Two bearings: 6903RS and MR18307-LLB

Today we're sharing a case reported by several customers who, when changing the bearing on their bicycle wheel, noticed an engraving on the seal that can be misleading when choosing the replacement bearing.

Some unscrupulous wheel manufacturers may in fact use the first seals available on their bearings, even if the marking does not match the part number. MR18307 and 6903 bearings have very similar dimensions: 18x30x7mm for the former and 17x30x7mm for the latter.

Both bearings share the same outside diameter...
...and the same thickness.

6903 seals can then be mounted on MR18307 bearings. You may then notice this 6903 marking when dismantling your bearings. Make sure you choose the right part number by measuring the inside diameter of the bearing. If this is 18mm, you need an MR18307 ball bearing. If the inside diameter of the bearing is 17mm, you need a 6903.

Inside diameter of MR18307: 18mm
Inside diameter of 6903: 17mm