T80 Motor build.

This is how it ended up so far.




I bought this motor, a T80, from BGF. I've learned the hard way that if you want a good dependable china girl motor you're better off if you completely disassemble your new motor, and build it up from the crank. I'll let you see a little of what I did.

Assuming you have your new motor in pieces, we'll start.

First thing to look at is the crank. It's the heart of the motor, and I've noticed they aren't very well made. There's 2 potential problems to be aware of. Most of the cranks aren't very true, so there's a lot of run out. And the balancing of the fly weights is a cruel joke.

The reason it's important to limit the run out is because if there's too much, the crank bearings will oscillate back and forth in the block halves, and cause the holes to go oval in the block. This ruins the block of course.

I don't know of a way at this time to true a china girl crank, so my method of finding a good one involves buying new ones until I get one I like. I ended up buying four crankshafts before finding one with only about 0.001" run out.
More than 2 thousandths and you can see the bearings waggle back and forth on the crank with your naked eye. I said naked haha.

Here's an example of typical crappy china girl crankshaft quality. Notice the flywheels aren't the same size.



A lot of the china girl motors I've seen have crankshafts that come with 2 piece flywheels. You can unbolt the fly weight and remove it from the crank for balancing purposes. The fly weight on the left is stock. The one on the right is one I started drilling to change the balance factor.



After some rough calculations it seemed apparent the flyweights were off by at least 50g. That's a lot. I stopped after removing about 40g. This is how it ended up.



Now that we have an acceptable crankshaft, we want to install it in the block. Do yourself a favor, and buy some good bearings from a bearing supply house. What you need are 6202VVC3 NSK, which are Japanese. These bearings will fit both the crankshaft, and the clutch shaft. When used on the crank, remove the plastic seals, but leave them in place on the clutch shaft.

You want to install the bearings in the block first. It's easiest if you heat the block with a propane torch, and then the bearing will drive in with little effort. Now you should be able to install the crankshaft into one of the block halves by hand. The bearing inner races aren't a very tight fit on the crankshaft. Of course use a new gasket between the block halves, and I used gasket sealer. Make sure the block halves are alined properly so the cylinder deck is flat, before you torque the block haves together. I used gasket sealer on a new cylinder base gasket too. None on the head gasket though.

This is what I used to compress the rings to get the piston into the cylinder. It's like a wire ty, but bigger. They usually put them around flex ducts. It was easy to use. To get it off, you just stick a small screwdriver into it, to unlock it and push it back apart. It worked so well I didn't even have to tighten it on the piston very much, which made it easy to operate and get back off. Make sure the rings in the correct positions.

Now I want to install the head. The stock T80 BGF head is the one on the left. The head on the right was on a X80A from LEB.



So both of those heads are crappy. The T80 combustion chamber looks like f, and probably will cause piston ping. The one from the X80A looks a little better, but probably not. I think there's so much room over the piston, there's not much squish effect with either head.

So this one looks good. A GEN2 MM high compression head. It calls for a 0.016" piston dome, and 0.003" squish.



For purposes of determining the squish, I used already been used gaskets for making the actual measurement.

The idea is to use solder on the piston top. Bolt the head down, and turn the motor over once. Then take the head off and measure how much the solder got squished down to. Hopefully 0.003". The solder I'm using is 0.036" dia. This head has M8 cylinder studs, torqued to 140 in lb.





Ok, with used gaskets we get 0.027". Close enough. I figure new gaskets, maybe 0.030". Hope the bearings don't get loose. Just kidding, not really though lol.

If you're using a stock piston, and head, and if you're using a new, base and head gasket, you don't need worry about the piston hitting the head. They'll be plenty of room.

Now install the crank seals. Then you can build the magnito side, and install the pinion gear.

Next you need to install the clutch shaft. I had a little more trouble with this than the crank for some reason. Maybe because it was my first time doing it. What I did was install a new bearing in the left block half. Use a propane torch to make it easier to install, and avoid damageing to the block. Then stick the clutch shaft through the motor from the primary side. The shaft should slide through the bearing race, into place. It seems like the bearings fit a little tighter on the clutch shaft than they do on the crank, but don't give up, it'll fit. Now heat the right block half where the bearing is going into. Use a appropritly sized socket to drive the bearing into place. Make sure it's all the way in.

The only other thing left to do is install the gears on the shafts. I see a lot of people having trouble both putting them on and taking them off.

If you install the gear on the shaft without the key, it slides right on. Both gears, and the drive sprocket are this way. The problem is when trying to install it with the key in place is a lot harder.

What I noticed is as you try to slide the gear over the key, the key shifts out of position and starts jamming the keyway. If you presist in forcing the gear on the shaft anyway, not only will it be hard to remove, but the gear could end up on the shaft a little cattywonkus.

I found if you keep at it long enough, you figure out how to get the gear over the key, without binding in the keyway. If you get the gears on right, they'll be easy to install and remove, and the bolts won't need to be breakwind tight. A small file can be very helpful.

That's my advise about that.

And this is how it ended up. a BGF T80, in my Cadillac AV Sport.

I've got about 10 miles on it now. So far it hasn't been over 27 mph. One thing standing out so far is how smooth the motor feels. The work I did on the crank seems to have had a positive effect. Hope it stays that way.

It has a ton of compression. On the first try the clutch just slipped when I tried to start it. I had to trim all the clutch pads to fit exactly right to get it to stop slipping the clutch. After that I did start right up, with no choke.

I'm running a NT carb with a 68 jet. NGK B6HS. The needle is in the leanest position, and probably needs to be moved richer. I'm running 24:1 mix dino 2 stroke oil.

Rocky_Motor said:

24:1? I'm not very experienced, but I thought break in was 16:1. I've run about 3 miles so far and it seems fine. Makes an odd sound, kindu like an electric motor sound not really sure but I think that is just part of the motor.

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A lot of us like 24:1 instead of 16:1. It's a lot less messy too.

Try lubing your pinion gear with grease to help with gear whine noise. You need to maintenance your pinion gear every few hundred miles after that.

maurtis said:

I second this! I plan on doing my first China Girl build soon, and am comfortable with working on anything above the crank (port cleanup, stud replacement, carb tuning), but thinking about opening up the bottom end scares the heck out of me. So much so, that I am pretty sure I will be rolling the dice on mine that the bottom end will be ok...

So, $165 shipped for a BGF kit, or $215 for a BNF (BikNutFast) kit? BNF for me all the way

BTW, are those MM Gen 2 heads still available? I cannot seem to find them online, and I like the look of them over the Pirate Cycle heads...

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The heads are available, but that doesn't mean anyone has them in stock. I waited about a month for mine.

Basement Cat said:

Could you give more detail on how you balanced the flywheel? I'm looking to do the same thing to mine because the vibrations are unbearable.

This is a pretty sweet build and thanks for the helpful guide

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Just remove about 40g total from your flyweights (20g each one) and you'll be pretty close. The flyweight on the left is stock. This is what it looked like after I finished with it.

wayne z said:

Hery Biknut, I'm impressed at how much you improved the little chinamotor with your work.
It takes a press and lathe to true up those crankshafts.
Press the pin out to seperate the flywheels. Then you can put higher quality bearings on the crankpin if you want.
Re-assemble the crank halfs on the pin just far enough that they can move on the crankpin with some interference.

Put one end of the crankshaft in the jaws of the chuck. Use a dial indicator on the other flywheel, and repeatedly strike the flywheel edge with brass drift or mallet, until you get minimal run-out. Then take it out and complete pressing it together . Put it back in the lathe an re-check for run-out.

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Wayne, thank you for the encouragement, and great instruction on china girl crank truing.

I was thinking about doing exactly what you recommend, but was lacking confidence to jump into the fire without some guidance. I'll have to buy a Harbor Slave 20 ton press too. I think I'll do it.

killercanuck said:

Sounds like the extra work/parts are worth it for a smoother running engine. She must be pretty torque'y with the extra compression and the length of your header pipe. Have you thought about extending your intake? Longer intake increases torque too(at the cost of top end), plus a level carb never hurts

Good job, thanks for all the pics!

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I think it's worth it, but if you find out your crank is bent that's a problem. If you don't want to know about it you better not look, lol. You could still balance it though. A bent crank that's balanced is probably better than a bent crank that's not balanced.

This motor has 20 miles on it now. It's very quite mechanically, both accelerating, and decelerating. It seems pretty tight too. When I hit the kill button, it stops moving right now. It feels like there's a lot of compression. When I let the clutch out it strains to hook up. At a stop blipping the throttle, it has extremely fast response compared to what I'm used to. It does have very good bottom end, and midrange power.

I had the exact same set up with the original motor on this bike, except without the high compression head. At first I had a modified stock exhaust. Then I changed it to this Xchamber system. Immediately I noticed a lot more bottom end and mid range. The top end was slightly improved, but was not as outstanding as the improvement lower down. Top speed improved from about 36 mph to about 39. I run a 36 T.

If this new motor cracks 40 I'll be happy with it. One thing that's apparent already, is that down where I ride most of the time, between 25 and 30, this motor is a lot smoother, and quieter than the original.

With only 20 miles, it's kind of hard to compare the power between this new motor, and the original X80, since I'm not winding it out. But, what I'm feeling on the bottom is every bit as strong as before, and I think even stronger.

The way I ride it seems to take my motors about 250 to 300 miles to seat the rings. Usually I notice the compression is a little better after the rings seat in. If this thing gets much better I may start having trouble starting, from the clutch slipping. Accelerating the clutching seems to be holding fine now.