Hello again. 

My 21 came with a 42T rear. I use the bike mainly on the street, including  long, straight highway portions (ideal for seizing) and occasionally may hit a track, but not more often than once a year.   

I've always felt it is a big sprocket specially after doing the wire splice. The engine easily reaches red line in 6th even uphill. 

I have now sealed exhaust, a fresh top end (in the post), and a set of main jets for airbox mod. 


 I am trying to decide between a 16 front, std 40 rear or something smaller, but again have no experience. I've read about the 16 being tight in the cases. 

Has anybody gone through this selection process?
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I've got the full selection of front and rear sprocket sizes. In your case i'd definitely get a 40T rear and leave the front as the standard 15T.

16T just catches on a lug on the cases which i think is a mounting point for the ignition cover, though once its scraped a few mm of (unimportant) aluminium away it doesn't catch on anything else and never causes a noticeable problem during use even when you first put it on.

You can run a 16T with a 40T without having to worry about your chain's length. That's if you want to increase your gearing more for highway use.

Beyond that it all depends on whether your engine will pull even taller gearing - like with a 38T rear etc.

I found i still ran out of revs in 6th at a race track with a long straight with the 16/40 gearing. I've since bought a selection of smaller rear sprockets but haven't been back there on that same track layout to try them yet.
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After years of moaning about immigrants now i am one...

Thanks, Hermit. I guess My question was:

Hermit wrote:

Beyond that it all depends on whether your engine will pull even taller gearing - like with a 38T rear etc.

I'll guess I'll test both 40 & 38 once the airbox is modified.
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Try 16 42 , it will be inbetween a 15 40 and a 15 39 . I think 15 38 is borderline for a stockish nsr as it may struggle to rev out

Hi, Neal

Yes, I did the math and 16 42 is just 1% lower reduction ratio than std. 15 39 is 2.5% lower. 15 42 is 6% higher than std. This looks like a big difference for street use.

What's easier to source, fornt or rear sprockets? either the 16 front or 40 rear will yield pretty similar results. Now, if I try the sdt gearing and still want some more speed, the next conbination is 15 39 or 16 41.

Oh, what a mess!

By the way, does "stockish" means "stock" or "near to stock"?

Near to stock

Hi, Neal

I contacted B&CExpress and they answeed: "None of our suppliers list a 16 tooth front sprocket for your bike, however we can supplier you with a Renthal 266-520-40HA (40 Tooth) rear sprocket."

intobikes.co.uk don't seem to carry 16T either.

I'll keep looking around, I still have some time until my pistons arrive.
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