TOPIC: lower unit trouble

I was using my 77 1500 and at about 5600 rpm and in the upper 30 MPH range it revved to the sky. I pulled the throttle back, put it back in gear when back toward the dock adding throttle and heard grinding. Stopped checked everything and restarted. The same happened when into gear moved at and idle and then added throttle and grinding. I limped to the dock and used reverse and now it is the only gear I have. I then went home. I removed the lower reinstalled all seemed well stunned it worked for a bit then the same grinding in forward. question to all what when wrong in this lower unit thanks for reading

It sounds pretty catastrophic in nature.
But before we pronounce the gears destroyed,...Check and see if the shift cable is still going all the way forward in the port side 'channel' that it slides in when the controller is shifted into Forward gear and some throttle applied (engine off) to insure the shift cable has gone as far as possible.
Look to see if something has falled into the 'channel' and is blocking the shift cable from going all the way forward.
Look at the arm on the upper shift shaft (under the bottom carburetor) and see if that arm is bumping into the base of the carburetor before it is completely in Forward gear.
With the controller in Forward gear & ignition key 'OFF' turn the prop and see if it simply clicks when turned clockwise as it is supposed to do - or - if other sounds are heard.
With the controller in Forward gear & ignition key 'OFF' turn the prop counter-clockwise and see if it refuses to turn without forcing the entire crankshaft assemble to rotate with it - or - if it makes other sounds.
Shift into Neutral and see if the unit makes any sounds at all when turning the prop either direction. It should turn both directions smoothly and quietly.
It doesn't sound good from where I'm sitting. (humor)
Thom (I have complete replacement units if needed)

Doc thanks so much for writing that out. I tried all that you said and it was right on. I then tore down the lower unit and found the shift cogs on the gears very rounded. This would explain why it would, at low speed work, and then apply more torque it lets it spin by the cogs. If you have, and wanna sell a shift cog and forward gear for a 4 inch housing with 2 to 1 gears I am all ears.
I am not to worried about reverse it worked ok. I doesn't look good from where I am sitting LOL One more question anyone every re machine the cog to square them again?

check the spring behind the follower. there is a reason for the failure.

RATS,...I had this all typed up and it vanished.
I have all the needed goodies, but I agree with Dave in that we need to identify specifically what caused this.
Has this lower unit recently been disassembled? Perhaps during re-assembly the propshaft and bearing carrier were properly seated, or perhaps something got left out of the ratchet assembly that allows the cam follower to be so deeply sunken into the fore end of the propshaft??
It's not unusual for the Reverse gear dogs to get thrashed due to a worn shift cam, etc, but it is unusual for the Forward gear to loose its dogs.
How does the Forward gear's main roller bearing and race look?
How does the needle bearing look at the front end of the propshaft?
Can you take a picture or two of the propshaft from the side so I can get a better look at the cam follower.
It looks like is is way too far into the propshaft.
Is the cam follower spring still in the propshaft?
Is there still a pin loading the sliding clutch?
Is the cross pin retainer spring still in place on the outside of the sliding ratchet?
How do the driveshaft bearings look??
Thom

I got the pics but having computer issues so hopefully I can get back soon

everything is still on the prop shaft the drive shaft is giving me fits to remove, but I haven't had much time lately. The shift cam looks marked up pretty bad maybe some binding.