Busted Transmission

Patrick12411

New Member
Feb 21, 2020
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I own a 79cc harbor freight predator motor for my beach cruiser. This morning when I went to perform some maintenance I found the gear on the transmission that usually turns the jack shaft now spins independently and the smaller gear on the shaft just comes right off, like a screw or something popped out.

Is this repairable or will I have to replace the transmission? Im not even exactly sure what is wrong with it as I dont know the finer details of exactly how all of its pieces fit together and what could be broken or missing.

Heres a video showcasing what is wrong: https://puu.sh/FH342/7723656002.mp4

Would appreciate tips for repair because Im not happy about paying 100 dollars for a new hunk of aluminum that could fail the same way in the future. Recommendations for better transmissions would be welcome!
 

Patrick12411

New Member
Feb 21, 2020
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Cheap fix.

Replace the missing setscrew which secures your sprocket to the shaft.
Im glad to hear its apparently cheap to fix. Where would the screw go? In one of the threaded holes? Can I find a screw like that at a hardware store or do I need to find a specialty vendor online?
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,521
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The setscrew goes into a threaded hole in the sprocket, and secures it to the shaft. Take the sprocket to the hardware store and get a setscrew or bolt to fit.
Mark the spot where the setscrew contacts the shaft. File a flat spot on the shaft, then drill a small divot for the setscrew to seat in. Locktite the screw/bolt.

Setscrew or bolt cost less than $1.00
 

Tom from Rubicon

Well-Known Member
Apr 4, 2016
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Rubicon, Wisconsin
A couple things to add to 5-7's advice which you should do. If the sprocket hub has enough thread use double set screws. Second one jams the inner one. Something I have done repeatedly, single set screws get you going, doubles keep you going.
Pat, there most likely is anti rotation key missing too. The shaft should have a slot that matches an internal slot of the sprocket hub. Still a cheap fix.
Tom
 

Patrick12411

New Member
Feb 21, 2020
7
1
3
24
The setscrew goes into a threaded hole in the sprocket, and secures it to the shaft. Take the sprocket to the hardware store and get a setscrew or bolt to fit.
Mark the spot where the setscrew contacts the shaft. File a flat spot on the shaft, then drill a small divot for the setscrew to seat in. Locktite the screw/bolt.

Setscrew or bolt cost less than $1.00
Would a screwdriver work for securing the set screw?
 

Patrick12411

New Member
Feb 21, 2020
7
1
3
24
The setscrew goes into a threaded hole in the sprocket, and secures it to the shaft. Take the sprocket to the hardware store and get a setscrew or bolt to fit.
Mark the spot where the setscrew contacts the shaft. File a flat spot on the shaft, then drill a small divot for the setscrew to seat in. Locktite the screw/bolt.

Setscrew or bolt cost less than $1.00
Would a screwdriver work for securing the set screw?
 

Patrick12411

New Member
Feb 21, 2020
7
1
3
24
A couple things to add to 5-7's advice which you should do. If the sprocket hub has enough thread use double set screws. Second one jams the inner one. Something I have done repeatedly, single set screws get you going, doubles keep you going.
Pat, there most likely is anti rotation key missing too. The shaft should have a slot that matches an internal slot of the sprocket hub. Still a cheap fix.
Tom
How would I find an anti rotation key? Can they just be bought online easily or do I need to know what to search for?
 
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5-7HEAVEN

Well-Known Member
Aug 2, 2008
2,521
136
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The anti rotation key is aka a keyway. Your shaft and sprocket must be machined for it. If it fell out, a hardware store probably has it. They sell it cheap, by the inch.

The setscrew is tightened with a screwdriver or allen wrench.

Use Loctite,
 
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