8n Hydrostatic lock ??? OR ???

Hows the ground braid? Hooked to clean metal?

Hows the starter to block mating? clean or cruddy?

What amps the starter pulling when cranking slow?

If ya put a new starter on and it cranked same way.. guess what.. it 90% probably aint the starter... it's probably a ground or wireing isue.. like insuficient wires / bad / dirty contacts.

Now that you have a battery and starter collection.. invest in a piece of sand paper , and perhaps new wires and or terminal ends and give er a test.

Soundguy

You write..... .."I put one end of the jumper cable on the stud going into the starter from the starter switch and the other end on the mounting stud of the starter"..... ...uh? That is confusing.

Jumping an 8N is really trivial. Make certain yer tranny is in NEUTRAL. Then clamp one of yer battery jumpers directly to the BIG starter motor stud where the flat copper buss-barr also connects. Then connect yer other jumper clippie to enny convenient GROUND. (I usually clamp onto the steering rod and wiggle a little thru the rust or paint) NOTE: starter motor don't care battery polarity. Now connect the OTHER end of eather jumper to enny battery post.

Check to see that yer tranny is in NEUTRAL. Ignition switch ON, carb-throttle lever about 1/4. And CONNECT the LAST jumper clippie to yer last jumper battery post. Starter motor should ZING. Iff'n NOT, ya most likely didn't gitta good clamp of yer jumper clippies. But iff'n yer certain yer jumper battery clippies are connecting good and it don't zing, then, yeah, bad starter motor.

ME? based upon yer convoluted description of jumping the BIG STUD TWICE, I'm gonna display my amazing abilities to mind read and GUESS you were actually jumping AROUND yer 3-terminal roundcan solenoid and that the INTERNAL GUTTZ of the starter solenoid are CRUDDED-UP. New 3-terminal tractor starter solenoid $10 (cheap)..... ..Dell, the amazing