michael h -

ANSI/ITSDF B56.1-2005, SAFETY STANDARD FOR LOW LIFT AND HIGH LIFT TRUCKS contains the following provisions:

4.2 Modifications, Nameplates, Markings, and Capacity
4.2.1 Except as provided in para. 4.2.2, no modifications or alterations to a powered industrial truck that may affect the capacity, stability, or safe operation of the truck shall be made without the prior written approval of the original truck manufacturer or its successor thereof. When the truck manufacturer or its successor approves a modification or alteration, appropriate changes shall be made to capacity plates, decals, tags, and operation and maintenance manuals.

4.2.2 If the truck manufacturer is no longer in business and there is no successor to the business, the user may arrange for a modification or alteration to a powered industrial truck, provided however, the user
(a) arranges for modification or alteration to be designed, tested, and implemented by an engineer(s) expert in industrial trucks and their safety
(b) maintains a permanent record of the design, test(s), and implementation of the modification or alteration
(c) makes appropriate changes to the capacity plate(s), decals, tags, and operation and maintenance manuals
(d) affixes a permanent and readily visible label on the truck stating the manner in which the truck has been modified or altered together with the date of the modification or alteration, and the name of the organization that accomplished the tasks

4.2.3 If the truck is equipped with a front-end attachment(s), including fork extensions, the user shall see that the truck is marked to identify the attachment(s), show the weight of the truck and attachment combination, and show the capacity of the truck with attachment(s) at maximum elevation with the load laterally centered.
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Standards Australia should have similar paragraphs in AS 2359.21985: Powered industrial trucks -operation. This standard contains Australian requirements for the operation, maintenance, repair and modification of industrial trucks and their attachments.