Refit Kits, also referred to as Upgrade Kits, Modification Kits or Modification Packages in various sources, are factory-produced packages, often by the original manufacturer, designed to modify a given unit such as a BattleMech from its original stock specifications into a different but standardized version. A modification from one set of specifications into another, or into a custom configuration, does not technically require a refit kit to perform, but the modification is more easily accomplished if a refit kit is available.
It is implied that, beyond the regular spare parts or components that could be expected to be available for maintenance purposes in the field anyways, such a kit or package would have useful extra content such as technical information, instructions, special tools or special parts involved in the modification.
Standard refit kits can be assumed to exist for any official variant of a specific design (if published in a Technical Readout) by default. It should be noted that this does not say anything about the availability of a given refit kit at any given point in space or time, however.
Applying the prefix "field" to any of the names used for refit kits, e.g. "field refit kit", implies that the modification from this package is easy to carry out, to the point where it can be applied in the field with only basic maintenance or repair facilities (if any), and without requiring technical support beyond what is usually available for maintenance in the field. By contrast, other refit kits may require proper maintenance facilities or even full-fledged factories to apply, depending on the intensity of the change and the type of components affected.
Refit kits are a staple of military logistics to improve or adapt available hardware, possibly even in the sense of a downgrade.
In BattleTech, the term is often (inaccurately) associated specifically with the upgrade kits that were produced by the Free Worlds League for the Draconis Combine and the Federated Commonwealth after ca. 3052, to upgrade their 'Mechs and other combat equipment to recovered Star League era technology levels: At the end of the conference on Outreach in 3051 Captain-General Thomas Marik struck a deal to supply the Federated Commonwealth, Draconis Combine, and Free Rasalhague Republic with Star League-era specs military hardware. In return, Thomas' son Joshua began undergoing treatment for his leukemia at the New Avalon Institute of Science. Remaining largely unaffected by the Clan war, the League's military industrial complex expanded more rapidly than those of the war-ravaged nations, with the AFFC, KungsArme and DCMS buying whatever they could, even after the Truce of Tukayyid. The bulk of these League manufactured BattleMech kits went to the Draconis Combine, and as the Combine's industries tooled up to develop their own versions of these kits, the FWLM also reaped the benefits to upgrade their own forces.  The Draconis Combine, after initially purchasing them from the Free Worlds League, also began producing similar kits for upgrading older machines domestically, to the point where the DCMS began stockpiling such field-modification kits.
The Free Worlds League branch of Earthwerks Incorporated invested heavily in producing Field Refit Kits. Also, as of 3067, Diverse Optics specifically works closely with Free Worlds League authorities to develop efficient and cost-effective upgrade kits and engineering protocols to further the FWLM's ongoing modernization, based on the development work and processes following the Clan Invasion. The Free Worlds League also developed a number of vehicle upgrade kits, however with the FedCom and Combine focused on 'Mechs, only the Capellan Confederation and St. Ives Compact showed any interest. This again allowed the FWLM to rapidly upgrade its armor units faster than its neighbors.
However, while this particular period was a prominent example for the overall concept, refit kits have existed before and are by no means limited to the post-Clan Invasion period. Examples for earlier refit kit programs include:
- In the lead-up to the Battle of Tukayyid ComStar created the Clanbuster refits, designed as field refits with parts they had in stock, especially type 300 XL Engines.
- In 3025, House Steiner engineers prepared new arm assemblies to upgrade the Zeus which contained a PPC (instead of the Autocannon/5) and an extra heat sink for distribution throughout the Lyran Commonwealth, changing the ZEU-6S into the ZEU-6T that would become the new baseline model.
- In 2688 Hollis Incorporated issued refit kits for Catapults in SLDF Royal units that upgraded the heat sinks and added more Ferro-Fibrous Armor and ammunition.
Strategic Operations rates refit kits A (easiest to apply) to F (most difficult to apply), with A and B refits being described as field refits, C and D as maintenance-level refits, and E and F as factory-level refits, with progressively higher time multipliers (compared to repair attempts carried out on the items in the refit kit), and progressively higher chances of failure.
Previously, the Mercenary's Handbook: 3055 described three types of refit kits, graded from A (the hardest to apply) to C (the easiest). Grade A refit kits usually require a repair platform; Grade B refit kits are possible (if difficult) to install even without a repair platform, and Grade C refit kits only contain weapons and electronics that can be installed or swapped in the field. Where the modification cannot easily be carried out in the field, it is suggested that installation of the upgrade kit at a suitable facility is part of the parcel.
- Strategic Operations, p. 188 "Customization"
- Field Manual: Free Worlds League, p. 18
- BattleTech Compendium: The Rules of Warfare, p. 36, "Wolfnet Archive File: 88675-RE-4-31/1/5 - Copy of a memo circulating in AFFC units"
- Field Manual: Draconis Combine, p. 14 "Arm of the Dragon - BattleMech Assets"
- Field Manual: Free Worlds League, p. 18, "The Modern FWLM - BattleMech Assets"
- Handbook: House Marik, p. 128
- Field Manual: Draconis Combine, p. 15
- Field Manual: Free Worlds League, p. 19, "The Modern FWLM - Armor Assets"
- Technical Readout: 3025, p. 112; Technical Readout: 3025 Revised, p. 106
- Technical Readout: 3039, p. 146
- Mercenary's Handbook: 3055, pp. 95-96