I just counted teeth and the output sprocket in my kit is actually 10t and the rear sprocket is 48, just a wee bit taller gear making 19.2:1 reduction.
Yes, 10T solid output sprocket on the newer 20T/80TFW as the freewheel is in the 80T pulley, hence the 80T
FW designation.
The solid 80T, 90T, 100T, etc use a freewheel output sprocket and as mentioned 12T
FW is about as small as you can go and they fail quite easily and you need a larger wheel sprocket to compensate.
It's tough to fit that new gasbike HS 142F 49cc with new 20T/80TFW/10T in a bike like the 26" Macargi Pantera 7S but it will fit if you have a welder, angle grinder with cutting wheel, big vice, and drill with proper decent bit and you tell me, I think they turn out nice like this one I just finished this week.
I don't have time go into detail right now so in short easy steps if you want to keep the bikes chain guard and not loose the ability to clean the stock air cleaner and cut down the choke lever I:
1. Undo the front chain guard screw and drop the dual mount down until the clearance above the chain is 1/2" or less including cutting off the bottom part if need be.
The goal here is to get the guard as low as possible so the engine mount base can go as low as possible.
2. Mark the frame mount spot on the front fender mound and drill a new hole for it's screw and get your chain guard back on.
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3. Take the whole front 4-bolt mount system off and discard if you like, you won't be able to get the front right motor mount drilled with it in the way and sticks out to far anyway.
4. Get yourself a muffler clamp that will fit the front down tube, in this case a 1 1/2" will squeak in but a 1 5/8" is easier to find and works just dandy.
5. Get yourself a piece of steel and make a piece a bit wider than the muffler bracket and about 2" up with a ~1/2" long spot to weld to the bottom of the stock mount with as little front space as possible.
6. Get the new front muffler clamp mount on and use the rear base adjuster to lower the base dead level with just a '
smige' of clearance between the chain guard and the base to eliminate any rattle noise.
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7. Get the funky wider pedal axle, bearings and pedal arms on.
I find putting both arms facing the same way so you can turn them both facing up and is how wide you can go.
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8. Set the engine with the 4G attached on the base and move it to where it actually fits in the bike.
9. Just holding it put the chain on to the back wheel sprocket.
Now just adjust engine laterally. If your system is anything like this one the 4G sprocket is in too far to fit past the tire to rear sprocket.
You cut the spacer bushing down some and flip the sprocket but thats it.
10. With the chain on and aligned, both pedals on pointing up for clearance, get you engine set for the perfect mount, mark it and drill a bit oversized mount plate holes and use wide washers so you have a bit of 'wiggle room'.
Here are a couple of pics of the mount.
Hope that helps.