VP 2000 series Gearbox Spline Wear

[Charieis]

Having read the various posts on this and other forums on the subject of worn gearbox drive splines, I thought I ought to check mine this winter. I seem to be one of the lucky ones - the wear I have on a 1985 engine is relatively small. However, I was intrigued by all the chat about torsional vibrations causing the problem. It seems to be glaringly obvious that the problem is probably due to mis-alignment of the splined drive plate bolted to the flywheel and the gearbox input shaft.
There is no precision alignment provided between gearbox and engine block (even a car has dowels on the bell-housing!) and the bolt holes in the splined drive plate are roughly machined and is attached with ordinary sloppy bolts.
The gearbox shaft will attempt to precess around inside the drive plate spline if the allignment is not perfect. A simple sum says that even a couple of thou of mis-alignment at 3000rpm will cause the equivalent of a 6 inches worth of metal on metal grinding in a minute. An hour equates to 30 feet and 200 hours around a nautical mile! Perhaps it is not surprising that they wear!
The "cure" of using resilient sleeves over cap-head bolts to "absorb vibrations" looks to me like curing the symptom rather than the fault, but I guess it should help anyway.
Cars have clutches that self centre and spigot bearings in the end of the crankshaft. Anyone know what alignment techniques are used by other marine engine manufacturers?

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[jleaworthy]

I am very interested to read your views, especially as I have just had the DB Marine mod done to my VP2002. I, too, was surprised when removing the gear box that there were no dowels locating it to the engine flywheel housing but then realised that the bearing carrier on the front of the gearbox is a machined fit into the bell housing and so acts as a large central dowel! Also the splined drive plate has a machined central spigot on the side fitted to the flywheel where it mates with a machined recess. It is true that there is no end support such as a spigot bearing for the splined shaft other than its fit into the splined drive plate. This would certainly allow some movement which could only increase with increasing wear. It was for that reason that I changed mine although it didn't appear any where near the point of failure. I was afraid that the wear would rapidly accelerate resulting in an earlier failure than the wear to date would have suggested. Incidentally I was very pleased with the service received from DB Marine. Their mod is a far better engineered solution than the original Volvo Penta effort.

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[Birdseye]

The gearbox on my 2 gm yanmar simply has a lip which fits inside the flywheel housing to give lateral location, and the usual bolts to prevent rotation. From memory (the gearbox is in my garage, but the engine is at the club) the flywheel has springs to give some degree of torsional shock absorbing.

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