Overhaul heavy goods and public service vehicle mechanical units

Business Sectors (Suites): Maintenance and Repair - Heavy Goods and Public Service Vehicles
Developed by: IMI
Approved on: 30 Mar 2021


This standard is about the bench-based overhaul of mechanical units, involving dismantling, assessment, repair, replacement or adjustment of internal components together with re-assembly and testing.

Performance criteria

You must be able to:

P1 wear suitable personal protective equipment throughout all overhauling activities
P2 use suitable sources of technical information to support your overhauling activities
P3 assess and prepare all the equipment required, following manufacturers' instructions, prior to use
P4 use the tools and equipment required correctly and safely throughout all overhauling activities
P5 carry out all overhauling activities following:

P5.1 the manufacturer's instructions
P5.2 your workplace procedures
P5.3 health, safety and environmental requirements
P6 work in a way which minimises the risk of:

P6.1 damage to other components
P6.2 leakages
P6.3 contact with hazardous substances
P7 ensure your assessment of the dismantled unit identifies its condition and suitability for overhaul
P8 promptly inform the relevant person(s) where an overhaul is uneconomic or unsatisfactory to perform
P9 use testing methods which comply with the manufacturer's requirements
P10 adjust the unit's components correctly to ensure that they operate to meet the vehicle operating requirements, when necessary
P11 ensure the overhauled units and assemblies conform to the vehicle operating specification and any legal requirements
P12 ensure your overhaul records are accurate, complete and promptly passed to the relevant person(s) in the format required
P13 complete all overhauling activities within the agreed timescale
P14 promptly report any anticipated delays in completion to the relevant person(s) 

Knowledge and Understanding

You need to know and understand:

Legislative and organisational requirements and procedures
K1 the legal requirements applicable to the units and assemblies overhauled (including road safety requirements)
K2 the legislation and workplace procedures relevant to 

K2.1 health and safety
K2.2 the environment (including waste disposal)
K2.3 appropriate personal protective equipment
K3 the implications on an Operators Licence of not carrying out repairs correctly
K4 your workplace procedures for

K4.1 recording overhauling activities
K4.2 the referral of problems
K4.3 reporting delays to the completion of work
K5 the importance of documenting repair information
K6 the importance of working to agreed timescales and keeping others informed of progress
K7 the importance of promptly reporting any anticipated delays to the relevant person(s)

K8 how to prepare and check the accuracy and operation of all the overhauling and testing equipment required
K9 how to use all the overhauling and testing equipment required

Mechanical unit overhauling activities
K10 how to find, interpret and use sources of information on overhauling procedures and statutory requirements
K11 how the relevant vehicle mechanical units and assemblies operate
K12 how mechanical units and assemblies are constructed, dismantled and reassembled
K13 the possible causes of faults in mechanical units and assemblies 
K14 vehicle operating specification for limits, fits and tolerances and where this information can be sourced
K15 how to assess the condition evident within unit sub-assemblies and components
K16 the cost-benefit relationship between the reconditioning, repair and replacement of components within units
K17 how to carry out overhauling activities for the types of unit on which you work
K18 the relationship between test methodology and the faults repaired – the use of appropriate testing methods
K19 how to test and evaluate the performance of overhauled units against the operating specification
K20 how to interpret test results
K21 how to identify the types and causes of mechanical unit and assembly failure
K22 how to make suitable adjustments to units and components
K23 how to work safely avoiding personal injury, damage to components, leakage and hazardous substances 


1. Overhauling activities are

1.1. dismantling
1.2. assessment
1.3. repair
1.4. replacement
1.5. adjustment of internal components
1.6. re-assembly
1.7. functional testing

Scope Performance

Scope Knowledge





This section contains examples and explanations of some of the terms used but does not form part of the standard.

Examples include, adjustments made to clearances, settings, pressures, tensions, pre-load and speeds.

Agreed timescales
Examples include manufacturer's recommended work times, job times set by your company or a job time agreed with a specific customer.

Examples include those for wear, damage, alignment, corrosion, leakage, distortion and balance.

Examples include hand tools, pullers and presses, measuring instruments, refurbishment tools, general workshop equipment and special service tools.

Functional testing
This refers to any applicable functional tests carried out after overhaul.

Heavy goods and public service vehicles
These are medium and large goods vehicles, buses and coaches of 3500kgs gross vehicle mass (GVM) and above.

Mechanical units
Examples are: engines, gear boxes, final drives, steering, suspension, chassis assemblies.

Testing methods
As prescribed by the appropriate technical literature.

Links To Other NOS

External Links

Version Number


Indicative Review Date

31 Mar 2024





Originating Organisation


Original URN


Relevant Occupations

Engineering, Heavy Vehicle Diagnostic Technician (Automotive), Heavy Vehicle Service Technician (Automotive), Vehicle Trades, Bus and Coach Mechanic, Bus and Coach Mechelec

SOC Code



Overhaul; units; mechanical; bench; adjustments; heavy goods vehicle; public service vehicle; bus; coach