1974 CHrysler 75HP 757HA Lower Unit Problems.

[fcross69]

I was out on the water yesturday after doing a prop swap installed how the Fleet Admiral guided me. As soon as i got going my motor started bucking banging on my transom. I pulled the cover to notice that the shifting rod was trying to move when motor was bucking. My cables are adjusted correctly I am wondering why my motor is trying to kick itself out of gear. I instantly trailored it and went right to the the drain plug on lower unit and out came the evil white milkyness. I have no metal flakes coming out at all even ran a magnet through the oil. I dropped the lower unit and hooked up a fitting so I could pressure test it for a water leak she holds a 100 PSI with no leak down while shifting lower unit into gear and spinning prop. Any ideas of what I should do next or would water in my oil make it try to kick out of gear under a load?? I even had the cover off with cable disconnected with my best friend shift gears for me by hand reverse gear worked with no issues but forward was still kicking it out even with him shifting by hand and I even lifted motor to make sure shifting rod was going all the way in and it was. So I am lost and pleading for help on this... I my serial number is 7039 my model number is 757ha I do believe its a 1974 but not sure I have no way to refference serial numers?

 

[Frank Acampora]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

All your symptoms point to worn drive dogs on the forward gear and dog clutch. If you are CERTAIN that the linkage is shifting fully into forward, then you must first drain the oil. Next remove the prop. Then remove the rear bearing carrier (held with two allen head capscrews behind the prop.) You must pull out the complete propshaft, reverse gear, and dog clutch assembly. Look in and inspect thje forward gear dogs. If they look decent, then leave the gear in. If they look rounded and worn, you need to remove the forward gear and dress them square. You need to remove the pinion gear in order to remove the forward gear.

In the first photo you can clearly see the three drive dogs in the forward gear (off the shaft)

Many times, the reverse dogs are in much better condition because reverse does not get nearly as much use. If this is so, again, sometimes you can simply turn the dog clutch 180 degrees and use the less worn dogs in forward.

Ultimately you will need to either dress the dogs square or replace the gearpak (Very expensive and time consuming.)

 

[fcross69]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

You are the master I so appreciate your help going to get some kerosene so I can clean everything up and post my finds and pics.

 

[fcross69]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

Forward dog gears are worn but so is face of forward gear. Going to remove the pinion gear and then forward gear and try dressing them but I am now on a quest for another lower unit. Will show picsof everything once its all apart.

 

[Frank Acampora]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

To dress them, use a Dremel and a straight carbide bit. Work carefully to maintain vertical faces and grind to the rounded wear mark on each dog. That way they will all be substantially equal.

If you would rather a used lower unit, see my private message to you.

 

[fcross69]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

I have the nut off the pinion gear and water pump removed how do I get the gear removed do I need to break down the 2nd part of lower unit??? What other models lower units will work on my 757HA just so I know for reference.

 

[Frank Acampora]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

The pinion is simply splined to the shaft. Once the nut is removed pull up hard on the drive shaft or wedge something into the gearcase to force up the shaft. No need to split the gearcase BUT---Water pump and top seal must be removed as the top roller bearing will come up with the shaft. I think you will find it necessary to go to franzmarine@aol.com in order to buy a new top seal.

The seals ARE standard so if you have a large bearing supply house near you they can get the number off the steel shell or compare. Be sure to specify ALL stainless steel--around 30 bucks if I remember correctly.

While you are there, you might as well replace the shift shaft seal--it is only 7 bucks. Also replace the fill and vent seals. Those are mostly standard and you can use Merc or OMC ones. 50 cents apiece. There IS an O ring sealing the lower gearcase to the upper portion but if you are holding 10 PSI, it is most likely still good. It doesn't get any movement or wear. The only time it would leak is if the gearcase was severely corroded.
I don't remember the number so find a maual on line or in the library (does anyone go there anymore?) or ask Franz. It is necessary to torque the pinion nut to a specific value to preload the two shaft roller bearings. Procedure calls for a new pinion nut but you can stake or re-deform the old one so it will lock onto the threads.

 

[fcross69]

Re: 1974 CHrysler 75HP 757HA Lower Unit Problems.

What other models lower units will work on my 757HA just so I know for reference.