Thamesway Marine Products Ltd




The TMP Gearbox does not readily lend itself to the D.I.Y enthusiast. Whilst not complicated it is however heavy and awkward to handle. Being of a mix of alloy castings and steel studs it is not unusual for the studs to 'grow' into their holes because of the salt water environment. Separation can require lots of heat, patience, and care to avoid damage to the faces of the castings.

The gearbox is attached to the engine bell housing by internal nuts thus one has to remove the gearbox / bell-housing as a unit. In most cases, as the bellhousing carries the rear engine mounts, the engine has to be supported.

It is a good idea, if possible, to undo the Output Coupling and Transfer Shaft nuts whilst the gearbox is still attached to the engine.

You will require a 7/8"A/F socket ( 1/2" drive ) with an outside diameter of 1.812 max O/D.

In order to disengage the gearbox drive from the engine a rearward movement of approx 2.0" is required. ( Mercedes installations need about 6.0" unless you undo the studs from the engine backplate. )

Where the gearbox has a SPLINE input no problem should be experienced in disengagement, however, in the case of the 8 bush input ( Perkins / Mercedes ) rust can seize the bushes into the fly wheel. When this occurs the housing 'bounces' as the rubber inserts in the bushes stretches.

A tried and tested solution to this problem is to cut a 2.0" square access hole in the housing. ( see sketch ) Squirt in WD 40 and use a lever between the gearbox flange and the flywheel, adjacent to each bush.

A Workshop Manual is available which gives detailed instructions for repairing the gearbox. yperlink here to order manual.)


For re-assembly you will require: -

Direct Drive :-

TM 12090 x 1, Oil Seal (Input)
TM 12020 x 1, Oil Seal (Output)
TM 12470 x 1, Gasket and 'O' Ring Kit
TM 12016 x 1, Neutral Brake Pad
TM 12009 or TM 12773 Oil Filter ( MK II / MK IV)

Reduction Boxes :_

TM 12090 x 1, Oil Seal (Input)
TM 12436 x 1, Oil Seal (Transfer Shaft)
TM 12436 x 1, Oil Seal (Output Shaft)
TM 12470 x 1, Gasket and 'O' Ring Kit
TM 12016 x 1, Neutral Brake Pad
TM 12009 or TM 12773 Oil Filter ( MK II / MK IV )

Check the following for wear: -

AREA. Fig 1

Input Flange / Input Shaft, Oil Seal running diameter.
Oil Pump Pressure plate face, Oil Pump Gears,
Oil Pump Shaft for ovality. Oil Pump body bushes.
Thrust Washer

AREA. Fig 2

Clutch Linings. Note the Ahead Lining tends to wear on the non-active face. This shows as deep witness marks of the clutch element "gramaphone groove". The lining may also have sunk into the Clutch element.
Replacement clutch linings now require bonding in place.
Check the Square section Sealing Ring.

Area. Fig 3

Planet Gear Bushes and Shafts. Excess wear causes noise and loss of oil pressure.
Check Bearing TM 12021 ref 3.18 ( MK II ).
Check faces of casting for distortion after removal / replacement of bearings

Area. Fig 4/5.

Check Oil Seal spacer (TM 12410) for wear on running area.
Clear water drain holes of any blockage.


Repair stripped threads with Helicoil thread inserts.

We are happy to answer your questions if you experience problems.