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Just speculation, but the 7 1/2 was the highest hp motor that had plain bearings so I wonder if they were concerned about bearing load(?)
I like this theory. I do know from what I have seen, nothing will throw a rod faster than an AD on a lean oil mix.
Long live American manufacturing!
After taking a look at the chart that oldemtr provided, a few things became apparent.
This chart is from 1963 or later. The OMC 7.5 Hp had stopped production in 1958. The other models continued to be produced through 1963, and improvements were being made. Taking a look at the section titled “Crankshaft Bearings – Diameter Clearance” the 7.5 Hp looks to have relatively larger bearing clearance numbers, when compared the 3 and 5.5 Hp. The 5.5 is listed as having “Roller Bearings”, so that may explain why it’s oil recommendation went from 16:1 up to 24:1. I don’t what the justification was for the 3 Hp change from 16:1 up to 24:1.
As Einstein famously wrote “Don’t forget CD and AD use 16:1” (but maybe not for the 1963 CD)?
I have the same chart Tubs has posted but I totally ignore the OMC recommended 24:1 mix given for the earlier AD and CD motors and stick with the original 16:1 mix for the older models. The 50:1 mix for the ’64 and later motors is OK as they all had needle bearings on the rod.
Notice to they say ’50:1 mix can be substituted for 24:1 mix as long as Johnson oil is used’. Was it possible the powers that be at OMC realised that sales of new six horse motors weren’t as good as expected and lots of their older motors were still being used regularly with no end in sight? Realising that a satisfied Johnson customer would probably come back and buy a new one again, slowly but surely eliminating the old motors off the face of the earth by purposefully and deceitfully shortening their life span would guarantee increased sales and profits and everything would be peachy keen again. After all, why wouldn’t they recommend the same 24 or 50:1 mix for the old TN series of motors? Possibly because they knew they would need replacing soon anyway or new parts for them were just NLA from their dealers.
Possibly I’m a bit too suspicious but I do have a box full of badly scored pistons I’ve collected over the years and the majority of them are from the 5.5 and six horse lineup. Sure, their are other causes but I also have a Fleetwin 7.5 crank and rod sitting here with close to an 1/8″ clearance on the lower rod journal. The rest of the motor was pretty worn to and I suspect it was caused by using a too light gas/oil mix.
As for the original question, stick with the recommended 16:1 mix and enjoy your summer! After all, oil is the lifeblood of any motor and is the cheapest thing you can put in it. Trying to save a few pennies now might cost big dollars later. Don’t be stingy with the oil as too much is always better than not enough!
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