Piston Propositions

Tell the truth. Did you seriously consider the piston choices when building your last engine? Most hobbyists concentrate on the head flow, the cam specs, the intake choice, carb size, headers and many other aspects of the combination. The most thought directed toward the piston choice is deciding whether or not to upgrade to forged slugs or adjusting compression ratio.

Many don't realize that pistons can make power...or not if you make the wrong decisions. In most cases more compression means more power. What are two of the main factors that determine the compression ratio? The chamber size and piston design. What aids in providing a complete burn of the mixture besides the chamber shape? The top of the piston design. What happens when a piston with poorly designed ring grooves makes the rings lose their seal? You lose power. Piston weight also contributes to or detracts from performance. Lighter pistons reduce reciprocating weight and allow the engine to rev more quickly, which gets you to the traps sooner.

So to make life a bit easier for your next engine build, HPP dropped in on Moe Mills of Ross Racing Pistons to find out how a forged custom piston is made.

Recognizing the need for a custom piston manufacturer to serve the racing industry Ken Roble (who has an incredible hot rod collection) and Moe Mills founded Ross Racing Pistons in 1978 in Ken's garage. Through tireless product development over the past 25 years, Ross has provided the industry with custom race pistons covering American Motors to Volvo and just about every make in between including Pontiac, from the 151 Iron Duke to the 455. Today, Ross' facility covers 25,000 square feet of So. Cal. real estate and a major upgrade in manufacturing equipment in 2002 enables the company to employ the latest technology for developing and manufacturing high quality racing pistons.

Since we had Moe's ear we decided to ask a few questions that we thought you may want answered. Here's what he had to say.

HPP: What should buyers look for in a custom piston manufacturer?
Ross: For a specialty engine such as a Pontiac, make sure that they have the correct forgings and proper tooling to do them. This of course is not a problem with small-block Chevys. Then make sure that the company has a good reputation and a fair price. The highest price doesn't always mean the highest quality.

HPP: What are common mistakes that hobbyists make when choosing pistons?
Ross: The first one that comes to mind is that many want too much compression for the combination that they are building. There are plenty of fast engines that run on 9.5:1. Another mistake is to use tool steel pins in street engines. They are real light and not recommended for the street because they flex in operation. The pins have excellent memory and return to their original shape but the flexing opens up the pin hole and the small end of the connecting rod, which causes the pistons and rods to crack. So the piston and rod is blamed but it's not their fault. Also, the tool steel pins are only good for about 10,000 miles, which is fine for a race engine but may come quick on the street.

HPP: How are pistons that are designed for nitrous, turbo or supercharged engines different from pistons for normally aspirated applications?
Ross: We make the piston thicker at the top, actual dimensions depend upon the engine type and the distance between pin bosses. The wider it is, the thicker the top must be. We also move the top ring down and the land between the top and second ring is made wider. Typically on carbureted Pontiacs the land is .125-.160. For nitrous pistons it would be more like.190-.250 wide. But remember this is for serious nitrous applications where the advertised increase with nitrous is a 250hp shot or higher. For engines with a typical 150hp shot or less, the piston we produce for straight gasoline engines is fine. If a lot of nitrous is run, like in a Beswick motor, a different forging is used to keep the skirts from collapsing. All of this extra material however adds weight to the piston.

HPP: What is the boost limit for turbos and superchargers before the piston you described above is needed?
Ross: Generally at 7-8 pounds this upgraded piston is recommended.

HPP: Do customers ever ask for the impossible?
Ross: Yes. Some customers ask for the heaviest duty piston possible and want it to be light at the same time. The problem is that much of the strength comes from using more material, which makes the piston heavier. Others request the deepest valve pocket possible but want 12.8:1 compression with 111cc heads on a 400. Of course, the material removed for the valve pocket is going to reduce compression.

Standard Features of Ross Custom Pistons
Compression ring grooves are machined to SAE D-wall specifications on bore sizes of 3.5 inches or higher meaning all Pontiac and Chevy V-8s.

* Radius valve reliefs: keeps the pistons from cracking on top. If you make the pocket with a real sharp cutter it can leave stress risers that cause cracking. This method eliminates that situation.

* Second land pressure reservoir: is an area where some compressed air/fuel mixture can accumulate. If you run a gapless top ring, it's not needed. But if you run gapped rings, on the power stroke, cylinder pressure escapes through the end gap in the top ring and gets trapped in the top and second ring. When the exhaust valve opens there is more pressure acting on the top ring from under it than from above it, which can unseat the ring from the cylinder wall and ring groove causing ring flutter. Ross' second land reservoir provides an accumulator area to hold the pressure and allow time for it to dissipate so the ring does not unseat during the exhaust stroke. The closer the top and second rings are to each other, the more important the reservoir is. Moe recommends, "If you run .018 gap on the top ring, you should run more gap on the second ring (instead of the common practice of running a tighter gap on the second ring) so that pressure will go into the crankcase. Yes this is blow-by but it's better than ring flutter. The higher the cylinder pressures the worse it gets. But a 400hp 400 is not going to produce enough cylinder pressure to cause this problem."

* Double Spirolox: are the most effective way of keeping the pin in the piston in full floating applications
* Machined side reliefs: to cut weight
* Lock removal indents: allow easy removal of double Spirolox with a small screwdriver
* Annular pin oiler grooves, which intersect the forced pin oilers: pin oilers scrape oil from the cylinder wall and force feed it to the pins. Two holes are drilled from the center of the oil ring groove into the pin boss then two holes are drilled from pin holes to intersect the first two holes. Next, a groove is machined into the pin hole that connects the two holes to spread oil much like a grooved main bearing.

Decisions That You Must Make

* Spacing between the ring grooves: If you are buying custom pistons, you have a choice of ring groove positioning. When you reduce the distance between the rings, the ring pack effectively moves higher on the piston creating narrow lands and a lighter piston. There is a practical limit however. If the lands become too narrow, detonation or high cylinder pressure will break them. Ross will make specific recommendations for your Pontiac or small-block Chevy engine based on experience to give you a safe land thickness.

* Compression height: is the distance from the center of the pin hole to the deck of the piston. Compression height can be adjusted when ordering custom pistons to work with stroked or destroked combinations, long rods, or blocks with much lower than stock deck height.

Compression height formula:
Compression Ht. = Block Deck Ht. - 1/2 Crank Stroke - Rod Length - Piston to Deck Ht.
For a 455 Pontiac the equation would look like this:
10.230 - 2.105 - 6.625 - .020 = 1.48 compression height

Extra-Cost Options
Moe told us that these processes are available for Pontiacs and small-block Chevys, but he doesn't feel that any of the milling operations would render results that justify the money spent to do them for these applications. In order to save you some cash, we still explained what they are so at least you know what you don't need as well as what you do need. * Window milling: consists of removing metal and creating "blank windows" on the inside of the skirts. This milling operation removes considerable weight but doesn't compromise skirt strength. It works best on long skirt pistons.
* Internal profile milling: using a vertical CNC machine the inside shape of the piston mimics the outside shape. Great for unusual custom pistons.
* Inside head milling: removes material not required to maintain piston integrity from underneath the dome.
* Pin boss milling: removes metal from the area between the top of the pin hole and the inside of the piston head. Not good for endurance applications.
* Maximum internal milling: includes all of the lightening milling processes that do not compromise strength.
* Valve pockets: special diameter, depths, angles and locations are available.
* Gas porting: consists of drilling small holes through the tops of the pistons, which allow cylinder pressure access to the back of the top ring to force it out making it seal more tightly to the cylinder wall. They are most effective with tight ring grooves and high ring positioning. Moe doesn't recommend vertical gas porting (holes drilled through the top of pistons) for street applications. First, the engine probably doesn't develop enough cylinder pressure to unseat the rings and second vertical ports plug up with carbon in a street engine after a short time. As a result vertical gas ports are most often use for drag engines and horizontal ports (drilled through the side of the pistons) are used on circle track and road race engines (and on street engines...sometimes). Unless you have 600 hp or more, Moe says not to worry about gas porting.

How a Custom Piston is Made
As with almost everything in life nowadays it all begins at the computer. The design parameters such as head type, bore size, etc. taken from the customer's order blank are entered into the computer. Then spreadsheet info that Ross has developed over the years, which contains all of the dimensions of the piston to be produced is entered into the computer. Using Pro Engineer software, a cyberspace model of the piston is created and displayed on the screen with all pertinent dimensions. From this, a job card is produced for the manufacturing machines to follow to make the piston. See the photos and the captions for the machining processes.

What Information Must the Customer Provide?
* Engine type, model and year
* Engine usage (road racing, drags, street, etc.)
* Bore size
* Type of fuel
* Will nitrous or forced induction be used?
* Compression ratio desired
* Crankshaft stroke
* Method of payment
* Method of shipment
To make ordering easier, Ross asks that you provide the following information only if the answer is not "stock":
* Rod length
* Head type
* Combustion chamber cc
* Amount heads were milled
* Valve diameters
* Intake valve location when it's on the seat, flush, amount recessed, amount raised.
* Camshaft lift at 6 degrees before and 6 degrees after top dead center at split overlap
* Type of rings you wish to buy, if you wish to use your own rings, ring width and radial thickness measurements must be supplied
* Type of pins you wish to buy, if you use your own pins diameter and length measurements must be supplied
* Options that you wish to add
* Gasket thickness and block height
* Type of rods
* Do you want Ross to pin fit your pistons?