What is Forging? The Ins and Outs of Manufacturing Forged Pistons


When it comes to overall strength, there's no beating a forged piston. But what is the process that yields the toughest parts in the racing world? We'll show you. 

When it comes to turning raw metal alloys into useful things, two processes dominate - casting and forging. Both have their place, but when strength and light weight are priorities, forging is the method of choice. Though it’s been around for more than six millennia, forging processes continue to advance the state of the art, bringing us everything from sharper, more durable kitchen knives to more fuel efficient jet engines, plus things much closer to our heart: lighter, stronger pistons.

Wiseco Forged Piston

Although forging is a metalworking process thousands of years old, it’s still the best method to produce components with the highest strength and durability.

Forging is defined as the controlled deformation of metal into a desired shape by compressive force. At its most basic, it’s a blacksmith working a piece with a hammer and anvil, and those first metalworkers toiling at their forges discovered something important about the pieces they were crafting – compared to similar objects made from melted and cast metal, they were stronger and more durable.

Though they knew the finished product was superior, what those ancient smiths didn’t suspect was that the act of forging was changing the internal grain structure of the metal, aligning it to the direction of force being applied, and making it stronger, more ductile, and giving it higher resistance to impact and fatigue. While a cast metal part will have a homogeneous, random grain structure, forging can intentionally direct that structure in ways that give a finished part the highest structural integrity of any metalworking process. 

Wiseco Aluminum Bars Wiseco Aluminum Slugs

Wiseco forged pistons start as raw bar stock in certified 2618 or 4032 aluminum alloy. Once they’re cut into precisely-sized ‘pucks’ they’re ready to be preheated in preparation for forging.

Although many performance enthusiasts might put billet parts at the top of the heap in terms of desirability, the reality is that the billet they are created from doesn't have the same grain properties of a forging. 

Wiseco Forging Press

The Wiseco Forging Process

Today’s state of the art in forging technology is far removed from the smith’s bellows-stoked fire and anvil. In Wiseco’s ISO 9000-certified forging facility, pistons begin life as certified grade aluminum bar stock, cut to precise lengths to form slugs. The choice of material is critical - conventional wisdom has always said that a forged piston requires additional piston-to-bore clearance to allow for expansion, leading to noise from piston slap until the engine gets up to temperature, but per Wiseco’s Research and Development Manager David Fussner, “Forged pistons do require additional room temperature clearance. However, the 4032 forging alloy we use has about 12% silicon content, and this significantly controls the expansion to nearly the same as a 12% silicon cast piston. The 2618 alloy expands a bit more and does require a bit more room temperature clearance than 4032.”

Wiseco_forging_TT_04.thumb.jpg.7e282a81e970d5a4fa4aae80ae4dccfd.jpg

Pistons are forged in a ‘backwards extrusion’ process where a moving punch presses the raw material into the die to form the rough shape. The process takes only a fraction of a second (longer in the isothermal press), and the speed of the press helps determine how material flows, and therefore the internal grain structure of the forging.

While 4032 is more dimensionally stable across the typical operating temperature range seen inside an engine, it does give up a small advantage in ductility to 2618, which has a silicon content of less than 0.2 percent. This makes 2618 a better choice for applications where detonation may be an issue, like race engines running high boost or large doses of nitrous oxide. The low silicon alloy’s more forgiving nature in these instances makes up for the tradeoffs in increased wear and shorter service life compared to 4032.

Once cut to the proper size, slugs are heated to a predetermined temperature and moved to the forging press itself, which is also maintained at a controlled temperature. There are two different types of presses employed at Wiseco; mechanical and hydraulic. Both have a long history in manufacturing, and each has specific strengths. Mechanical forging presses are well-suited to high production rates, helping to keep the overall cost of high-quality forged components affordable. Hydraulic presses have the advantage of variable speed and force throughout the process, allowing greater control of material flow, which can be used to produced forged components with even more precisely controlled physical properties.

Wiseco Forging

Wiseco’s isothermal hydraulic press forging machines use precise digital control of the temperature of the raw material, the punch, and the die, as well as the pressure exerted during the full motion of the forge. This allows very close control over the physical properties of the finished forging.

Regardless of the type of press, pistons are forged using a “backwards extrusion” process where the material from the slug flows back and around the descending punch to form the cup-shaped forging. Picture the stationary part of the press (the die) as the mirror image of the piston top, and the punch as the mirror image of the underside. As the punch descends, the puck is transformed into the rough piston shape with material flowing up along the sides of the die and punch to form the skirt. This entire process takes place on the scale of milliseconds (on the mechanical press), and the all-important flow stresses of the material are determined by the strain rate (or speed) and load applied by the press.

In addition to three mechanical forge presses, Wiseco also has two isothermal hydraulic presses in-house. These state of the art forges maintain the temperature of the piston slug, the die, and the punch very accurately through computer control, delivering more precise dimensions and geometry for the finished pieces, as well as allowing for more complex designs to be successfully forged, and even the creation of metal matrix composite forgings.

Wiseco Piston in Process

Once the puck (left) has been transformed into a forged blank (middle), it still has a ways to go before becoming a completed piston (right).

The Heat Is On

Once the forging process is complete, the components next move to heat treatment. Wiseco’s aerospace-grade heat treatment facility is located in the same plant as the presses, and here the pistons go through a carefully controlled process of heating and cooling that relieves stress induced during forging, increases the overall strength and ductility of the metal, and provides the desired surface hardness characteristics. 

While casting can deliver parts straight out of the mold that are very close to their final shape, forgings require a bit more attention in order to get them into shape. Fussner explains, “In a dedicated forging for a specific purpose, the interior of the forging blank is at near-net as it comes off the forging press.  And in some cases, we also forge the dome near-net with valve pockets and some other features. Other than these items, most other features do require machining.”

Wiseco Clutch Basket

Pistons aren't the only thing Wiseco forges and machines in-house. Wiseco clutch are also forged and machined, as well as finished with hard anodizing. The forging (left) allows the basket to closer to the final shape before machining. The basket shown here is just post-machining.

One basic forging may serve as the starting point for many different types of finished pistons, unlike castings which are typically unique to a single design or a small group of very similar designs. Regardless of the manufacturing method for the piston blank, some degree of final machining needs to take place to create a finished part. “As a ballpark percentage, I would say about 75% of the forging blank would require machining.” Cast pistons also require finish work on the CNC machine, but this is almost always less extensive than a similar forged piston. “That’s the main reason why forged pistons are more expensive than a cast piston,” Fussner adds. 

Another reason for the added expense of forging is the significant cost of the initial tooling for the die and punch, which must be made to exact specifications and be durable enough to survive countless forging press cycles. Per Fussner, “We control these costs by making all our forging tooling in house at Wiseco headquarters in Mentor, Ohio.” The ability to make their own tooling, doing their own forging, and their in-house heat treatment facilities make Wiseco the only aftermarket forged piston manufacturer in the United States with these unique capabilities.

Once the machining process is complete, Wiseco pistons can also receive a number of different proprietary coatings to fine-tune their performance. These include thermal barriers as well as wear reduction treatments.

Though forging is a technique literally as old as the Iron Age, it’s still the undisputed king of manufacturing techniques for light, strong, durable components. Wiseco continues to refine the process with the latest methods, materials, heat treatment, and machining to provide the highest quality aftermarket components available, at an affordable price.

Wiseco_forging_TT_06.jpg.958c1615e1b1fe5f4820982b9f486dc0.jpg

Wiseco forged pistons provide superior quality and performance at an affordable price thanks to the company’s close control over every step of the manufacturing process.

12 people like this




User Feedback


Man, I can only imagine the investment #wisecohas made to be able to produce the massive line of parts you guys sell. Pretty impressive ops! Good info to understand when buying shiny bits of aluminum for your bike, especially internals. Thx for sharing!

Share this comment


Link to comment
Share on other sites
On 5/31/2018 at 9:24 AM, Bryan Bosch said:

Man, I can only imagine the investment #wisecohas made to be able to produce the massive line of parts you guys sell. Pretty impressive ops! Good info to understand when buying shiny bits of aluminum for your bike, especially internals. Thx for sharing!

Thanks Bryan! It's been over 75 years of investment. There's no replacement for experience!

54 minutes ago, DHix said:

You have sure gone a long ways, From the pistons you made in the 1970's

Thanks, DHix!

Share this comment


Link to comment
Share on other sites

That is so interesting , i used to run the big ajax forge press there years ago it is great to see this company thrive i loved that job and am glad to see pics of the forge shop, keep up good work gentlemen and ladies i will always support your products sincerely 

Paul Stephen Adkins

Edited by Paul Stephen Adkins
1 person likes this

Share this comment


Link to comment
Share on other sites


Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


  • Similar Content

    • By TheHumbleBraap
      So just picked up a 2002 Yz250 a few days ago. Bike was running ok and all seemed to be in pretty decent shape for a 2002. Was going through a full service on the bike. Plan on going through most of the whole bike. Went to pull the head to see what the cylinder was looking like and found what I would call a stain all along the cylinder walls. When I run my finger along the walls everything feels smooth. I feel no ridges or rough spots everything feels smooth. I don't have a bore gauge so I cannot measure the bore. I planned on putting a fresh top end in it, but now I am wondering if I should just go ahead and send the cylinder off to Millennium and let them get the cylinder back up to spec before I put a new piston and rings in. Just wondering what you guy's might think could have caused the discoloration.
       



    • By ThumperTalk
      Whether you're a seasoned wrench who justs wants to compare notes or a top-end rebuild No0b, here's a great article on the subject, including pics. Give it a read, share any comments in the comment section, and share with an any 2 stroke ridin' buds that might benefit from it.

      https://thumpertalk.com/articles/how-to-rebuild-the-top-end-in-your-two-stroke-r878/

       
    • By Rob@ProX
      Rebuilding a top end is a task most two-stroke owners will run into at one point or another. Here, we go over critical steps and key tips to installing a new piston and ring(s) in your two-stroke.
      Periodically, if you own a two-stroke, there will come a point where you need to rebuild the top end of your engine. Hopefully, this won’t come as a surprise to you and will be part of your planned maintenance schedule versus experiencing an unplanned engine failure. While two-stroke engines are relatively simple mechanical devices, rebuilding them requires knowledge of how they work, attention to detail, and a systematic approach.

      We’re going to cover numerous tips pertinent to two-stroke top end rebuilds. These tips will be discussed chronologically and will encompass all phases of the build from pre-rebuild prep, to disassembly, through post build. The tips we’re going to share shouldn’t be considered inclusive of everything that has to be done, but are tips that focus on things that are either often overlooked or incredibly important. Let’s get started!
      Pre-Teardown
      Diagnosis  - Before tearing the engine apart, are there any signs that a specific problem exists? If so, are there any diagnostic tests such as compression or crankcase leak down that are worth performing?

      Before tearing your engine down, asses the specific problem with you're engine if you're rebuilding due to a running problem.
      Clean Machine - Take time to thoroughly clean the machine before opening up the engine, especially if you will be servicing the top end without removing the engine from the machine.
      Service Manual - Performing engine maintenance without an OEM factory service manual is not recommended. Make sure you have a manual for your machine prior to starting work. The manual is the only place you’ll find service limits, torque specs, and other key data.
      Disassembly
      Limit Contaminants - Once the cylinder has been removed wrap a clean, lint-free rag around the top of the crankcase. Dirt is one of the leading causes of engine wear, and limiting the opportunity for dirt to enter the crankcase is very important.

      Keep a lint-free rag at the top of the crankcase at all times while it is open and exposed to potential contaminants.
      Piston Removal - Easy piston circlip removal can be accomplished by using a pick and needle nose pliers. Insert the pick into the dimple in the piston and behind the circlip. Then use it as a lever and pry the circlip out partially. Once out partially, grab the circlip with needle nose pliers. During this process, be careful not to scratch or mar the wrist pin bore as this will make removing the wrist pin much more difficult.

      Use tools as needed to aid in circlip removal, but be careful not to mar the pin bore so the wrist pin can be easily removed.
      The ease of pin removal will be largely dependent on the engine design and condition of the bore. If the pin can be removed by hand, great, if not, light tapping while supporting the rod is permissible. Otherwise, a pin puller should be utilized which can be bought or made. In its simplest form, this can consist of an appropriately sized bolt, nut, and socket. Once the wrist pin has been removed, the piston can be removed from the rod.

      Hopefully, the wrist pin can be removed by hand once the circlip is out. If not, an appropriately sized socket with some light tapping from the opposite end can help break it loose.
      Power Valve Disassembly - Prior to taking the power valve system apart, spend some time reviewing the procedure in your service manual. For additional insight into how the components interact, review the exploded views in the service manual and look at part microfiches, which can be found online.

      Online microfiches can be very helpful to double-check reassembly of the power valve. They can be found on many motorcycle dealer websites.
      When removing the power valve system, consider laying the components out on a clean rag in an orientation that correlates to how they are installed in the engine. This is a relatively simple thing to do that will help you remember how they are installed later. When it comes to cleaning the components, clean them one at a time or in small batches so that they don’t get mixed up.

      Lay out all the parts of your power valve assembly as you disassemble it. This will help you keep everything organized, and make sure you get it back together correctly.
      Inspection
      Reed Valve - Don’t forget to check the condition of the reed valve petals, cage, and any stopper plates. Most service manuals will detail the acceptable clearance between the petal tips and cage as well as the stopper plate height. Ensure any rubber coatings on the reed cage are in good condition.

      Inspect all reed valves components thoroughly before reassembling the top end. Any parts showing signs of excessive wear or damage should be replaced.
      Intake Manifold - Check the intake manifold for cracks. Cracks are more common on older engines, and propagation all the way through the manifold can lead to air leaks.
      Exhaust Flange - Check the condition of the exhaust flange and ensure that it is not excessively worn. An excessively worn flange will make exhaust gas sealing difficult, hamper performance, and leak the infamous spooge. 
      Power Valve Components - Take a moment to review the condition of all the power valve components. Significant wear can occur over time and lead to performance losses.
      Rod Small End - Check the small end rod bore for surface defects such as pitting, scratches, and marring. Any severe defects in the bore will necessitate rod replacement.

      The rod small end is a critical point of inspection. Any damage to the inside surface could affect the small end bearing, leading to a chain of top end problems and potential failure.
      Sourcing New Components
      When freshening up the top end in your two-stroke, it’s important to reassemble with quality components. A deglazed and honed or bored and replated cylinder is a critical component to ensuring reliable performance from your new top end. Your local cylinder shop should be able to handle the bore and replate when necessary, and a simple deglazing can be accomplished with a Scotch-Brite pad. Be sure to retain the 45-degree honing mark angle.
      There are a lot of choices for new pistons from the aftermarket out there, but many people choose to stick to OEM. However, when ordering from the OEM, every individual part must be ordered separately, including the piston, ring, pin, clips, gaskets, etc. Dealing with all these part numbers and chancing forgetting a component can be a pain, and get expensive.

      ProX two-stroke pistons are manufactured by OEM suppliers, and come with the piston, pin, ring(s), and circlips all under one part number.
      ProX two-stroke pistons are manufactured by the same OEM-suppliers to exact OE specs. They are available in A, B, C, and D sizing for most applications. ProX pistons come with the piston, ring(s), pin, and clips all in one box. Complete top-end gasket kits can even be ordered under one part number. ProX pistons provide an OEM-replacement option with less hassle and less strain on your wallet.
      Find ProX pistons for your bike here.

      Even though ProX pistons are made by OE suppliers, the quality control difference is evident. On the left is a ProX piston for a Honda CR250, and on the right is a brand new piston out of the box from Honda. Which would you choose?
      Measurements
      The number of measurements that should be taken throughout the top end rebuild will be discretionary. At ProX, we strive for excellence and err on the side of caution when it comes to engine building, so our builds consist of numerous measurements and inspections prior to reassembly. For us, this ensures a high level of confidence and safeguards against external oversights. We recommend the same to anyone building an engine.
      Below is a list of measurements that we routinely make when rebuilding a two-stroke top end:
       Piston ring end gaps  Piston-to-cylinder clearance  Rod small end diameter Out of these measurements, confirming or adjusting the ring end gaps is by far the most important, followed closely by ensuring the cylinder bore is within spec with respect to diameter, straightness, and roundness. Understandably, some measurements may be difficult for the average home builder to execute, usually due to not having the right equipment, however, a competent shop should be able to assist.
      Ring end gaps can be checked by installing the ring in the bore without the piston, and using a feeler gauge to find the measurement. Correct ring end gap is listed in the installation instructions that come with a new ProX piston.

      ProX rings often do not need to be filed as they are pre-gapped, but it's always a good idea to make sure your end gap is within the provided spec.
      Piston-to-cylinder is another measurement that should be checked before final assembly. For this, use a bore guage and a set of calipers to measure the bore size. Next, grab a set of micrometers and measure the piston. ProX pistons should be measured perpendicular to the wrist pin, a quarter of the way up the piston skirt from the bottom. Subtract your piston size measurement from your bore size, and you have your piston-to-cylinder clearance. ProX pistons come with a chart on the instruction sheet that shows the range your clearance should be in.
        
      Measuring piston-to-cylinder clearance is a smart precaution to help ensure you won't run into any unexpected issues with your new top end.
      A final measurement we recommend taking is the rod small end diameter. This is important because sometimes these can get worn out and create free play for the small end bearing, resulting in damage to the bearing and most likely the entire top end. It can be done using the same method as the bore diameter. Compare your measurement to the acceptable range in your owner's manual.

      Making sure the diameter of the small end of the rod is within spec is often overlooked, but can prevent a serious top end failure.
      Prep Work
      Cylinder Cleaning - Once the cylinder has been deglazed or has come back from replating, it should be cleaned one final time. There is almost always leftover honing grit that will need to be removed. To effectively clean the cylinder, use warm soapy water and a bristle brush, followed by automatic transmission fluid or a similar cleaning solution and a brush or lint-free rag. To check the cleanliness of the cylinder, rub a cotton swab around the bore and look for contaminants. Clean the bore until no contaminants are visible on the cotton swab. Any honing grit that remains in the cylinder will facilitate premature wear of the piston rings.

      A clean, de-glazed, and properly honed cylinder is key to piston and ring function and longevity.
      Power Valve Function - Cylinders that have been exchanged or replated should have the power valve system reinstalled ahead of final installation. Often times, excess plating can inhibit power valve movement. To correct this, the excess plating must be carefully removed. On cylinders utilizing blade style power valves, the blade position with respect to the cylinder bore should be checked to ensure the blade does not protrude into the bore.

      Assemble the power valve before installing the new piston and reinstalling the cylinder. Be sure to check that the power valve is moving as it should, and not protruding into the bore.
      Piston - It is usually easiest to prepare the new piston as much as possible by installing one of the circlips and the ring pack ahead of joining it to the connecting rod. Unless your service manual dictates which circlip must be installed first, choose the easiest installation orientation. Typically, your dominant hand and preferred work orientation will dictate which side you choose to install the circlip on.
       
      It's easier to install one clip and the piston ring(s) before fixing the new piston to the connecting rod.
      Reference your service manual to determine the correct orientation of the circlip. Usually, the open end of the circlip should be oriented to the 12 or 6 o’clock position. Temporarily install the wrist pin and use it as a backstop so that the circlip is forced to move into its groove. Installing the circlip should be done by hand to limit the chance of deformation. Orient the circlip to the desired position, then push the open ends of the circlip into position first. Be careful not to scratch or mar the wrist pin bore in the process! Once installed, use a pick or screwdriver to confirm the circlip is fully seated and does not rotate. Any circlips that can be rotated must be replaced because they have been compromised and deformed during installation.

      Make sure to note the orientation of each clip after installation. Some manuals may recommend specific positions depending on the piston, but always be sure the gap is not lined up with or near the dimple(s).
      Rings - The compression ring(s) will be directional, and the top of the ring is typically denoted by markings near the end gaps. Apply a thin coat of oil to the ring, then carefully work the ring into position, making sure to line up the ring end gaps with the locating pin in each ring groove.

      Install the ring(s) with the marking(s) facing up, and make sure the ring end gap is lined up with the locating pin in the ring groove.
      Installation
      Piston - On the top of the piston, an arrow will be imprinted, which typically denotes the exhaust side of the piston. Consult your service manual to confirm the proper orientation of the arrow and piston. Apply a light amount of assembly lube to the small end bearing and wrist pin bore on the piston, then install the bearing. Align the piston with the small end of the rod, and slide the wrist pin into place. Once again, use the wrist pin as a backstop, then install the remaining circlip into position. Use a pick or screwdriver to confirm it is fully seated and does not rotate.

       Don't forget to apply some assembly lube to the ring and piston skirts before assembly!
      Cylinder to Piston - In most applications, a ring compressor is not required to compress the rings and install the piston into the cylinder. Lightly oil the cylinder bore with assembly lube or engine oil, then lube the piston skirt and ring faces. Prior to installing the piston and rings, confirm one final time that the piston ring ends are oriented correctly to their respective locating pins.
       
      Once the new piston is installed on the connecting rod, apply some assembly lube to the cylinder wall, and carefully slide the cylinder over the piston. Squeeze the ring with your hand as you slide the cylinder on, simultaneously making sure the ring end gap remains aligned with the locating pin.
      Position the piston at or near TDC then carefully lower the cylinder bore down onto the piston. Use your fingers to compress the ring(s) and ensure the cylinder bore is square to the piston. Feel how easily the cylinder slides over the piston and rings. The installation of the cylinder should be smooth and offer little resistance. If resistance is felt, stop immediately and assess the ring pack. Occasionally one of the rings may come out of position in its groove and snag the cylinder bore. This typically happens as the ring transitions out of your fingers and into the cylinder bore.
       
      Once the cylinder is safely over the ring, slide it all the way on keeping the piston at top dead center (TDC). Don't forget to torque your cylinder and head nuts to the specification listed in your manual.
      Post Build
      Torquing - Your cylinder and head nuts should always be torqued to the specifications outlined in your service manual. Double check all the nuts are set at their corresponding specs.

      Spark Plug - Don’t forget to install a new spark plug and if necessary gap it appropriately.
      Air Filter - Be sure to install a clean air filter prior to start up.
      Crankcase Leak Down Test - As one final precautionary measure perform a crankcase leak down test. A crankcase leak down test will help confirm all the seals, gaskets, and joints are sealing as they should.
      Break-In - When running your new top end for the first time, keep the engine slightly above idle, with slow and mild revs until the engine starts to get too hot to touch. Then, shut the engine off and let it cool until it is warm to the touch. Repeat this process, revving slightly higher and letting the engine get partially hotter each time. After 3 cycles like this, let the engine completely cool, then check all your fluids and re-check the torque on your cylinder and head bolts.
      Once that is squared away, you can begin break-in runs riding the bike. Make sure to keep the RPMs varied while riding for the first time, not letting the engine lug or sit at idle. A safe bet would be to ride the bike like this for 5 minutes, then 10 minutes, and finally 15 minutes, with adequate cooling in between. This will ensure your piston ring(s) are evenly and properly broken in. It’s never a bad idea to double check your fluids and torque one more time after complete cool down.