Several knowns here that you must stay with:
- Your engines require the 2.15:1 ratio lower units (ratio change occurs in the lower unit). There is no substituting this ratio for another ratio.... it simply will not work with the 4 cylinder engines.
- Your engines require the single exhaust down tube, of which also require the single exhaust relief transom shields. Any 4 cylinder transom shield of the large shaft collar steering fork design will work. (i.e., the 280 or 275 shields)
- Your 1984 PDS aft splines are likely fine spline.... so to prevent changing the PDS's, stay with a transmission that has the fine spline female yoke.... or simply change the yoke to fine spline.
- All transmissions in this year range (i.e., mid size driven gear bearing retainer) are of the same gear reduction, so any transmission will work. However, best to stay with a 4 cylinder transmission since they have not seen V-8 power put to them.
- All Intermediate housings and main suspension forks are the same between the 275, 280 and 285.
- A common misnomer here is that the 270/275 lowers are short shafts, and that the 280/290 lowers are long shafts! All prop shafts net the same length of exposed prop splines when the correct spacer is used. Propellers interchange with the only issue being that 270 and some early 280 prop shafts are not threaded for the spinner center keeper bolt.
The 270/275 2.15:1 lower unit exhaust outlet housing doubles as the prop torque fin/trim fin.
The 280/290 2.15:1 lower unit prop torque fin/trim fin is a separate part... exhaust exits above this through it's own outlet.
Of the two, the 280/290 trim fin is more effective at countering prop torque and is easier to adjust, IMO.
NOTE: there are some "A" transmissions (275A/290A for example) that I prefer over the pre- "A" transmissions. Has to do with the Bearing Box and collar, and method of sealing this area. If interested, I can explain later.
If you go with the 290 2.15:1 lower unit, it will have the exhaust back-flow prevention flapper on the drive. This means that you can eliminate the exhaust bellows flapper.
The 280 2.15:1 lower unit can be retro-fit with a 290 flapper.
Any and all interchanging of Lowers/Uppers to a non-original Intermediate housing requires that shimming be re-calculated and performed.
IOW, DO NOT swap a lower or an upper unit onto a different Intermediate housing without re-shimming.
One last thought.
4 cylinder drives in particular, rarely go bad if the maintenance has been kept up on. The exact same engineering, bearings, shafts, etc are used in the 2.15:1 drives as are in the V-8 drives......, meaning that you should not fear a good used 2.15:1 drive. You almost have to deliberately attempt to ruin one for, one to go bad on you.
In a perfect scenario, and if this were mine and wanting to stay with the same drive system, here's what I'd look for.
Two 280 or 275 single relief transom shields, collar steering forks, helmets, etc.
Two 275A/290A/SP-A transmissions.
Two good 275 or 280 Intermediate housings with the correct suspension forks, reverse latch units, etc.
Two 2.15:1 290 lower units, or two 2.15:1 280 lower units retro-fit with 290 flappers.
Any of these purchased separately would need to be shimmed correctly.
If you want power trim, then we'd need to go later in year model.
Here's a 290 power trim drive....... but I'd prefer the 290A
IMO, the best of the AQ series would be around the "C" and later drives... but these are considerably more money.
BTW, I just happen to know of a man in California who has two new 2.15:1 drives... but like said, I see little need to go brand new on these.
If interested, I could put the two of you in touch with each other.
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