[How to Tutorial] Upgrade 520 chain & sprocket kit on GV125-S EFi (GV125 S 2019+ Models)
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Having been tempted by the promises of immense top speed by shiny things in the shop i purchased the upgraded sprockets and chain for the GV125S-EFi.
It duly arrived and promptly work got busy and i put it on a shelf until last weekend and then i started the install.
Started with getting the bike up on the trolley jack. Yep i’m too tight to get a lift/paddock stand/etc but got the bike up and stable.
Next job was to remove the front sprocket cover (3 bolts).
You then need to loosen the front sprocket and rear sprocket nuts. No photo’s at this stage as much frustration ensued in trying to remove the front sprocket. I ended up cutting a bit of wood down to a size i could slot through the rear wheel and swingarm to prevent the rear wheel moving and putting a lot of weight on the front sprocket. Rear sprocket nuts came undone ok.
Main thing here is to keep the chain on so you can chock the rear wheel and loosen the sprocket nuts.
Then rear wheel off, reflect that size does matter…
Old chain and sprockets were in good condition (as they should be after only 4000miles!)
New rear sprocket on (old one laying on top), nuts torqued up to 20-30Nm.
The new front sprocket is substantially larger than the original
Original 428 chain was 130 links (OEM is 138 links as per Hyosung Korea)
This particular GV125-S model has a lightly shorter swing arm (for whatever reason, Korea has some explaining to do!)
Anyway because the 520 kit was sold for bikes that originally had 138 links 428, you can see we had to cut 1 or 2 links off to match my shorter swingarm.Admin Edit = Currently investigating 2-3 customers that have shorter swingarms and why Korea ships bikes with 138 links and some owners report their original bikes have 130 links , sometimes 134!. I personally believe it is something to do with early batches of swing arms being used early production models, but we will have more answers soon. It is not a big deal, it is better to be a bit longer than shorter , so that there is 1 extra link to cut off if it is needed! ❤
New master clip installed. Note that the closed end should be in the direction of chain rotation.
Then it was time to torque up the front sprocket nut (80-100Nm) so back in with the wood chock (the one i didnt take a picture of earlier!) so that i can use the chain and rear wheel to lock the sprocket up.
At this stage i realised the chain was too long. After some messages with Marcel whilst waiting for a chain splitter to come i had to take a couple of links out.The new front sprocket is 14 teeth, rear is 46 teeth , and the length from front sprocket centre to rear sprocket centre on my bike is 24-1/2 inches. After talking to Marcel it appears that i likely had a shorter swingarm from standard out of the factory as the supplied chain should have been fine (typical…i have all the luck!)
Admin Edit: (though you can always request different size sprockets based on your riding style, this one is balanced=it will have more top speed and also accelerate the bike faster , magic ? Probably! We need more GPS proof because text is not enough isn’t ) – We won’t ever send 15T because it is silly (12T, 13T and 14T are good enough!)
So we will be looking to do 44T rear! 😀
The 14T 520 is a lot bigger than a 14T -> For the same revolutions the engine makes , you get more out of a 520 kit. A larger “CIRCLE” rotates less than a small “CIRCLE” in basic science terms , so engine will scream less in 1-3rd gear , and larger sprocket is 46T (big circle) vs smaller (428 circle) sprocket. Go big or go home ? 😉 …. It is funny people say 14T on any 428, 520 , 525 has the same “rotating” circumference need to go back to school! when evidently a 14T 520 looks almost the same size as a FIFTEEN TOOTH 428 (yes 15T 428 is similar size to 14T 520).Yet 14T 428 vs 14T 520 is evident which one will make the engine LOWER its raves for the SAME SPEED. This is why buyers should be careful listening to the uneducated wannabe facebook mechanics. Rant over 😉 , sorry for digressing on your topic Paul!
Front sprocket installed and rattle washer knocked flat
Everything back on, double-checked and went out for a 10mile shakedown ride. Initial impressions are good, picked up speed well, and cruised at a higher speed for lower revs. I’ll report back with an update at the weekend when i get out for a proper ride.
Oh, and whilst the bike was out of use, i gave it an oil and filter change and new brake pads all round…oooh red!
Thanks Marcel for the parts and the support!
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-
Having been tempted by the promises of immense top speed by shiny things in the shop i purchased the upgraded sprockets and chain for the GV125S-EFi. It duly arrived and promptly work got busy and i put it on a shelf until last weekend and then i started the install.
Started with getting the bike up on the trolley jack. Yep i’m too tight to get a lift/paddock stand/etc but got the bike up and stable. Next job was to remove the front sprocket cover (3 bolts). You then need to loosen the front sprocket and rear sprocket nuts. No photo’s at this stage as much frustration ensued in trying to remove the front sprocket. I ended up cutting a bit of wood down to a size i could slot through the rear wheel and swingarm to prevent the rear wheel moving and putting a lot of weight on the front sprocket. Rear sprocket nuts came undone ok. Main thing here is to keep the chain on so you can chock the rear wheel and loosen the sprocket nuts. Then rear wheel off, reflect that size does matter… Old chain and sprockets were in good condition (as they should be after only 4000miles!) New rear sprocket on (old one laying on top), nuts torqued up to 20-30Nm. The new front sprocket is substantially larger than the original Original 428 chain was 130 links (OEM is 138 links as per Hyosung Korea) This particular GV125-S model has a lightly shorter swing arm (for whatever reason, Korea has some explaining to do!) Anyway because the 520 kit was sold for bikes that originally had 138 links 428, you can see we had to cut 1 or 2 links off to match my shorter swingarm.Admin Edit = Currently investigating 2-3 customers that have shorter swingarms and why Korea ships bikes with 138 links and some owners report their original bikes have 130 links , sometimes 134!. I personally believe it is something to do with early batches of swing arms being used early production models, but we will have more answers soon. It is not a big deal, it is better to be a bit longer than shorter , so that there is 1 extra link to cut off if it is needed! ❤ New master clip installed. Note that the closed end should be in the direction of chain rotation. Then it was time to torque up the front sprocket nut (80-100Nm) so back in with the wood chock (the one i didnt take a picture of earlier!) so that i can use the chain and rear wheel to lock the sprocket up. At this stage i realised the chain was too long. After some messages with Marcel whilst waiting for a chain splitter to come i had to take a couple of links out.The new front sprocket is 14 teeth, rear is 46 teeth , and the length from front sprocket centre to rear sprocket centre on my bike is 24-1/2 inches. After talking to Marcel it appears that i likely had a shorter swingarm from standard out of the factory as the supplied chain should have been fine (typical…i have all the luck!) Admin Edit: (though you can always request different size sprockets based on your riding style, this one is balanced=it will have more top speed and also accelerate the bike faster , magic ? Probably! We need more GPS proof because text is not enough isn’t ) – We won’t ever send 15T because it is silly (12T, 13T and 14T are good enough!) So we will be looking to do 44T rear! 😀 The 14T 520 is a lot bigger than a 14T -> For the same revolutions the engine makes , you get more out of a 520 kit. A larger “CIRCLE” rotates less than a small “CIRCLE” in basic science terms , so engine will scream less in 1-3rd gear , and larger sprocket is 46T (big circle) vs smaller (428 circle) sprocket. Go big or go home ? 😉 …. It is funny people say 14T on any 428, 520 , 525 has the same “rotating” circumference need to go back to school! when evidently a 14T 520 looks almost the same size as a FIFTEEN TOOTH 428 (yes 15T 428 is similar size to 14T 520).Yet 14T 428 vs 14T 520 is evident which one will make the engine LOWER its raves for the SAME SPEED. This is why buyers should be careful listening to the uneducated wannabe facebook mechanics. Rant over 😉 , sorry for digressing on your topic Paul! Front sprocket installed and rattle washer knocked flat Everything back on, double-checked and went out for a 10mile shakedown ride. Initial impressions are good, picked up speed well, and cruised at a higher speed for lower revs. I’ll report back with an update at the weekend when i get out for a proper ride. Oh, and whilst the bike was out of use, i gave it an oil and filter change and new brake pads all round…oooh red! Thanks Marcel for the parts and the support!hi mate what year is your bike ? I just orded 1 for my sons bike but his is the 2025 model just hope i dont have same problems with chain been to long lol.
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