons are well-suited for teams embarking on new engine development programs. These programs often require last-minute design changes that can affect bore sizes, cylinder head configuration, valve sizes, valve pocket depths, pin boss dimensions or load paths.

Usually billet pistons are available in a range of finishes. Diamond furnishes them in a natural finish or hard-anodized or with a ceramic crown coating and a moly skirt coating. Hard-anodized coatings help prevent scuffing and galling of the cylinder bores under extreme conditions.
In the early days of motor sport, slang for a piston was "slug", a term that suggests a limited amount of engineering—how false that is. With the introduction of affordable billet pistons, engine development just took another huge leap forward.

Text by Ray T. Bohacz; photos by Moore Good Ink

Diamond Pistons

Forgetting cast pistons, forged pistons have been the mainstay of the aftermarket for years. However, there are some limitations involved with their use.
Billet pistons such as the one on the left are machined from a solid bar of aluminum. They are ideally suited to teams who require frequent changes to cylinder head configuration, to bore and valve sizes, and to valve pocket depths, pin boss dimensions or load paths
While it may look like a lot of work to whittle a finished piston from a bar of aluminum, with the advent of today’s CNC machining centers it takes very little effort once the original design is programmed into the machine.
Whether forged or billet, the piston must be machined in an oval manner with a taper to the piston skirt to allow for expansion, due in part to the differences in heat applied at varied areas across the piston. However, changes in the design of struts and buttresses or extra material requirements for the underside of the piston crown are much easier to accomplish with a billet product.
Before custom design begins, the Diamond people send out to each customer a job card which identifies, among other details, the fit band—the widest part of the piston. The fit band is usually placed between .500in and 1.300in beneath the oil control ring.
The billet piston presents unlimited design opportunities: it allows for the creation of a complete custom piston—struts can be placed anywhere the engineer desires. For those developing a new engine program, billet pistons offer the better prospects.
While there might be no appreciable differences in strength between a forged and billet piston, the billet offers the engine builder greater versatility in structural changes and also much prized exclusivity—top engine builders prefer their design secrets to remain safe.
NASCAR teams might have been the first to use billet pistons, but that technology has spilled over into Pro Stock drag racing as well as other classes. Along with a measure of exclusivity, it’s given teams the ability to experiment with piston designs hitherto not available with a forging.
Coatings have also played a role in piston design and the process begins by submerging the pistons in a detergent acid bath and in rinse tanks to prepare them for the coating phase involving an electrolysis process and several rinse cycles.
Embraced by NASCAR and Pro Stock, production billet pistons are now available to all with engine development programs.