Firstly, thanks to the guys who posted threads much appreciated. For Fun Times the manual 39 has no additional information than manual 28 procedure is the same but definitely worth a look thank you.
Ok to complete my part on this post. I have found the markings on the gears and they are as per mercruiser standard markings. They are however extremely faint and I had to use wet and dry sandpaper and a torch at a 45-degree angle to see them. Perhaps the tech who stamped them had a big weekend on the turps and was tired when mine got stamped. They are as mentioned very difficult to see.
So now I have remarked (re-stamped) them and the unit is back together as per merc requirements. I was however thinking about the “why” earlier as in why are they stamped. I made an early assumption they were balancing marks not match marks for gear mesh. My leg is a bravo diesel 3 ratio 2:00 to 1. I have not been able to find much on the internet about this so have come to the conclusion they are indeed balancing marks not gear-mesh marks. If anyone knows different from a marine tech or Merc themselves feel free to post on after this. So, my conclusion for balancing marks not mesh marks is based on my experience with industrial gearboxes not marine but for my 2 cents worth here goes.
The gear “set” I am referring to here is only top set in the upper half of the drive unit but is typical for any similar sets. Therefore (the “set” being the single input driving gear and the two mating forward and reverse gears) if these gears had a 1 to 1 ratio, then one may assume marking are for match marking and it is also good engineering practice when reassembling used gears to ensure they go back in the same position to keep consistency in wear patters etc. In the merc manual it only references the two forward and reverse gears to ensure they are aligned; it makes no mention of the input gear and “matching” this with the two forward/reverse gears. For match marking the input gear and both for/reverse gears would all need to be put back in the same position.
My leg however has 27 teeth on the single drive gear and 32 teeth for both forward/reverse gears, Simple math ratio’s shows this can not be broken down further 27/32 therefore one revolution of the input gear would turn the two mating gears only .843 of a revolution. This means that it would take 27 revolutions of the input gears before the teeth would come back around into the same mesh as the start point. So, no I don’t believe the -/+ markings are for a “gear mesh” it just doesn’t add up.
Now for balancing which is what I believe they are for. I have made this conclusion on the following. The “top box “gear set would spin in my case with the 27/32 ratio at 84% of the maximum engine speed. Mine is a Diesel and WOT can do 4300rpm. Bear in mind guys with petrol engines can run significantly faster that that. These legs fit all sorts of applications and merc has to do it’s best to ensure its legs are suitable across a wide range of applications. So, the single drive gear is mated into two cup and cone bearings and is well supported for any out of balance, the gear is symmetrical in design so let’s say well balanced by design. The two forward and reverse gears are not as well supported within their bearing assemblies but let’s say fit for purpose. The gears themselves are symmetrical and would be well balanced in design except for the thread that has been machined onto it for the bronze bush to be installed for the shift lever to actuate it. The bronze bush itself is also symmetrical except for the thread. All threads have a start and finish point therefore depending on this point the weight would differ in its centrifugal balance on both the gear and the bush. Each gear has a plus or a minus symbol stamped onto it, some also with a “H”’, mine is next to the plus symbol. I would reasonably assume that once the bronze bush has been installed onto the gear the unit would be put onto a balancing machine and the heavy and light sides stamped with a -/+ symbol for assembly. Now the Merc manual makes it very clear you must pair a plus with a minus either matched top or bottom. (Note they don’t care which) you must not pair two minus or two plus markings. These being balancing marking would ensure the unit is assembled with the “heavy” side of each gear at 180 degrees to each other. This also makes sense as to why merc don’t care whether its top or bottom it’s about having the two heavy sides opposed to each other. The manual makes note gearbox failure can result if assembled incorrectly. What Mercruiser have done is halved the centrifugal force acting of the gear set assembly by installing the heavy sides of each gear opposite each other. The comment on gearbox failure would result from the additional load put across the bearing assemblies trying to retain the additional centrifugal (imbalance) force from an incorrect assembly. (Think all the weight on one side when spinning)
OK so if that doesn’t make sense above think about this. If both heavy gears are incorrectly set together when they are spinning, they will exert twice the centrifugal force as being opposed to each other. Whilst this balance may seem small you need to multiply it by – (let’s say one of the powerful petrol speed boat engines) multiply it by up to 6000 times. Therefore, imagine the gears trying to throw themselves out of the gearbox due to the combined or “double” unbalance” six thousand times minute. This will exert more load across all the drive components and will lead to premature failure of the unit.
As mentioned at the start of this text. These are my assumptions only and how have come up with my conclusions. Happy for anyone to add more particularly if they have additional vendor specific information which seems to be hard to find on the net.
