BritBike Forum logo
BritBike SponsorBritBike SponsorBritBike SponsorBritBike SponsorBritBike Sponsor SteadfastCyclesBritBike SponsorBritBike SponsorBritBike SponsorBritBike SponsorHepolite PistonsBritBike SponsorBritBike Sponsor
Upgrade to: Gold Membership | Premium Membership | Vendor Membership | Site Sponsor Membership
Words from our sponsors..
Vintage Motorcycle Auctioneers!
Vintage Motorcycle Auctioneers

CLICK HERE to see VIDEO
How the Electric Roller Starter works.

ORDER on eBay. CLICK HERE!
Buy your Electric Roller Starter here
ShoutChat Box
Comment Guidelines: Do post respectful and insightful comments.
Buy BritBike staff a coffee
Buy BritBike's staff a coffeeStill here since 1996 serving BritBike enthusiasts..
Search eBay for motorcycle parts in following countries
Australia, Canada, France, Holland, Italy, United Kingdom, USA
Random Gallery photo
Member Spotlight
Jeff23
Jeff23
Connecticut
Posts: 51
Joined: January 2009
Show All Member Profiles 
Newest Members
Bill Walker, Phil McClusky, 1957 Road Rocket, Payne, Mysticbus
11018 Registered Users
Top Posters(30 Days)
franko 135
koan58 66
DavidP 63
Rohan 50
Popular Topics(Views)
1,149,213 mail-order LSR
a word from..
Forum Statistics
Forums35
Topics68,949
Posts691,020
Members11,018
Most Online14,755
May 5th, 2019
Who's Online Now
64 registered members (998John), 2,260 guests, and 955 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Iron b31/b33 race engine questions #796824 01/28/20 8:27 am
Joined: May 2012
Posts: 56
J
jimq Online Content OP
BritBike Forum member
OP Online Content
BritBike Forum member
J
Joined: May 2012
Posts: 56
Hi all. I am collecting bits to build a pre unit single race bike. I have bought an incomplete thick flange zb31 cased motor which is said to have a b33 crank and has a small fin b33 top end. It is a bit of a bitza and hopefully doesn't turn out to be scrap. Anyway I have been told by a guy who seemed to know what he was on about that for my purposes a 350 would be a better proposition than a 500 for an iron race motor and a 350 is best built with a 500 crankshaft as it is heavier. My plan at this stage is to build it up as a 350 with the 500 crank, a carrillo or similar short late gold star rod and a cut down big fin b31 cylinder. I intend to run the engine on methonol. Does this sound like I have been given good d advice to anyone in the know? Also I am unsure at this stage how to tell the difference between a b31 and b33 crankshaft other than if it is a later short rod b33. Can anyone give advice or pictures there please. Cheers, Jim.

Support Your #1 BritBike Forum!

Check out British motorcycles for sale:
British Motorcycles on e-Bay UK
British motorcycles on e-Bay North America
CLICK HERE to see VIDEO
How the Electric Roller Starter works.


ORDER on eBay. CLICK HERE!

Buy your Electric Roller Starter here
Re: Iron b31/b33 race engine questions [Re: jimq] #796828 01/28/20 10:02 am
Joined: Mar 2014
Posts: 3
glenno69 Offline
BritBike Forum member
Offline
BritBike Forum member
Joined: Mar 2014
Posts: 3
G'day Jim,

Not sure that you would want to use the 500 crank for a 350, Being heavier is not an advantage for a race engine I wouldn't think.
You also may have trouble with the balancing of the crank with the smaller 350 piston.

I have heard of racers back in the day doing the opposite in fact, and using the lighter 350 crank flywheels in a 500 engine
to make it spin up/ accelerate faster.

Not sure about how to identify a 350 as opposed to the 500, I would assume the 500 flywheels are thicker, I do have a 500 crank in a set of cases that was a race bike, but I think they have been lightened, will try and find out thru my contacts if no one else knows.

Not sure if you saw my post about my bike, on your other thread re wheel choice etc .

Regards
Glenn


1974 Mk2a Commando
1968 Bonneville
BSA B31 Racer
Norton/BSA 500 Racer
DBD 500 Goldstar Project
Re: Iron b31/b33 race engine questions [Re: glenno69] #797055 01/30/20 11:53 pm
Joined: May 2012
Posts: 56
J
jimq Online Content OP
BritBike Forum member
OP Online Content
BritBike Forum member
J
Joined: May 2012
Posts: 56
Hi Glenn.

Thanks for the reply.

Yes i did see the reply to my other thread but not for a while after you posted it.

I am thinking i will go with the 19 inch wheels as they will add a bit of authenticity to the bike and you can get good Avon's for them. the same tyres guys are running on their Manx's.

What you say about the lighter crank makes sense but i note that the gold stat cranks look heavier again than the B33. I think i can see the difference now. it is in the rim that runs around the outside of the flywheel being thicker on the 500 crank.

I guess i will wait until the next time i am at the track to quiz some of the guys i have talked to previously about what they had said.

My bike will be fairly similar to yours but the class that will allow me to run a methanol burning pre 63 350 over here is the modified class which allows any drum brakes so i plan on running a 69-70 type BSA / Triumph 8 inch tls.

I am in a planning and collecting parts as i find them stage of this project at the moment so not pressing into anything too seriously. I am pretty keen to find a frame though, then i would have most of a motor and a frame which make me feel a bit more like i am actually getting somewhere with it.

At this stage i am hoping to have my B50 finished for a meeting in october.

Re: Iron b31/b33 race engine questions [Re: jimq] #797065 01/31/20 12:51 am
Joined: Sep 2001
Posts: 3,139
Gordo in Comox Online Content
BritBike Forum member
Online Content
BritBike Forum member
Joined: Sep 2001
Posts: 3,139
These two cranks are 1956.

Gordo

Click to enlarge

The B31 crank has casting part # 66-610

[Linked Image from i.postimg.cc]

The B33 crank has casting # 66-635

[Linked Image from i.postimg.cc]


The roadside repairs make for the best post ride stories.
Re: Iron b31/b33 race engine questions [Re: Gordo in Comox] #797070 01/31/20 1:17 am
Joined: May 2012
Posts: 56
J
jimq Online Content OP
BritBike Forum member
OP Online Content
BritBike Forum member
J
Joined: May 2012
Posts: 56
Hi Gordo.

Thanks for that reply.

I have been trying to figure out what the visual difference between B31 and B33 cranks is for a couple of weeks now.

What you show is what i was thinking about a week ago until i managed to convince myself that the 350 and 500 cranks look similar ( to what you show as the B31 crank ) but when viewed from the side that rim that goes around the outside of the fly wheel is wider.

My information all comes from scrolling through pictures i have found on the internet.

What you are saying is the b33 crank looks like what i was calling a gold star crank.

I am sure you will be right and i will be wrong. That will make it alot easier to tell what i have when i pull the thing apart.

Cheers.
Jim.

Re: Iron b31/b33 race engine questions [Re: jimq] #797082 01/31/20 2:49 am
Joined: Sep 2001
Posts: 3,139
Gordo in Comox Online Content
BritBike Forum member
Online Content
BritBike Forum member
Joined: Sep 2001
Posts: 3,139
Jim: then to complete the story this is a 1952 Gold Star crank. The rod and wheels are polished and the drive side shaft is stepped to take a larger bearing.

I would be surprised if a B31/33 case had been modified to take the larger bearing.

Gordo

click to enlarge

[Linked Image from i.postimg.cc]

[Linked Image from i.postimg.cc]

[Linked Image from i.postimg.cc]

This is a DBD crank for comparison

[Linked Image from i.postimg.cc]

Last edited by Gordo in Comox; 01/31/20 7:37 pm. Reason: Photo added

The roadside repairs make for the best post ride stories.

Moderated by  Allan Gill, Jon W. Whitley 

Home | Sponsors | Newsletter | Regalia | Calendar | Bike Project | BritBike Museum | Spiders Cartoons | DVD- Manuals & Parts books
Upgrade to: Gold Membership | Premium Membership | Vendor Membership | Site Sponsor Membership
Powered by UBB.threads™ PHP Forum Software 7.7.3