PRM 140VR2 Gearbox rattle

[Carronade]

I have a PRM 140VR2 Gearbox coupled to a Thornycroft 90 engine. The power unit has been installed in the boat since 1986 but has only had 750 hours running. The gearbox had developed a 'rattle' which was diagnosed as a drive plate failure. The drive plate was removed (spring plate early version) and replaced with a R&D plastic bonded coupling. The gearbox has been replaced in the boat and the engine restarted. The gearbox 'rattle' is still present. Has anybody out there got any experience on a likely fix for this problem.

The engine is in fine condition and one thought is to replace the gearbox with a modern box anyone done this for the Thornycroft 90?

 

[NorthUp]

Is the rattle at idle- what speed is your engine idling at?

 

[fisherman]

Did you see the condition of the gearbox splines, that go into the drive plate? Long term 'rattling' usually wears them down, but not 750 hours I would think, unless maybe this engine is a bit lumpy on tickover. Some of these boxes had a flat blade/slot drive for the oil pump, extensive slow tickover would wear that out, but the result was operation failure. Can you hear where in the box the rattle is by using a sounding device, ie screwdriver handle/broom handle poked in the ear and touched against the casing.

 

[Carronade]

The rattle is noticed at idle and does not fade until speed is increased to about 1000 revs it then reappears at 1400 revs then fades at about 2000 revs. Performance in astern is the same but noise is "damped"

 

[Carronade]

The condition of the splines in the old drive plate is not bad though some "polishing" is in evidence. To be honest the old drive plate looks to have been OK no doubt some noise was created by the springs which do seem a bit slack in the nmounts. The engine is run most weekends even if the boat is on moorings. Care is taken to avoid tick over too long and run up to 2-2500 RPM (in drive) until engine is warmed through. Difficult to identify location in the 'box' as the power unit is deep in the moulded keel area and noise is reflected even with a sounding stick. Sound is more like chatter than bearing noise which would normally be present through out the speed range. As I mention above the noise is not present at all revs.

If there is a problem which needs attention then the box will have to come out once again and then no doubt be stripped and rebuilt with new parts. (some parts are no longer available) the 140 box is obsolete. This is not going to be a cheap job and the price of a modern 260 D might be a more sensible alternative?

 

[Carronade]

Engine idles at 5-600 RPM yes the noise is at idle and is less evident at higher revs though does not fade significantly at any speed. Noise is reduced in astern. ...Clutch plates?