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1973 Johnson 50 with “Powershift” piston assisted shifting

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dh_user

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Hi all - I have a 1973 Johnson 50hp, model 50ESL73R. The shift rod design is 2 sections: an upper shift rod that goes up through the exhaust housing and connects to the linkage on the power head just like nearly all shift rods do; and a lower section with a cylinder/piston/valve that lives under the o-ring seal and inside the gear case. The bottom of the lower section is a diagonal key that fits into a matching-face diagonal key at the forward end of the prop shaft.

My understanding is that this shift rod design lasted exactly one year (lucky me). It’s not an electric shift as used in prior years… no wires or solenoids. It’s not a mechanical shift, as used in following years (and according to the marine engine diagrams and my factory manual, on the manual-start version for 1973). It must have been a winner😀.

Which brings me to my issue: the NOS gear case housing I bought from marine engine has the same casting numbers as my old gear case, but the cavity where the shift cylinder belongs is maybe 1/16” or 3/32” narrower than the diameter of the shift cylinder, so the lower section of the shift rod won’t fit down in the gear case.

Anyone ever seen this?

Thanks,
Dean
 
I hate to tell you but the casting number & final part # are not necessarily the same. May have used the same casting & machined it differently between the years, so it would be assigned a different P/N.

Perhaps a machine shop could machine it to the correct dimension?
 
Also used in 1974 models.----Nothing wrong with the design either.----Changed in 75 to mechanical shift as a cost cutting measure.----Send the gearcase back and get the right one.-----Picture of your oled case ?----Find one on E-bay ?
 
Droid said:
I hate to tell you but the casting number & final part # are not necessarily the same. May have used the same casting & machined it differently between the years, so it would be assigned a different P/N.

Perhaps a machine shop could machine it to the correct dimension?
Click to expand...
Thanks Droid … I’ve experienced that too. Other than this cavity for the shift assist cylinder, the NOS gear case seems to match. I ordered it by the OMC part number from marine engine, but the old and new ones are definitely different in that one respect. I’ll post photos in a few minutes (tried last night but got a file size error).

I thought of carving out that cavity a little myself. It’s all inside the “wet” part of the gear case, so it should be fine I guess.
 
racerone said:
Also used in 1974 models.----Nothing wrong with the design either.----Changed in 75 to mechanical shift as a cost cutting measure.----Send the gearcase back and get the right one.-----Picture of your oled case ?----Find one on E-bay ?
Click to expand...
Thanks recerone; appreciate your experience. I’ll call marine engine today and talk through it with them. Posting photos in a few minutes.
 
Racer one, if I got tired of messing with this, what would it take to use the same NOS gear case and convert the shifting mechanism to mechanical?
(I'm somewhat committed to this gearcase, as all 11 lower pieces are primed/painted/clear coated.)
My first guess is to:
- replace the prop shaft with one for the manual start gear case (different spring and detent design, different front plunger for the shift rod linkage)
- replace the oil pump with the linkage parts on the mechanical
- get a one piece shift rod from the same year manual start parts diagram
Am I wading into a big giant science experiment here, or is it that simple?
 
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What was wrong with the old gearcase ?-----Myself I would switch it over to a gearcase from the 79 to 88 models.---Requires changing shift linkages on the block.-----A doner motor would be the way to go.
 
CORRECTION: I can no longer edit/correct my initial post - so I'm correcting one thing here:

The NOS gear case I thought I got from marine engine - I actually got from pine lake parts on ebay. It was sold as OMC part 385560, which is shown as superseded part 393752 on me.com (and both those part numbers are in the gearcase diagram for my engine)... so I thought it would be compatible.

I'll reach out to Pine Lake Parts, but it's been almost 3 years (I got sidetracked by a few projects in the meantime), so I think my options will be:
1) have Marineengine measure and confirm the size of the cylinder on their NOS part 393752... order that one, re-do the paint job, and wrap this up
2) machine the gearcase I have to widen the hole (this sounds janky to me, but I get it could work... it's not like it's a bearing housing or anything)
3) stick with the gearcase I have and convert to manual shift (can anyone weigh in on my idea in post #6 above?)

Any thoughts based on your experience?
 
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racerone said:
What was wrong with the old gearcase ?-----Myself I would switch it over to a gearcase from the 79 to 88 models.---Requires changing shift linkages on the block.-----A doner motor would be the way to go.
Click to expand...
I broke off 2 bolts (that hold the the driveshaft bearing housing) inside the old gearcase. 2 local machine shops looked at it and passed on the extraction project.

Hey - love this idea to switch out the gear case to a newer platform... you have a lot more experience here than I do... what do you like about that year range (why did you suggest that specifically), and what parts would I be swapping? Realizing a donor is ideal, I'm just trying to get a feel for whether it's the switch-over parts in post #6 above plus the powerhead linkage, or way more involved than than.
 
Have you verified that forward gear bearing / gear placement is the same on your new gearcase ?----I would say drill out the broken bolts on the old gear case.----That is what I would do here.----You would need to bore out ( servo cavity ) the new gearcase to an exact depth to get correct shifting.
 
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racerone said:
Have you verified that forward gear bearing / gear placement is the same on your new gearcase ?----I would say drill out the broken bolts on the old gear case.----That is what I would do here.----You would need to bore out ( servo cavity ) the new gearcase to an exact depth to get correct shifting.
Click to expand...
I've installed the oil pump, forward gear/thrust bearing/washer, plus the drive shaft/pinion/nut... and fit seems correct. The hole in the oil pump (facing up the gearcase) that accommodates the bottom end of the shift-rod (from the assist cylinder) is linked up correctly. The bottom of the shift rod just barely fits into that hole, before the cylinder bottoms out because it won't fit in the cavity.

Wondering your thoughts on the best long-term solution here...
1) find another gear case that fits and use it (whether I drill the bolts out or find another case)
2) keep current gear case and carve out the cavity
3) convert current gear case to mechanical-shift parts

Thanks,
Dean
 
I took the new gear case to a good machine shop, to have the cylinder cavity milled out a little wider. Will post photos when I get it back.
 
Update; after a good long time, I got the gear case back from the machine shop. He did really good work, and the gear shift rod/piston assembly fits now. Here are a couple of photos. (I also asked him to dig out the broken 1/4-20 bolts from the old gear case, so I’ll have that one too.)

Big thank you to everyone for the options and advice!
 
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