Modular Flashlight Guide by OVEREADY, Inc.

Modular lights offer a tremendous variety of options and capabilities, but sorting through them all can be daunting. We hope this guide helps in your selection process. If something isn’t clear, please let us know. Modular lights consist of three parts:

  • The Chassis or host – Bezel, Body, and Tailcap.
  • Light Engine -LED Drop-in or Incandescent Drop-in
  • Fuel – One use Alkaline or Rechargeable Lithium Battery Cells

Light engines and batteries are electronics that become obsolete, burn out, or run out. Invest in a tough, durable chassis and replace the light engine and batteries when you’re ready for what’s next. Put them together and you’re ready to go:


Surefire® (SF) makes a variety of high-quality torches, the design of which has become the basis for compact modular lights industry-wide. Choosing one depends on your needs and budget, but there are important characteristics to consider. In Surefire® vernacular, numbers describe the length and letters describe the style.

Length – Length originally described the voltage produced by the number of cells (6, 9) but changed in later models to simply the number of cells (2, 3). 6Pis equivalent to C2 and 9P is equivalent to C3. Translated into high capacity rechargeable batteries, 2 cell lights support a single 65mm long cell and 3 cell lights support a pair of 50mm cells. The choice between them is primarily about voltage. One cell produces about 4 volts, two cells produce about 8. Which is better depends on your needs, but one cell models are shorter and two cell models are more versatile.

Diameter – SF lights are designed to be used with 16340 cells, more commonly referred to as CR123A. 16mm in diameter, they offer tremendous advantages over alkaline cells. Chief among these are energy density and energy release. Changing from 16 to 18mm cells multiplies these same advantages while adding capacity and rechargeability. But SF flashlights aren’t designed for such large cells. This is where boring comes in, a process that enlarges the inside diameter of the original battery tube. The result is high output, high runtime, and “guilt free lumens,” all at the same time. While tested for strength and durability, including drop and crush tests, bodies are modified from their original specifications and cannot retain all of their original strength.

Style – Surefire® makes a variety of model families (series). Each has a specific emphasis. Made from the same aluminum with the same thread specifications, in many important ways they are interchangeable. Where they differ is style. P series are round with knurling and are meant to be held with a standard grip. Grip rings can be added to support other holding styles. C series are square and can be held with the standard grip but also allow for the Rogers Technique. Pocket clips and grip rings are built into all C models. Z series are optimized specifically for the Rogers Technique, and are a favorite among law enforcement, but are not as versatile for pocket carry as are the C’s.

Switches – Traditional flashlights have clickies. Push the button a little and the light comes on. Push it a lot and the light stays on (with a click sound). Most SF lights come with a combination of a momentary button that does not stay on and a twisty cap that does – both methods are silent. SF has clicky switches but they are generally not standard. Most models here are available in factory twisty and custom clicky. These are all forward clickies, with the light turning silently on at the start of the press. Many non SF clickies are the reverse, where the light does not activate until you both press and let go.

 – Raw aluminum is rather soft. To toughen it up, finished bodies go through a process called anodization. Ironically a kind of rust, it’s not so much a layer of something added to the aluminum, but an alteration of the aluminum itself. The result is a hardened shell, ready to stand up to years of heavy use. There are 2 finishes popular on aluminum flashlights, type II and III. Both involve bathing the metal in electrified chemicals, but type III has an extra temperature component that is more difficult and expensive. Also known as a hard coat or hard anodize (HA), type III is even harder and more durable than type II. SF generally makes type II products in black and type III products in gray. For more variety of colors, like Safety Orange, also offers CeraKote hard baked ballistic grade ceramic paint on select chassis. With its unique ceramic backbone. CeraKote provides excellent extreme protection, abrasion resistance, hardness, & unmatched durability on our custom modified chassis.

Please note: While purchased new at retail, these lights are modified and may not be eligible for warranty service.

Light Engines

The key to modular lights is matching your drop-in to your preferred host/battery voltage. Incandescent drop-ins depend on the voltage of the bulb, which can be changed. LED drop-ins are fixed and come in three voltage categories:

Low voltage requirement (1 cell only)
use in C2/6P (18650×1) for high output, with no dimming/warning on protected cells
recommended: Malkoff drop-ins

Medium voltage requirements (1-2 rechargeable cells)
use in C2/6P (18650×1) for maximum runtime with dimming/warning on protected cell
use in C3/9P (18500×2) for high output, with no dimming/warning on protected cells
recommended: Malkoff drop-ins

High voltage requirements (2-3 rechargeable cells)
use in C3/9P (18500×2) for maximum/flood output, with no dimming/warning on protected cells
recommended: Malkoff drop-ins

Warm vs Cool
High power LEDs are based on a design that emits light with a blue tint. This is called cool. To make it less blue, manufacturers can put what is essentially a yellow filter on top (called a phosphor). When struck by light energy, the phosphor releases yellow light. The blue and yellow average together, creating white. All LEDs with this phosphor treatment are described as warm. Add enough phosphor to equal how blue an LED and it creates neutral. Keep any phosphor and it creates warm (yellow). But neutral and warm LEDs are confusingly described as warm, but ‘warm’ typically refers to the neutral variety. When it comes to most color rendering, white is better than either blue or yellow.

Deciding between them is a personal choice. Blue light is particularly challenged outdoors with the greens and browns common in nature. But the phosphor conversion process needed to make white light taxes the available lumens. Warm drop-ins are about 25% less efficient than cool, with a corresponding drop in lumen output given the same amount of energy (and battery drain). People not sensitive to blue light may not even notice the difference while those with a high degree of color receptivity often abandon cool LEDs once they’ve tried warm/white.

See our complete color guide for additional information and examples.


Batteries are first defined by size. Generally described with a 5 digit number, digits 1 & 2 are the diameter (in mm), and digits 3 & 4 are the length (in mm). So a 16340 is 34mm long and 16mm in diameter while an 18650 is 65mm long and 18mm in diameter. In our case, a single 17670 is right at home in a flashlight built to manage a variety of (two) 16340 (aka CR123A) cells. Make (modify) the tube a bit wider and 18650’s are right at home.

Batteries are also defined by chemistry. There are two lithium rechargeable chemistries favored by us, Lithium Cobalt (LiIon) and Lithium Manganese (IMR). The first requires protection circuits, offers more capacity, and is better suited for low to medium power applications. The second is safer (not needing circuits), offers faster discharge rates (along with less capacity for a given size), and is better suited for high power applications (over 3 amps). See here more information.

Please note: In addition to extra energy, lithium based rechargeables have over twice the voltage of ‘normal’ batteries and will overload many AAA, AA, C, and D battery driven devices. See Sanyo® Eneloop® batteries for these uses.

For more information on flashlights in general, and modular lights in specific, please visit the CandlePowerForums