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Shaft is true, and under power there is no vibration. Engine mounts are good. When I had the engine out a few years back I did a lot of re glassing around the bearers. The lever has the proper amount of travel and fully engages in ahead and astern. In every failure the issue has been the cones shedding that gritty stuff they coat them with. Sounds like overloading. Installing the new box over the weekend. Lift and hold on Wednesday and will put my smaller prop back on. Oh, and I'm fussy about oil levels,too.

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If the clutch cones have been consistently wearing their friction material away they could (must) have been frequently creeping / slipping a little...too much torque or the oil is too slippery to let them "lock" in to their neighbour.  the helical spline that the cones run on would clamp up the clutch more with increased torque (up to a point) but if there is super slippery oil involved they may not . the other thing of course is the linkage adjustment but thats been double checked. ponder ponder.

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Oh no, not again Chris. This is nuts and I imagine bloody annoying.
You need to get the supplier to bring in the big guns on this. They spec'd it. It should not cost you, apart from the difference between the 80 and the 90 if they decide to up spec it.
I simply cannot see any problem on your install. And even if you had an alignment problem or a prop too power hungry problem, it does not really matter. The input is the critical part and the engine is well within spec. You are using the Commercial application figure and even then, you are right on spec, not over. If you use the pleasure spec, then you are well within the max. Even if the prop is overloading the system, it does not cause a problem that the box cannot handle without it also causing a sever load issue in the engine. And the engine is not loading down.
Alignment issues would not cause this issue either, but we know that is well and truly all good.
This really is head scratching and the supplier needs to get this sorted for themselves as well as for you. It simply does not make sense.
This is also why IT and I said it is important to get the supplier or installer to advise on Oil and also it's fill level due to mounting angle. Now you can go back to someone and say, we did everything as you all advised and it failed. Now they need to find out why and fix it.

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The oil is ATF Dextron 2. That's what the manual said and that's what's in it. Concerning the amount which is supposed to be important, I filled the gearbox to the dipstick mark prior to installation when it was sitting level, installed the box and marked the inclined position on the dipstick. Thankfully the new gearbox has the dipstick located exactly in the middle instead of the forward end so that should make things easier.

I think that gearboxes are the forgotten component in marine drivetrains. All that we ever hear is to make certain that the engine can achieve max rpm at WOT. Nobody seems too bothered about whether the gearbox can cope with the loading or not.

At WOT my engine is starting to labour and producing smoke consistent with unburnt fuel so the propeller is sucking up every one of those 29 horses produced. The old gearboxes were rated at 32hp, but thats at its max 4400rpm. At my WOT of 3600rpm the gearbox can take 26.5hp so it is clearly overloaded at WOT.

The assumption is that when you throttle back to 2000-2800 cruise rpm the propeller hp requirement will drop below the engine output. Without hard data as to the actual propeller loading for a given number of revs this would be, in my case anyway, an expensive supposition.

As to why I think marine gearboxes are somewhat ignored in the drivetrain, I don't think that I have heard of an auto transmission failing but I have heard of a lot of marine gearboxes slipping. The new PRM90 I'm about to install has a permissible loading very much in line with the engines power output curve so I hope it will cure my ills.

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Looked at a little Sole 3 cyl marine engine and gearbox a day or two ago. Very similar to mine, also rated at 29hp with an almost identical power curve. At 2000rpm it puts out 16.3 HP and at those revs the Twin Disc gearbox can take 18hp.

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Chris ,did you check that the gearbox morse control cable correctly engages the gearbox, ie has sufficient throw.

I too have had some gearbox dramas post a recently completed self done diesel heart lung transplant.

Cone gearbox TMC40. 

Original gb ratio not correct so run in period consisted of inability to run up to correct revs, changed prop to seek a solution(easiest) but still had black bum, still not correct.

Changed gb ratio (always carry a spare gb :thumbup: ) and voila all was sorted, until at 200 hours gb failed to engage and drive in forward, removed and examined oil, metal in oil,bugger.

Changed out engine control morse cable etc, replaced gearbox as other one was toast and then paid very close attention to the engagement throw length.

Here is what I learnt, over my entire life I have had almost a obsessional aversion to paying anybody to do anything,so I tackle with a reasonably successful hit rate most of all life's tasks.

I now have a great respect for marine engineers and the contorted and complex environment they encounter daily.

My next boat will have the engine in the saloon and I will make the tea whilst the engineer goes about his business.

Part with the $$$ and pass the original problems ownership on to a professional.

Hah, wife has new car, I,m banned from lifting the bonnet.

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So I hope this will be the final postscript to the whole sorry gearbox affair.

I mentioned in a couple of posts back that I was s bit suspicious of PRM in that whereas they had added a few new units to their stable of gearboxes over the years,the only one to be discontinued was the PRM 80 (mine) and which had been replaced by the PRM 90. Same external dimensions but with a slightly improved output.

Today I went to the supplier for some parts and the boss was not there but one of his mechanics was. In the course of an interesting conversation he mentioned that quite a few of the 80s had come back faulty over the last couple of years, all with the same problem. Slipping cones, so he said.

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