Vintage Dirt Bike Q & A

Hello all, I am helping get a 74 CR125M ready for a vintage race and could use a little advice on gearing. I know stock is 14/49. When we got this thing it was 14/51. We're going with a 520 chain conversion and when the new 51 tooth rear sprocket arrived I noticed it's as big as a pizza pan. Way bigger than the old 51 tooth-428 chain setup. With our lack of recent experience we didn't see that coming. We're thinking of going with a 13/47 setup for similar reduction. Some specifics are,
Track is small and tight
Riders are 180+ lbs and kinda old.

Any advice you could lend here would be appreciated.

I measured 14T OEM Honda sprockets for a CR125M (428) and MT250 (520), and came up with a diameter conversion factor of .80 for 428-to-520. This gives 11.2T / 39.2T in 520 for the stock 14/49 gearing in 428. For your tight track / 180+ rider situation, I'd suggest 11/40 (lightest), 12/43, or the 13/47 you mentioned. You may want to keep an eye on chain/tire clearance, it's always an issue on my CR even with 428 chain.

For what it's worth... I considered going with 520 on my vintage bikes, but stuck with a heavy-duty 428 instead (D.I.D. 428NZ) and have been very glad I did. The NZ chain and 428 sprockets are lighter and have noticeably less drag than the non-O-ring 520 setups on my EVO/modern bikes, and my current MR175 NZ chain has over 3,600 mi. on it with little stretching or apparent wear (I replaced the last one too early at 3,300 miles, it was barely worn at all). Maybe something to keep in mind at the next replacement.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I did a few races with a 520 then switched to a HD 428 and never went back.
As for the gearing I used the stock ratio because it worked just fine for me. On occasion I might have changed the counter up or down 1 depending on the track conditions.
For me changing cranking pressure, carb diameter and pipe was how I changed my top or bottom end. 32 for slow tight stuff 36 for WFO. DG down for top end, up pipe for low end.
You will have to see what works for you at the track of the day, carry extra chainwheels and counters, test in the area before the race and fine tune at the race.

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1994 AHRMA National Champion Sportsman 125 Intermediate.
1996 AHRMA National Champion Classic 125 Expert.

This is really good stuff. Thank you friends.