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Q: Do I need forged pistons?
A: Forged pistons may not be absolutely necessary in all applications. However, the minor additional cost of a forged piston can save your motor from disaster in a marginal situation.
Q: What is the difference between the FPS (Factory Performance Series) and the SRS (Sportsman Race Series) series of Probe Pistons?

A: Probe FPS Forged Pistons are designed for heavy duty and high performance use when the application requires a piston that is stronger than a Cast or Hypereutectic Cast Piston. They are suitable for heavy duty, high performance street and some race applications. Compression ratios are similar to factory applications and weights are generally lighter than the TRW equivalent. The FPS series is manufactured using the same ultra-modern machining techniques as the SRS and series pistons.

FPS - Technical Summary

  • Forged of VMS-75 High Silicon Aluminum for lasting performance.
  • Light weight forging design is typically about 70 grams lighter than TRW’s.
  • CNC Diamond machined skirts and ring grooves for increase power.
  • Tight piston to wall clearance design for better ring seal.
  • Reinforced piston skirts to eliminate piston skirt collapse.
  • Radial Anti Detonation Grooves reduce detonation by clipping pressure spikes.
  • Pressure Equalization Channel enhances ring seal, making more power and increasing fuel economy.
  • Increased valve relief depth for high performance camshafts.
  • Offset wrist pins for reduced noise and lasting operation.
  • Stock size rings grooves and press fit piston pins.

Probe SRS Forged Pistons are designed for performance and race applications where a strong, lightweight forged piston is an advantage. With well over 200 part numbers now in stock for popular Chevrolet, Ford, Chrysler, and Sport Compact applications, the SRS Series are a full-featured piston and are extremely popular for use in street and most racing combinations.

SRS - Technical Summary

  • Forged of 2618 t-61 Aluminum alloy for the ultimate strength and fatigue resistance.
  • Light weight forging designs for quick revs and power.
  • Tight piston to wall clearance design for better ring seal.
  • Fully CNC machined domes and valve reliefs for perfect fit.
  • Radial Anti Detonation Grooves reduce detonation by clipping pressure spikes.
  • Pressure Equalization Channel enhances ring seal, making more power and increasing fuel economy.
  • Full floating lightweight wrist pins and double spiro locks for durability.
  • Narrow piston rings for the ultimate in power.

Q: Can Probe pistons take NOS (how much)?

A: Now there’s a loaded question! Standard weight Probe FPS and SRS Series are manufactured with sufficient deck thickness to handle even large nitrous systems. The ring lands are moved down to provide additional strength in that area. Having said that, NOTHING can protect a piston from a nitrous lean-out.

Q: Can Probe pistons take boost (how much)?

A: This is the same area as Nitrous. As the minimum thickness is .200” in the standard weight pistons, they are normally used in the 15 to 18 lbs. range with no problems, though the same caveat applies for a lean-out.

Q: How much horsepower can Probe pistons handle?

A: There really is no limit. Probe forged pistons (SRS Series) are machined on 2618-t6 blanks and can handle as much horsepower as any piston made by anyone. The truth is, it’s all about the tune-up. If the mixture and timing are correct you’ll never have a problem. If they are not, that’s when you run into problems no matter who’s pistons you are ruining (we mean running).

Q: What piston to wall clearance do I run?

A: Proper piston to wall clearance depends on the power level of the engine. Use the chart below as a guideline. Minimum clearances are fine for naturally aspirated street applications. However the clearance should be increased for higher output engines, especially with nitrous and forced induction (turbo and supercharged).

Recommended Piston Clearance - FPS Series

Bore (Inches)MinMax

Recommended Piston Clearance - SRS Series

Bore (Inches)MinMax
Q: Where do I measure the piston skirts?

A: For the most accurate measurement place your micrometer at .500” from the bottom of the skirt 90 degrees from the pinhole.

Q: How deep are the Valve reliefs?

A: The valve pocket depths vary by make and application. Most FPS Series forged pistons have valve reliefs that are .030 deeper than stock. The SRS series are designed for camshafts in the .640” to 670” lift range for small blocks and about .780” lift for big blocks. This can vary somewhat due to alteration to the duration and centerlines of the camshafts.

Q: What will lighter pistons do for me?

A: Light pistons have several benefits. They reduce stress on the rotating assembly and block, especially at high rpm. They allow quicker wind up of the engine, much the same as a lightweight flywheel would. They make balancing easier, reducing or eliminating the need for expensive Mallory metal.

Q: What ring end gap should I run?

A: A proper ring end gap is essential for achieving maximum power output. Too large of a gap can allow combustion pressure to escape past the rings. Too small of a gap can cause the rings to end butt and damage the rings and cylinder walls. Both conditions will reduce the performance level of the engine. The proper end gap increases with the power output of the engine. Please use the following formula:

Top Ring: Multiply the bore size by .004 (ex. 4.000 x 0.004 = 0.016)

Second Ring: Add .003" to top ring calculation (ex. (4.000 x 0.004) + 0.003 = 0.019)

For power adders, increase gap by 0.005"

Q: How many Spiro-locks do Probe pistons take?

A: With the exception of a few specialty pistons, all Probe pistons use two spiro-locks per side. Never leave one out. That would allow the pin to hammer its way out of the piston. Needless to say, that would not be a good thing.

Q: How far down is the top ring?

A: Where it is physically possible, the standard is a minimum of .200” for small blocks and .350” for Cleveland’s, Chevy and Ford big blocks. If this dimension is a cause for concern, please call our techs for more information.

Q: How thick is the deck of the piston?

A: The FPS Series pistons normally run deck thicknesses in the .350” + range. The SRS series usually utilizes a deck of about .200”, The PRS series are as thin as .150”.

Q: How much RPM can Probe pistons handle?

A: There is no practical limit for RPM. In general, the higher the RPM the lighter the piston should be (33 ounces at 10,000 rpm is 20 TONS). For the SRS Series pistons 8,500 rpm is common.

Q: Can I use aftermarket heads with Probe Pistons?

A: Yes, Probe forged pistons are application specific and have valve reliefs designed for most OE and aftermarket heads including Twisted Wedge, SJC, Kaase, GT-40, Blue Thunder, Edelbrock, Dart, Airflow Research, World, Patriot, Canfield and many more. Piston descriptions online and in our catalogs will designate specific application usages. In some cases where modified valve reliefs may be required, Probe offers this as a service to our customers as an upgrade cost.

Q: How big of a cam can I use with my Pistons and or Engine Kits?

A: Most kits and short blocks will handle solid or roller cams in the .600+ lift range. But depending on the particular valve placement in the cylinder head and other cam specifications, deeper or altered valve reliefs may be required. Probe offers this as a service to our customers as an upgrade cost.

Q: Can Probe Balance my Engine Kit if I purchase one?

A: Yes, Probe Industries offers complete Engine Kit Balancing Services at a Nominal charge. Balancing services typically take 1-2 business days. All un-balanced and balanced Engine Kits are available to be drop shipped to your business or your customer for your convenience.

Balancing Types are Available in:

  • 28 oz.
  • 50 oz.
  • External
  • Internal
  • Neutral