Shimano BB-UN300 Bottom Bracket - English | 73 x 118mm Spindle - Square Taper JIS
Bottom Brackets
Shimano BB-UN300 Bottom Bracket - English · 73 x 118mm Spindle - Square Taper JIS
Shimano BB-UN300 Bottom Bracket - English | 73 x 118mm Spindle - Square Taper JIS
The Shimano BB-UN300 73 x 118mm is a square taper bottom bracket for mountain and wide-shell bikes, a cartridge bearing design that delivers Shimano quality and smooth, consistent rotation in a sealed unit compatible with square taper JIS cranks. Read the overview
At a glance
- Shell
- English threaded, 73mm
- Bearing Type
- Cartridge
- Model
- BB-UN300
- Interface
- Square Taper JIS
3 in stock
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Overview
Which bikes use the Shimano BB-UN300 73 x 118mm Bottom Bracket?
The Shimano BB-UN300 73 x 118mm is a square taper bottom bracket for mountain and wide-shell bikes, a cartridge bearing design that delivers Shimano quality and smooth, consistent rotation in a sealed unit compatible with square taper JIS cranks. The 73mm English threaded shell and 118mm spindle cover the common mountain bike and wider Q-factor configuration, so if your frame has a 73mm threaded BB shell and square taper JIS cranks, the UN300 in this spindle length is typically the correct replacement. Shimano specs the square taper spindle in steel.
This bottom bracket ships without crankarm fixing bolts, which are sold separately, so add a pair if you do not already have them. Cartridge bearing construction means the bearing assembly is sealed and not user-serviceable, replaced as a complete unit when worn rather than repacked like cup-and-cone designs. It fits cranks with a JIS square taper. A straightforward, proven replacement for any 73mm square taper threaded bike needing fresh bearings.
All the specs
MPN EBBUN300C18X
- Model
- BB-UN300
- Interface
- Square Taper JIS
- Shell
- English threaded, 73mm (1.37 x 24t)
- Spindle Length
- 118mm
- Spindle
- Steel
- Bearing Type
- Cartridge (sealed, non-serviceable)
- Crankarm Fixing Bolts
- Not included, sold separately
- Product Code
- EBBUN300C18X
FAQ
Good questions
Get to know Shimano
There's more to Shimano than this one product. Our Shimano page has the full Shimano lineup we carry and more about the brand.
Visit the Shimano page →Do I need a 68mm or 73mm shell version?
Measure your frame's threaded BB shell width. Mountain bikes commonly use 73mm, while many road and hybrid frames use 68mm. This is the 73mm version. If your shell is 68mm, choose a 68mm BB-UN300 instead.
How do I know if I need the 118mm spindle?
Spindle length sets your chainline and crank clearance. The BB-UN300 comes in several lengths. Check the spindle length stamped on your existing bottom bracket or consult your crank and frame spec before ordering this 118mm version.
Are crank fixing bolts included?
No. This version ships without crankarm fixing bolts. Reuse your existing bolts or add a pair to your order.
Can I service the cartridge bearing?
No. The BB-UN300 is a sealed cartridge unit and is not user-serviceable. When the bearings become rough or noisy, replace the entire unit rather than repacking it.
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