What causes lower unit failure?

As the actuator is positioned now, is the leg in a position that the boat would be going straight forward?

 

If not water on the gears its torque above design spec, rpm doesnt seem an issue as long as its doesnt get too hot

 

The biggest killer by far is water entry caused by old seals, fishing line, engine overheating and corrosion.

Tne next are shock loads from impacts, prop ventilation etc.

Dog clutches are problematic due to poor shifting habits, adjustment and damaged cables.

Brand and model have a big say in robustness. In my experience Mercury build the toughest gearboxes, followed by Yamaha and the other Japanese brands, with the almost extinct OMC/Bombardier units right down the bottom, mainly because they always assumed their machining was too good to require proper gear backlash adjustment.

This is what Mercury do with racing gearboxes, no shifting, about 1:1 gearing, long narrow housing. This one is a dual prop unit, hence two driveshafts.
360apx | Competition Outboards | Mercury Racing https://www.mercuryracing.com/engines/competition/360-apx.html

Your question is too broad, with no idea of your plans given, except over speeding. For example outboard sizes range from ~2-600Hp in multitudes of year/models with different gearcases, some robust, some fragile.

 

They always used the same legs, the old OMC 25 & 35 had different legs.

On gear oil, it doesn't have to be marine, but stick to synthetic rather than pure mineral oil.

 

Mine failed from old age and it blew a seal, losing lubrication. ( It was a 1972 and failed in 2020, so not too bad. ) A friend who ran a marine repair service told me, that the most failures he had seen (this was in the 80's) were due to people hitting stuff, mostly rocks, with the lower unit. I have heard people say to use ATF as lubricant, but I did not try it. I followed the owners manual to the letter.

 

I got to say the Volvo Penta was the best, in spite of how complicated they were. The shift smoothly with the cone clutches instead of dogs like a Mercruiser. Also, they are reversible.

 

Agreed Gonzo, I was talking outboard there, but in sterndrives, Volvo was always tougher in the outboard gear, however Mercruiser had superior engines early on, and when Volvo went to Chevys, Mercruiser had much tougher exhaust components, just don't moor them in salt or the boat sinks.

 

Ike OMC (Johnson/Evinrude) specified ATF as a performance alternative on portable engines, though I forget the HP cutout, maybe up to 6 or 9.5hp; and they demanded it for larger solenoid shift gearboxes (pushbutton controls). These were in use prior to that 1972 period up until ~1975. It's kinda like you lost a knot with the 80W90 diff oil but gained a fair bit of protection over ATF.

 

I owned 2 350 hp Volvo D6 (diesels) DPH ( duo props) and was not impressed
I had the DPH-A gen and when coupled with the 350 engines, the legs did not stand up very well. The original DPH were designed for much lower horsepower when they first came out and
had lots of issues with the gears overheating. They have been upgrading the DPH series and are now up to DPH E

I had a couple of discussions with water taxi owner that used twin DPH series in 3 of their vessels. He said that they always keep a rebuilt spare unit available to enable a quick change as
their longevity between rebuilds was not very good.
Here is the breakdown of the DPH series by year and revision based on industry records:

• DPH-A (First Generation): 2004–2006
• DPH-B: 2007–2010
• DPH-C: 2011–2019
• DPH-D: 2014–2020 (overlapping production with C)
• DPH-E: 2020–Present (current, often used as replacements for older DPH setups

The issue appeared to be the fact that the original Generation A was not designed for up the the 400 hp that the diesels that Volvo is using now.
A company at the Seattle boat show had improvised a pitot style pickup through a tube that would spray water on the upper gear case area.

I cannot speak to the current Generation but I was not impressed with the DPH-A legs

Odd that they worked on trying to gain reliablility with 4 new generations.

 

It was red but it was never atf
OMC type C oil

 

Getting on, but I had it lodged in memory that OMC issued a service bulletin regarding this. Maybe early to mid 80s.

I had a quick look and couldn't find anything, maybe I got it wrong. Perhaps they were never scanned, as Bombardier wouldn't have been interested in 40 year old superceded tech, and OMC had a lot on the plate in latter years; with their FUCHT technology.