Puzzled: ISO or JIS bottom bracket?

gom

Senior Member
Location
Gloucestershire
Looking at other threads, there seems to be some good knowledge about, so if you will bear with me on what is nearly a repeated question …

I was hoping to replace the left-hand crank on my Brompton as the folding pedal can’t be removed, neither by me or LBS. Taking the crank off and trying both my spares I find neither will go down far enough, and end up further from the bottom bracket than the Brompton original.
Looking at Sheldon, this says to me that the BB is JIS and the spare cranks ISO.

However, the Brompton is from 1999 and everything I read says that it should be ISO. (The BB has been replaced by a LBS, but the crank is original & made in France.) One of the spares is unbranded, but the other is Shimano Alivio, bought c2009 – and I can’t see it can be anything other than JIS.

So what’s going on? Am I doing everything while standing on my head?
 

weareHKR

Senior Member
 

weareHKR

Senior Member
That’s the puzzle. Sheldon says I’m fitting an ISO crank to a JIS spindle ”If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out....”, but from the info I have about the parts it’s the other way round.
🤔
May I suggest the Info you have may be incorrect?
 
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gom

gom

Senior Member
Location
Gloucestershire
That’s what I’m wondering. Made in France implies ISO, Shimano implies JIS, but could it be that my particular parts are the other way round?
I think I shall have to live with a (to me) mystery, hoping that one day further fettling will enlighten me.
 
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gom

gom

Senior Member
Location
Gloucestershire
Excellent, many thanks @rogerzilla.
It now looks like everything is JIS 😮. Brompton‘s and my tourer’s Shimano BB both measure very close to12.7mm.
The Brompton crank sits well on my tourer‘s BB in place of a Shimano crank - just not the other way round. Possibly all down to tolerances.
I begin to wonder whether there has been some adjustment of the Brompton parts. Given the handmade nature of 20th century Bromptons I would not be too surprised.

I fear this doesn’t get me any closer to getting another crank to fit the Brompton, but I feel a lot less confused.
 

rogerzilla

Legendary Member
What sort of BB did the LBS fit, and is yours the old style crank with the permanently swaged-on chainring?

Tolerances for square taper can be really bad and are in the same ball park as the ISO/JIS difference for cheaper stuff.
 

Milkfloat

An Peanut
Location
Midlands
I would go back to the root of the problem, removing the pedal from the existing crank. Have you tried another LBS, one with longer extension bar or a bigger hammer?
 
OP
gom

gom

Senior Member
Location
Gloucestershire
What sort of BB did the LBS fit, and is yours the old style crank with the permanently swaged-on chainring?

Tolerances for square taper can be really bad and are in the same ball park as the ISO/JIS difference for cheaper stuff.
Don't know what the BB is alas. The Brompton parts are definitely old-style, so really should be ISO.
I guess things aren't as clear I decided above, and tolerances mean it will remain unclear. I’ll need to spend some more time on this sometime, but meanwhile you’ve given me some useful info. Thank again.
 
OP
gom

gom

Senior Member
Location
Gloucestershire
I would go back to the root of the problem, removing the pedal from the existing crank. Have you tried another LBS, one with longer extension bar or a bigger hammer?
I need a flat-faced 24mm socket which I’m thinking of buying. LBS should have one as used for suspension forks, but “the current situation” means LBS visits not so straightforward 😕.
Pedal has not been removed in 22 years so will be feeling happy where it is. I’m probably lucky I could get the crank off.
 

T4tomo

Guru
I need a flat-faced 24mm socket which I’m thinking of buying. LBS should have one as used for suspension forks, but “the current situation” means LBS visits not so straightforward 😕.
Pedal has not been removed in 22 years so will be feeling happy where it is. I’m probably lucky I could get the crank off.
although having got the crank off, you can now get it in a vice and apply some decent leverage with an extension bar to the offending pedal
 

rogerzilla

Legendary Member
Where are you in Gloucestershire? I must have the right socket as I've fitted and removed these pedals many times.
 
OP
gom

gom

Senior Member
Location
Gloucestershire
If it's the old style Stronglight chainset, I should have an old left hand crank in the spares heap. Yours if you want it.
Many thanks for the offer - but I have now bought something on ebay that should be ISO, although not arrived yet.
If it fits then good, if it doesn't then I think I'll have to wait till things are more normal and pester LBS at some time when they are not busy. Based on all the info above, who knows what I have. And it may be at tolerance limits as well.
 
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