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OMC 40hp Gearboxes - late 1960s

Posted: Tue Oct 20, 2020 2:49 pm
by Rapier
Having stripped and reconditioned my 1964 Johnson 40hp RD-26 gearbox, I wondered what to do with the 2 others that a kind CMBA member had passed on to me; the options being to strip both and keep the parts, or recondition the two. Parts are expensive now and I've noticed a lot of 'consumables' are just no longer available. Our trawls of part sources are becoming fruitless...I'd spoken to a long term supplier who'd always been happy to help source scarce bits from his bins and he was quite dismissive, citing the advanced age of these machines. So, it didn't really matter what I did with them..the outcome would be the same.

Anyway, it occurred to me as I progressed through the stripping, evaluating and rebuilding process that every 40hp box I looked at was different, even if the internal components were virtually the same. I knew this already, but had never had such a variety in my Backyard Boatyard to compare at the same time. I'd bought and sent countless motors to Mike Tarry for him to build that (famous) Johnson he owned, and I'd serviced an early 35 and owned 4 other OMC 40hps, aged from 1960 to 1971. Frankly, all I wanted was a reliable running motor and they all were: the variations in internal bits didn't interest me at all, and I could see no reason why I'd need to document them. Still don't, but this may be an aide memoire for someone going down the route...

The first box was a late 1960s Evinrude version (it was painted blue, under the blue hammerite). Draining the oil produced the normal gush of water and oil the colour of tar. The water pump housing, impeller wear plate and under plate area was crusty with sand and salt deposits and as always with lower units in this condition the front bolt broke off leaving the stud behind. I had tried 50/50 ATF and acetone, heating and cooling cycles, etc, but it was almost welded in position. The gears, thrust washers, all bearings, pinion and other bits were in excellent condition, which was a surprise. These were cleaned in solvent, along with the gear housings. The housings were stripped of all paint, with proper paint stripper (the stuff that you don't notice you've dripped on you until it's suddenly very painful). All the seals were replaced as a matter of course, being hard and cracked, as were the spaghetti seal and prop shaft 'O' ring. The whole lot was reinstalled with a coating of oil; pinion roller bearings put in one by one, with bearing grease, so they didn't drop through the drive shaft hole. I used Permatex to seal the casings and let it set for 24 hours before trimming the excess that had squeezed out of the seam. New drain and fill washers were fitted.

The difference between this post 1967 version and the '64 RDS-26 is the:
- Water intake pipe runs directly from the pickup to prime the water pump.
- There is no intake grate / grid plate on the port side of the motor.
- The water pump housing does not prime from the above, so this area is a different shaped base below the impeller wear plate (in the upper gear housing).
- It had a later propeller shaft with spring and detent ball bearings and grooved clutch dog (makes a more 'positive' shift).

The impeller was the same as the '64 (part 3/4in in height)

The next box was an early 1970s Evinrude version (it was painted pale blue, under white hammerite, or house paint..). Draining the oil produced water and oil the colour and smell of tar, with bits in it... The water pump housing, impeller wear plate and under plate area were suprisingly clean and as there was an unworn impeller (without any set) I assume it must have been serviced. The gears, thrust washers, all bearings, pinion and other bits were when cleaned evidence that it had had a mishap at some stage, compounded by silt and lack of oil. All parts had got hot, the shaft scored, the bearings damaged by the mixture and thrust washers scored too. Both gears had problems, the forward gear showing it's brass pushing had been pressed in and the reverse gear damaged by the dog clutch. I suspect that they'd been swapped around as a means of preventing the gears from jumping out, a common fix. Unfortunately, the bushing between reverse gear and the prop shaft was seized - this typically moves around the shaft and the lucbricating gear oil flows down a circular intake into an inner channel, between bushing and gear. The forward gear bushing has a thrust collar and the same lubrication, however this one is press fit and needs to line up both circular intake holes. Both bushings had been starved of oil and got hot, with some transference of material showng on the propeller shaft.

Given the tolerances are not fine and a new old shaft is £20, I went down the process of pressing the bushes out of the gears, discarding the damaged reverse (the ex-forward gear..) putting the old forward gear back to the reverse end and fitting the forward gear bushing to a spare new gear (that I hoarded from a boat jumble 15 years ago). The dog clutch is unworn, so I assume the episode that led to the damage on the gear casings meant it had been replaced, but the cradle that sits on the dog clutch was beaten up by that takes little load, so it'll stay. Both bearings were damaged; the roller bearing was noisy from wear and the tapered roller that sits at the point of the gearbox has signs of trying to beat off the grit that was circulating in there, over time. The FAG roller bearing is £2.50 on ebay from SKF (made in Romania) and the tapered roller from Timken £35, but a generic with the same dims, £28. Both are far cheaper than the recommended OMC part.

The difference between this 1971 onwards version and the '67 is the:
- Water intake pipe runs from the pickup round the side of the gear casing and up under the wear plate to prime the water pump - like the 64
- There is an intake grate / plate on the port side of the motor, like the '64.
- The water pump housing is smaller in height and of a different pattern to either the '67 onwards and the '64
- It had a later propeller shaft but without spring and detent ball bearings or grooved clutch dog - clutch dog is the same as '64.
- There is no drive shaft seal in the waterpump housing. Drive seal is positioned below the impeller wear plate.

The impeller is smaller (part 1/2in in height)

pics to follow..

Re: OMC 40hp Gearboxes - late 1960s

Posted: Fri Apr 30, 2021 12:36 pm
by LitchboroughLitchbor
Thanks for this helpful report. I've got 3 of these gearboxes, which will probably all need attention at some stage. Your insights will be useful. I wonder what you did with that "welded in" bolt?

Re: OMC 40hp Gearboxes - late 1960s

Posted: Fri Apr 30, 2021 4:01 pm
by Rapier
I think I managed to get the vice grips on it and remove the stud.

Re: OMC 40hp Gearboxes - late 1960s

Posted: Fri May 14, 2021 1:50 pm
by LitchboroughLitchbor
I was interested in the broken stud, because it's such a perennial problem!

I've been taking the exhaust cover, exhaust pipe and cylinder head water jacket off my 40hp and have broken at least 7 bolts in the process. I got fed-up with my little butane torch and invested in a MAPP torch, which does a much better job of heating it up.

I've managed to get 5 of the 7 out, using Mole grips and patience. But 1 exhaust pipe screw and 1 exhaust cover screw are sheared off flush, so I'll try and drill them out - or just leave them!

I'm planning to use Copper Ease grease for reassembly of all these fasteners - but don't really know whether it will be the answer.

Re: OMC 40hp Gearboxes - late 1960s

Posted: Fri May 14, 2021 2:20 pm
by floater
I too have battled with many seized bolts whilst wrestling with outboards, even stainless screws are no guarantee. I have usually resorted to drilling them out once broken, I've never had much success with any of the special extractor tools which promise much but deliver only a hardened sheared off stub for the user to then deal with. I use copper slip in many applications but I've never been keen on using it on aluminium outboards because copper and aluminium are not happy bedfellows, corroding viciously when in close contact. I've often cringed when when seeing copper pipe used on marine engines, probably ok in a closed circuit with corrosion inhibitors but on a raw system it must be suicide. I have however never been able to establish whether copper slip actually contains an amount of raw copper to be a problem in an outboard but I have found other products that are copper free for applications where galvanic corrosion due to the presence of copper is an issue. I once asked a very well qualified corrosion expert at work whose advice was to avoid using copper slip on aluminium. I therefore use a graphite / lithium product on the outboards.

Re: OMC 40hp Gearboxes - late 1960s

Posted: Sat May 15, 2021 10:34 pm
by Rapier
I'd stay away from copper slip. I've always used Quicksilver 2-4-C, which has PTFE in it's formulation. Lubriplate 105 for assembly (it's originally a gearcase grease for pre 1962 Mercs and other antiques with grease filled gearcases, has been renamed now - is white and slippery, looks like mayo) and Quicksilver 101 which is green and sticky, with zinc chromate (so no longer made in it's old formulation for H&S reasons..) & with teflon as per 2-4-C. 101 helps where you have opposing metals and salt water environment. Quicksilver also does a great needle bearing grease, just the right viscosity to prevent needles from dropping into the crankcase.