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How can I tell what bore

1004 Views 15 Replies 9 Participants Last post by  five0addict
I purchased a used XR50 and the kid selling it to me said he thought it had a 81cc BBK. It definitely has a BBK (I have a stock bike as well) but I was wondering how to tell which one. I would assume the size of the piston. The cylinder is still marked 49cc, but there are a couple of kits out there that sell a BBK with the cylinder still marked 49cc.
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you cud take this barel off and look at the size ov piston
it could be a 75, i dont know if theres a 81cc sleeper cylnder.
yes, most likely it is a 75cc becuase I don't think any of the 81cc cylinders have 49cc marked on them
it could be an 88, my local mx shop has an 88 kit bran new in the box that says 49 cc on the cylinder
The 81s also come in cheater kids I believe, and the cylinder is black as well. (I think)
ya, I know the 81cc kits come with black cylinders as well, just didn't know if any of them had 49cc marked on them
my 75 has 49 on it...but it looks diff than maybe u can check that?
Well I'm pretty sure it is a 75 or an 81, but it seems the piston size would still be 52mm. So, how do you know? I'm curious more than anything.
dude a 52mm piston is an 88, all diff cc sizes have a diff size piston so if u have a 52mm piston then u have an 88
Well, that should make it easy then. I was looking around on line and found this 75cc kit that said it came with a 52mm piston. The kit was from Race N Paint.

BK1075B -- 75cc Light Bore Kit

[BK5075B] $199.00

52mm steel sleeved aluminum cylinder

52mm piston, rings, circlips, and wristpin

High performance roller bearing cam

Complete gasket kit

Combo kit option adds:

Keihin 20mm carb and intake

Standard throttle assembly w/ cable, and mounting hardware

The link is -- 75cc Light Bore Kit&products_id=128

So, what size piston comes with a 75cc kit?
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Looks like I found it.

Kit comes with: 75cc CYLINDER, 47.5mm Piston, Pin, Rings, Circlips, Copper Headgasket, Base Gasket. Correct Main Jet for stock carb. Midrange Camshaft. Thrust Chamber for stock muffler.

so, nevermind. I'll look at the piston and found out what's in there.
what qualifies as the "correct mainjet" for the stock carb?lol
a 62 main, the thing im wondering about is the thrust chamber, whats that?
a thrust chamber makes your stock muffler act like a disc style add it on and its supposed to increase top end and still allow you to use your stock sparky.
so like a supertrapp?

wheres a picture of this!!!
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