White Industries ENO Bolt / Cap Screw Replacement (M8-1.25 x 16mm)

The other day, the White Industries Eric’s Eccentric Eno hub was giving problems. The chain was not tensioned enough, and the hub was slipping and slanting the rear wheel. A few attempts to realign the wheel (fixed gear side) and tighten the cap screws resulted in the same problem. Upon closer inspection, one of the cap screws was bent and the threads were stripping. After measuring the dimensions, it was determined to be a M8-1.25 × 16mm cap screw. However, using a die to straighten and clean the threads was not enough. A replacement was required.

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A Longer Cap Screw?

In the past, the cap screws would be barely threaded into the hub to fit it onto the bike. Afterwards, the eccentric axle would be rotated to tension the chain, and the screw caps would be tightened. Looking at the frame and hub, a longer screw could easily be used.

Cap Screws M8-1.25, $0.34-$0.85

This was a common part found at the local hardware stores. Prices varied depending on length, grade and quantity, but they’re cheap. Either $0.85/each or $18.08/50.

M8-1.25 × 16 mm (2) and M8-1.25 × 20mm (2) were bought at Canadian Tire. Grade Class 12.9.

A2-70 vs Class 12.9

The original cap screw was rated as A2-70 (stainless steel). The hardware store cap screws were rated as 12.9 (Alloy Steel, Quenched and Tempered).

Marking Min. Yield Strength
(MPa)
Min. Tensile Strength
(MPa)
A2-70 450 700
12.9 1100 1220

  • Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.
  • Yield Strength: The load at which a material exhibits a specific permanent deformation.

Mountain Bike Test

The new M8-1.25 × 20mm cap screw was threaded into the hub. It was used on two different winter mountain bike rides for a total of 4 hours. No adjustments were required.

Bolt vs Cap Screw

A bolt and cap screw are two different things.

  • Cap screws are generally used in precise applications (dimension tolerances)
  • Bolts are generally used for their mechanical properties (stress loads)
  • A bolt is assembled with a nut
  • A cap screw can be tightened by wrenching down on the head or on the nut