Can someone tell me what gear ratios were used on Merc 800, 850 and 80 motors from 1970 -84.
My 1980 ish Merc 80 is a bit of a Heinz 57 being an amalgamation of two engines with another thrown in to plug the gaps. The gearbox on it appears to be early style with a rounded back edge to the cavitation plate whereas the powerhead appears later leading me to wonder if the power head and gearbox are miss-matched.

As far as I'm aware, there all the same ratio, 2.3-1, there may have been a very few very early ones with 2-1, but chance of finding one is slim, however, 6cyl gear boxes fit and the majority of these are 2-1, does this help ?
And on a different note, thanks for organizing the meeting at Ivy Lake last week, as ever, very good.

Thanks for the reply, just what I was looking for. If ratios were different across the years I might have something to worry about but as they are all the same, no worries.

Thanks for your thanks but thanks to you and the other attendees too that came along and made the event happen

I'm a great believer in "If it fits,it'll work" I have a gear unit from an '84 75HP 2.3:1 on my '69 Merc800. Before that I had a unit from a 650/60HP with a gear ratio of 1.64:1.Just a pitch change on the prop to accommodate.
To work out your gear ratio,mark the tip of the prop at the skeg and mark the flywheel and see how many revolutions it takes to turn the prop one full revolution
2 flywheel revolutions : 1 propeller revolution is 2.1, 1.64 flywheel revolutions to 1 propeller turn is 1.64:1.

Answering my own question here and confirming the replies. However I finally got around to messing about with my Merc gearboxes so took the opportunity do the sums. One is an early one from a mid 70s Merc 800 and has a more vertical look and rounded trailing edge to the cavitation plate. The other is a later one from a mid 80s Mariner 75, this one has a more raked look and a flater trailing edge. Both are, as Se7en said earlier 2.3:1

..."I had a unit from a 650/60HP with a gear ratio of 1.64:1"

If that's a three cylinder, it's an entirely different set up and won't fit.

Jeff

Hi Jeff, far from marineengine you are It was from a 4cyl,of unknown origin to me,but the guy I got it from is very much a Merc tech and into his old Mercs,won't work on them except his own,and it fitted perfectly.He told me it was off an old 650 with 1.64.1 ratio,so I took his word for it.Now it's not beyond the realms of possibility of me mishearing him,however I am 100% sure he told me it was a different gear ratio,it fitted,and as I trust him.

I agree with fastjeff, if it was from a triple, it won't fit, it may have been from the older 650 four, from the mid to late 60s, but not sure what the ratio is without looking in the manual, I'm sure I remember a "service bulletin" back in the day ( around 1965 ) re a change of gear ratios in production and for replacement on the 650,900,950,1000,1100 models, which implies they all fit, I seem to remember the early models had a taller gear ratio,and early 800 gearboxes have more in common with the 650 4 gearbox, than they do with the later 80/85 gearbox. Will look it up in the next couple of days.

This is seriously embarrassing .I got off my high horse today and pulled the old gear unit and checked the gear ratio,2.3:1,stupid or what. The gear were the same up to '71 then changed to the triple,either at the end of '71 or early '72.
I think it's time I invested in a hearing aid