All Motor - OEM or Forged Pistons??
The Final Lap - Mar. 2009 Newsletter ver. 1.0


To all of our patrons and supporters,

This month, I am going to touch on the differences between OEM and Forged pistons, and what is an appropriate application to use them in.  Maybe it's because we constantly have a daily onslaught of technical questions or lately there has been a huge spike in engine rebuilds, but I've been sounding like a broken record lately, suggesting specific OEM pistons in popular B-series N/A (Naturally Aspirated) applications.  Majority of our customers drive their vehicles daily.  Therefore, reliability is always a factor to consider when building a street-engine.  The single best way to obtain a healthy compression ratio, stay very reliable and make good "useable" horsepower, is to use OEM pistons in your build.  Why not forged pistons, you ask??  They are lighter and stronger, so why can't we use them?  The answer:  They ARE NOT designed to be used in a daily driven application.  Well, what are they designed to be used in?  The answer:  A RACE ENGINE.  And what is done to a race engine after a race or after a certain number of races?  They are checked from top to bottom and rebuilt if needed.   Anyone who builds a street purpose motor has no intentions of tearing that puppy down every 10k just to see how things are doing in there.

Now to make sense of all this.  The reliability factor between OEM and forged pistons is in the material and how it behaves.  Forged pistons are made from two different alloys.  2618 aluminum alloy and 4032 aluminum alloy.  The difference is that 4032 has more silicone built into the piston, allowing for LESS thermal expansion, and allowing you to run a tighter piston-to-wall clearance.  What is  piston-to-wall clearance?  Piston-to-wall clearance is just what it says.  It's the area between the piston and the cylinder wall.  This clearance is built into the bore so the piston can expand correctly.  2618 aluminum alloy has less silicone and expands much more than 4032.  However, 2618 has proved to be stronger and can take much more abuse (detonation).  Under detonation, I have seen the "ring lands" become brittle and crack in 4032 alloy pistons.  The disadvantage to having 2618 alloy is because of the loose piston-to-wall clearance that must be run.  Typically, most manufacturers of 2618 alloy pistons require a wall clearance of .0030 to .0045in. (3 to 4 1/2 thousandths of an inch).  When it is this loose, there will be a considerable amount of "piston slap".  When you first start the engine and it is cold, you will hear the pistons rocking back and forth in the bore, giving off a "clackity clackity" sound.  As the engine warms up, this sound will diminish or dissappear entirely, depending on the clearance.  What's so bad about that??  Well, imagine this event in a daily driven engine.  The car is started on an average of 2-4 times a day.  Multiply that by a number of days, then weeks, months, etc.  What ends up happening is your blocks cylinder walls become "chaffed" and "out-of-round".  Thus, leading to improper sealing and oil consumption.  Equalling an oil drinking motor, with crappy compression numbers, decreased power and loss of performance.  All of this, after you spent a few thousand on your engine build and it's only been about 20 thousand miles??  Race teams understand this and use the piston for what it is intended.  To accept the abuse from the rigors of racing.  The engine will be checked and if it needs new pistons and bearings after 3 races, then so be it....that's just how racing is.  As a matter of fact, in some all motor drag race applications, I know of a team, who at the beginning of the season, will start off at a 84.5mm bore, but by the end, will be at 86mm bore.  They purposely build the engine "loose" to get as much horsepower as possible, but this requires them to "correct" the chaffed cylinder and go larger on the bore on each rebuild.  At the end of the season, the block is thrown away and a new build started on a fresh block for next season. 

Ok, so i've thrown all this at you about forged pistons, now what about OEM pistons?  Where do they stand in all of this?  OEM pistons are used in many "autocross" and "roadracing" applications and obviously, should be used in your street builds.  OEM pistons are used in roadracing over forged pistons, because of the effective sealing properties and reliability that they provide.  As you may know, many B-series pistons are interchangeable between B-series blocks and you can achieve a great performing and reliable build with them.  Remember, that not all of the build is in the bottom-end (block).  You still have the cylinder head, camshafts and most of all, tuning.  For those running a 81mm or 81.25mm bore, the Type-R series pistons are ideal to run.  Depending on your block, you can run Japan spec Integra Type-R pistons or Civic Type-R pistons.  What about 81.5mm or 82mm??  Well on an "un-sleeved" block, 82mm is the biggest I would suggest.  We have our own "il4 performance" pistons that are cast by the same manufacturer that casts them for Honda.  This company casts the US Spec Integra Type-R piston for us in 81.5mm, 82mm and 84mm, 84.5mm for B20 applications.  OEM Honda is all you will ever need for your daily driven, all motor build or for your autocross, roadrace build.

Give us a call at 714-903-1898 for pricing and info on other services such as connecting rod service, crankshaft service and custom piston machining.

In the next newsletter, I will discuss parts researching, buying quality and not just the name.  InlineFour.com