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Lister 3G Marine Gearbox

The Lister 3G Marine Gearbox is most commonly found on four and six cylinder marine versions of the Lister H Series engines; HA, HR, HRW, HW.
Be aware however that it was not the exclusive fitment on the HR/W series engines, so you do need to check if looking for information or parts. If you have
the turbocharged 6 cylinder version, you definitely won't have a 3G box (at least not originally).
It is also fitted to the rather more scarce FR Series four and six cylinder marine engines, although whether it was called 3G at that point is not clear.
Unlike it's slightly smaller relative (the 2G), the 3G box is mounted more conventionally at the flywheel end of the engine.

The 3G was available as a direct drive unit or with a reduction box of 2:1  or 3:1 ratio.  The photograph shows a reduction box fitted. It is
important to note that it is not readily possible to convert between direct drive and reduction drive in an existing installation as there is a difference in height of several inches in the output flange.. Otherwise, conversion is relatively simple. Where a reduction box is fitted to a 3G box, there is provision for air
or water cooling of the reduction gear.
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Lister 3G --  How it works:
The 3G is a relatively simple, three position gearbox:  Forward, Neutral, Reverse.  Power is transmitted to the gearbox from the engine by a deep gear
with a drilled centre. The gear is secured to the nose of the crankshaft with studs and nuts and the end of the crank is drilled to allow the passage of
engine oil under pressure to lubricate the gearbox.  There is no drive plate between the engine and gearbox. The input gear mates with
a set of internal epicyclic gears which are part of the gearbox drum and hub assembly. In neutral with the engine running, the hub assembly spins but no drive
is transmitted. When forward is selected, a pair of toggle levers are pushed over centre, acting on a carrier which pushes together a multi plate clutch.
The clutch discs have serrated edges which are permanently in contact with serrations on the inside of the hub; the end pressure plate is keyed to the
mainshaft. With the clutch discs pushed together, drive is transmitted.

To obtain neutral from forward, the toggle levers are pulled back over centre in the opposite direction, removing the pressure from the clutch discs
which are then able to spin relative to the pressure plate  -- no drive being transmitted.

Reverse is achieved by the action of a brake band gripping the outer face of the hub/drum assembly. With the drum unable to turn, the epicyclic gear train comes into play, transmitting drive in the reverse direction via a gear on the forward end of the mainshaft.

Forward and neutral are actuated by an internal forked lever operated by a gear selector cross shaft. Reverse is actuated by a lever, roller and rocker arrangement, also operated by the selector cross shaft. All these actions are performed by using an external, manually operated lever. The external lever
has its own external limit and detent bracket which limits its travel when selecting forward or reverse and gives a positive centre location for neutral.
It is common for these boxes to be controlled remotely. Whatever means is chosen to achieve this it is essential to confirm full and positive engagement
of the desired position. Lister offered a hydraulic servo kit for operation of the box but this is a vanishingly scarce option that we have seen just once.

In either the direct drive version or the reduction box version, a thrust bearing is fitted immediately behind the output flange capable of taking all forward or reverse thrust.

What are the common problems ?

The good news is that generally these are not a problematic gearbox, but inevitably age and use will catch up with them, so watch out for ---

Any sign of slipping, either in forward or reverse. The relevant adjustment should be checked as soon as possible.
If adjusting the forward engagement, be sure to get the toggle screws as even as possible.
Satisfactory adjustment of the reverse brake band can be difficult as often there seems to be insufficient scope to let the roller get to where it need to be.
It seems to vary from box to box; we suspect machining tolerances may have left something to be desired.

Not  a common failing, but not unknown; the brake band metal can completely fracture. Replacement is the only fix.

Still with the reverse band, it is also not unknown for the post that carries the actuating rocker to pull its studs from the gearbox casing.
However you choose to fix this, do it well or you will be doing it often (if you use reverse).

General wear is the next most prevalent issue, including worn bearings, bushes, gears and sometimes the hub/ drum itself. The good news is unless left to an extreme condition, none of these things will rob you of drive. A useful and simple test is to grasp the output flange (with whatever it is coupled to unbolted) and check for any lateral movement or evident rotational noise or roughness when turning by hand in neutral. Readily perceptible lateral movement, noise and roughness almost inevitably indicate a box in need of some renovation.

Bear in mind however that the gearbox is pressure lubricated from the engine oil circuit, so worn bushes along the mainshaft will leech away main engine oil pressure as surely as wear in the engine itself. As they share the same oil, any contaminants arising from the engine will find their way into the gearbox   and vice versa.

Things to do and not to do:

DO --

Adhere to the engine oil and filter change intervals. 

Remember that where a reduction box is fitted, this does NOT share the engine oil. It uses SAE80 or SAE90 gar oil and the level should be checked using the dipstick.

           Make sure the engine installation is such that the gearbox output flange and whatever is connected to it are perfectly aligned. The flanges should be perfectly flat to each other and the bolts go through their holes, by hand, fully. Misalignment here will cause premature wear to the gearbox and likely
the stern gear. Flexible  propshaft couplers are no substitute for basic correct alignment, unless they are a properly engineered double jointed assembly, using a separate thrust plate.

          If the engine appears to be labouring and / or black smoking. With the engine turned off and the gearbox set in neutral, check that the propshaft rotates freely.  If not, check for a fouled propeller.  Make sure no one can start the engine or engage a gear while you are doing this.

DON'T  --

Try and free a fouled propeller  by constantly alternating between forward and reverse. If a good blast in reverse does not clear the prop, best to
physically remove whatever is causing the problem. 

Leave the gearbox in gear if you are not using the boat for some time. If you go to your boat but are not moving it, it is a good idea to start the engine and run the box through its positions to keep things free in the  box.

Make any adjustments to the gearbox selector rollers unless there is a good reason. Any signs of slippage in forward or reverse should be investigated                 quickly, but otherwise ---- if it's not broken, don't fix it.
Use any additives in the engine oil. As the engine and gearbox share the same oil, the additives may glaze the linings, causing permanent loss of drive.

Fire the engine up with the gearbox removed.

                 Throw away your old Lister gearbox if you are swapping it out for something different.  Give us a call.
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Parts Availability.

 With the exception of the bearings and seals, every other part of the gearbox is a Lister specific item and the majority have not been supported for some
                              time. We keep stock of the clutch friction discs and brake bands plus bearings, bushes, seals and gaskets. We also stock core unit                                                   gearboxes and  reduction boxes for full reconditioning.
                             If you are shopping around for bearings, you need to be aware there is a dramatic difference in quality between those from a reputable                                manufacturer  and those (typically) of Chinese origin. Some of the bearings are significantly expensive, but given the work involved in a rebuild and the effort of removal and refitting it is an area where it is absolutely not worth economising.  As a rule of thumb, if the bearings aren't made in Europe, Japan or North America, you probably don't want them.

Top Tip !
                            We all like a bargain and if your box is playing up you may be inclined to find a usable second hand unit.  Just remember that the                             youngest of these boxes is likely to be pushing 40 years old, so you do need to be sure before buying that your prospective purchase
                            actually works.
                            If it is being sold  as rebuilt or having had attention, don't be afraid to ask to see the receipt as evidence of what has been done.

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