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Posted: Sun Nov 14, 2010 7:36pm

Post Subject: Gearbox slippage

I have a 1973 40ft Minden Narrowboat. It has a Lister SR3 air cooled engine with a LH 150 gearbox with final drive. On canal cruising, the above unit works well. The problem Iâm having is when cruising on rivers where more speed is permissible, overtaking or cruising into the flow. The engine revs well, but the prop and shaft do not keep up and there seems to be some slipping either at the gearbox or final drive. The oil levels are both O.K. (would old or incorrect oil be the cause of this?). It was suggested it could be worn drive plates. At present it only occurs when revs are increased to the top third of the rev range (no rev counter). Below this it behaves normally. Can you think of any other causes? A new stern tube was fitted last year but slipping occurred prior to this.


Posted: Mon Nov 15, 2010 9:22am

Post Subject: Gearbox slippage

The LH150 gearbox is different from most other hydraulic boxes in that it uses oil pressure to give you astern and neutral. In ahead there is no internal oil pressure in use. On any other box I would suggest that you make sure the oil level is correct and that it is using the correct grade of oil. In this case EP80 gear oil. However as no oil pressure is involved in ahead it can not be related to the oil. I am 99.9% certain that these Listers do not use a drive plate like a modern engine so that can not be the problem. Ahead gear is obtained by allowing a set of springs push a cone clutch into engagement (in astern and neutral oil pressure keeps the springs compressed). They do this via a lever with an adjustment at the top under the gearbox cover with the filler and domed section on it. AS the cone wears the clearance between the end of the lever and the operating piston gets smaller and I hope that you will find the gap has closed right up and the adjuster is holding the clutch partially engaged. If you are mechanically minded I am sure that you will find the adjuster (it is mounted horizontally, the vertical nut and screw under the dome is for astern)and if there is no clearance between the bolt head and lever give it some. The manual says 3/32 of an inch (2.38mm) and although there was a gauge available many say it is about the thickness of an old florin. If you are not mechanically minded you will need the manual. A copy of which can be fund on ( the end of the SL/SR engine manual. If that is not the problem then I fear the friction surface of the cone clutch may be worn or a circlip holding the cone in place may be worn or falling off. I suspect parts for these boxes may be getting hard to find so you need an company who really understands them (you can break an unobtainable part if you are a bit ham fisted refitting the gearbox to the engine) and the contact for parts is Marine Engine Services of Uxbridge if your local company can not source them. Tony Brooks

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