For most modern Trek frames

Enduro 1.5" 36 x 36 Degree Stainless Steel Angular Contact Bearing 40mm ID x 51mm OD x 6.5mm H.

ABI Headset Bearing for new builds or replacements.

• Model: Headset Bearing
• Brand: Enduro
• Manufacturer Part Number: ACB 6808 SS, 290413-BAG
  
• UPC: 810191012634

Specs:

• Thickness: 6.5
• Cartridge Bearing Angle: 36x36
• Bearing O.D. (mm): 51
• Bearing Type: Cartridge
• Bearing I.D. (mm): 40