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PRM 120D Drop Centre Gearbox 2.04:1 Ratio
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PRM 120D Drop Centre Marine Gearbox | Ahead Ratio 2.04:1

Part No. PRM-120D2

This gearbox has been discontinued by PRM and replaced by the PRM-125D2 Gearbox.

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Brand PRM
Country of Origin United Kingdom
Gearbox Manufacturer PRM
Construction Heavy Duty Aluminium Gearcase
Hydraulic or Mechanical Mechanical
Gearbox Model PRM 120D
Ratio Ahead / Stern 2.04:1
Shaft Arrangement Drop Centre
Nominal Power Rating Pleasure 1.67 BHP / 100 RPM
Nominal Power Rating Light Commercial 0.91 BHP / 100 RPM
Approximate Dry Weight 16kg (35lbs)
Capacity Oil Capacity: 0.8 litres (1.4 pints)
Gearbox Spline 10 teeth B10 x 23 x 29mm DIN 5464
Gearbox Coupling 4", 4 holes, 3-1/4" PCD, 2-1/2" female register
Warranty (Years) 2


The PRM 120 Marine Gearbox is designed for use in pleasure craft where a simple lightweight transmission is required.

The cone clutches for both ahead and astern are mechanically operated and require no adjustment.

The PRM 120 gearbox is purpose built to operate with standard rotation engines, i.e. providing a clockwise rotation of the gearbox input shaft when viewed on input. In forward drive the output shaft rotates opposite to input shaft therefore requiring a right hand propeller.

The reduction ratios offered are 2.04:1, 2.5:1 and 2.94:1 with a common astern ratio of 2.5:1

The heavy duty aluminium gearcase is externally ribbed for strength and heat dissipation.

Direction actuation is by a lever with positive detent positions for ahead, neutral and astern. It is suitable for use with proprietary remote control operating systems which must be single lever dual action controls which operates the engine throttle in conjunction with the gearlever.

Nominal Power Ratings, PRM 120 Marine Gearbox

Ahead Ratio Pleasure Light Commercial
2.04:1 1.67 1.25 0.91 0.68
2.50:1 1.34 1.00 0.91 0.68
2.94:1 1.11 0.83 0.77 0.58
Max. Input Power 67 50 47 35
Maximum operating speeds : 5000 rev/min

Maximum operating speeds : 5000 rev/min

Note: These powers are expressed in BHP and kW per 100 rev/min engine operating speed, and are measured at the engine flywheel. Ratings have been established to ensure the long trouble free life of the gearbox which should not, therefore, be used at powers in excess of those shown.

Service Classification Definitions - Pleasure

Limited to craft used exclusively for pleasure boating; operation at full engine throttle should not exceed 5% of total time, with balance of usage at 90% of full throttle engine speed, or less. Maximum operation 500 hours per year. The selection of PRM120 according to this classification for any commercial boat is not approved by PRM Newage Ltd.

Service Classification Definitions - Light Commercial

Planing or semi-displacement craft used in pleasure or commercial applications may qualify for light commercial rating if annual usage is less than 1500 hours and full throttle operation is limited, with most operating time at partial throttle.

Important Note

  • It is essential for the engine, transmission model, reduction ratio and propeller size to be correctly matched so that the engine can attain its rated speed appropriate to the relevant service classification without labouring.
  • It is also necessary to ensure the torsional compatibility of the complete propulsion system from engine through to propeller, since disregarding this may result in gear noise, particularly at low speed operation, and may even result in damage to the engine as well as the transmission components.

PRM Newage Ltd will provide all possible information and assistance to help find solutions to potential torsional problems, but it is the ultimate responsibility of the person assembling the drive and driven equipment to ensure that they are torsionally compatible.

Oil Cooling

The normal Operating temperature should be in the range 50° to 120°. ATF Automatic Transmission Fluid is recommended.

Propeller Free Wheeling

The PRM 120 gearbox output shaft can be rotated continuously with the gearbox in neutral.

It is therefore not necessary to fit a propshaft brake when running with the engine shut down, for example when sailing.

Approximate Weight & Oil Capacity

Approximate Dry Weight 16kg (35lb)
Oil Capacity 0.8 litres (1.4 pints)

PRM120 Gearbox spline: 10 teeth B10 x 23 x 29mm DIN 5464
PRM 120 Gearbox coupling: 4", 4 holes, 3¼" PCD, 2½" female register

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