Square Taper Bottom Bracket Basics

The scope of this document is to provide basic information for those interested in maintaining bicycles with square taper bottom brackets.

Shell Width:

Common bottom bracket shell Widths for cottered and cotter less Square Taper Bottom Brackets:

Width (mm)	Standard - Use
68	ISO - Road, Mountain, Hybrid
70	Italian – Road, track
71	Raleigh - Road
73	ISO - Mountain
76	Raleigh - Folding

Threading:

Common Bottom bracket threading for cottered and cotter less Square Taper Bottom Brackets:

Standard	Thread	Left Adjustable Cup	Right Fixed cup
British	1.370” x 24 TPI	Right Hand Thread	Left Hand Thread
ISO	1.375” x 24 TPI	Right Hand Thread	Left Hand Thread
Italian	36mm x 24 TPI	Right Hand Thread	Right Hand Thread
French	35mm x 1mm	Right Hand Thread	Right Hand Thread
Swiss*	35mm x 1mm	Right Hand Thread	Left Hand Thread
Raleigh**	1 3/8’ x 26 TPI	Right Hand Thread	Left Hand Thread

*  Some Motobecane
** Nottingham, Except Carlton and Asian

Tapers:

Not all square taper bottom brackets are created equal. There are two primary standards, JIS and ISO. What makes comparison of these two standards difficult, is that they do not specify the exact same dimensions. 

In practice, the small end of the JIS bottom bracket is slightly larger than its ISO counterpart. Also, the tapered part of a JIS crank arm is slightly longer or thicker.

The net result is that an ISO spindle will appear to go further in to a JIS crank that the correct JIS spindle. A JIS spindle will appear not to go in to an ISO crank as far as the correct ISO spindle.

JIS – Japanese Industrial Standard Specification: JIS D 9410:2010

By far the most common. Used on Asian, European and domestic.

·         Square end with 2° taper toward end of spindle.

·         Length of taper, 15 mm.

·         Width of small end, 12.65 mm.

ISO – International Organization for Standards Specification: ISO 6695:2015

Less common, used on Campagnolo from 1994, and some earlier European makes, such  as Stronglight and TA.

·         Same square end with 2° taper toward end of spindle.

·         Length of taper 18 mm.

·         Length of taper 15mm

·         Width at small end, 12.73 mm. measured 1.5 mm from end.

·         Length of taper in crank arm, ≥ 12 mm.

JIS Low Profile – Shimano

Uncommon. Used on some Shimano Dura-Ace cranksets. Replacements available from  Phil Wood .