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Pre unit single racing bike questions. #787448 10/18/19 11:30 pm
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jimq Offline OP
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I am considering a new race bike project and have a couple of questions about pre unit swing arm singles I would be interested to hear people's opinions on.

One is wheel rim size. Would one build such a bike with 18 or 19 inch wheels.

The other is are b31/b33 crank cases any good for racing.

The bike I am picturing may have an iron top end running on methonol or may get alloy top end but is unlikely to get a genuine gold star engine.

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Re: Pre unit single racing bike questions. [Re: jimq] #788004 10/24/19 9:10 am
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Ginge Offline
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Gidday Jim,

I used to knock around the Dunedin Street race circuit during festival week. They don't run bikes anymore but when they did they ran some old Brit stuff and some early Japanese stuff. Mostly 350 and 500 Brit. Some 250 and 500 Japanese. An occasional European.

I met a bloke in the pits one year running a B33. We had a good chat.

It wasn't the prettiest bike but he had a lot of fun on it. It was in standard trim less guards and lights etc. Cafe/racer seat. Clip on bars. Original forks up front. Total loss battery ignition ( back before we had magic black boxes). He was competitive but not a winner. Had a beard as long as your arm. He raced Teretonga, Dunedin, Waimate and Levels. The whole kit went in the back of a Hiace van which he also slept in.

I think the B series cases will be fine. The bike I saw was a methanol bike, but hadn't been tuned to the edge. Just an old BSA on meth.

I think it comes down to budget and how you want to race. Race to have fun, and B31/B33 will be just fine. Race to win, you need something with the right cams, the right gear box, tyres, carbs, magic black boxes etc etc. Add in the box trailer and dancing girls and pretty soon you are too fast for the NZ racing circuit.

Just build something that suits the circuits and hill climbs near you. B series will attract attention every time you crank it up. You probably won't beat the Japanese bikes anyway so who cares what you ride. Classic racing in NZ is so small that you will end up dicing with others outside of your class. Sad but true.

Plenty of B series bits around. As for wheel size, I'd build whatever you can get for tyres. I'd rather race with good tyres, than the wrong ones on the right sized rims. Rim size is fixable with gearing and sprockets.

You've got me motivated to build my sprint/hill climb bike now.


'51 C11 in a '54 C10L frame..."The Bantam Eater"
'70 Triumph Trophy 500. Bottom end rumble awaiting diagnosis
'72 Triumph Tiger 650. Back on the road...
Re: Pre unit single racing bike questions. [Re: jimq] #788009 10/24/19 10:26 am
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I run 18" in mine as theres a better choice of race tyres,b31 cases are fine , I would recommend an after market conrod though,goldie cams and b44 piston is a good combination in a b31


BSA lightning
BSA B50MX
TRIUMPH TR6C
BSA BUSHMAN
BSA Gold Star Daytona
BSA Gold Star Scrambler
BSA Rocket Gold Star
BSA C15S
BSA Cyclone
Triumph T120
Triumph T100 Daytona
Triumph 5TA Trials
Triumph T100 Scrambles
Cheney 560 TT
Re: Pre unit single racing bike questions. [Re: jimq] #788016 10/24/19 11:30 am
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Andy Higham Online Content
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The cases are more than strong enough until the 60 years old rod lets go, fit a modern rod like a Carrillo.
The crank can squirm around a bit, especially when the tapers have been apart a couple of times. If you can afford it buy a Pearson, Absaf or NEB crank.
I would recommend running on methanol, in a racing situation heat will be a problem in an Iron motor with petrol


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
'35 & '36 OK Supreme
Kawasaki ZZR1400 "Kuro no senshi"
Re: Pre unit single racing bike questions. [Re: jimq] #789195 11/04/19 11:55 pm
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jimq Offline OP
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Hi.

Thanks for the reply's everyone.

With this info and having spoken to a guy at a recent race meeting i am now satisfied that iron motor bottom ends are fairly robust.

The wheels i think i will make my own mind up about in time.

as this is my first foray into pre unit singles (most of my brit bike experience is in B50'S and T100Rs) I feel i really need to find a good source of information on the bikes and what modifications were made over the years so i know when buying parts what to look for and what to avoid i.e i am just trying to get my head around the "thick flange crankcase" thing now

Re: Pre unit single racing bike questions. [Re: jimq] #790325 11/17/19 12:11 pm
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glenno69 Offline
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G'day Jim,

This is my B31 which I have been racing here in Perth for about 5 years now, though I only get to about 3 or 4 meetings a year. Sometimes less.
It is essentially standard with a few upgraded parts, carefully assembled and has been pretty reliable.
Happy to pass on any info to help another "Iron Head" racer.

I went with 18in flanged Excel rims and use Avon tyres, they are excellent.

The cases are 1958 B31, thick flange, alternator type,
Standard Crank, with Gold Star big end, I don't believe an expensive aftermarket crank is necessary for a 350.
Standard Rod, crack tested and shot peened,
650 Triumph piston, ( same bore size as B31 ), I was using standard cast ones ( cheap ), but this year I have gone to a forged one from MAP, they are the business.
I replace the piston every 1-2 years, depending on how much use it has had, BUT, I replace the Gudgeon every season, as they are only 15mm dia and have broken 1, with disastrous results.
Biggest inlet valve I could get in,
Modern style springs to prevent coil bind,
27mm AMAL Mk1 Concentric, running on Methanol
With the cast pistons I was using around 10.5: 1 compression, which was fine with the Methanol and nice and safe for starting out.
This year with forged piston, I have gone up to 12:1

Anything else I can help with, just ask,
I took up racing at 45 years, and it is the best and most exciting fun I have had, I just love it.

Attached Files 015.jpg

1974 Mk2a Commando
1968 Bonneville
BSA B31 Racer
Norton/BSA 500 Racer
DBD 500 Goldstar Project

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