Why One Oil Filter Fits Both Harley Twin Cam & Milwaukee-Eight Engines

Do Harley Twin Cam and Milwaukee-Eight engines use the same oil filter?

Yes. Both engines share the same oil filter specification: a 3/4-16 UNF thread, identical gasket dimensions, and internal valve requirements including an 11–17 psi bypass valve and a silicone Anti-Drain Back Valve (ADBV). OEM filters such as 63731-99A, 63798-99A, and 62700296, along with high-efficiency equivalents like the AMSOIL EaOM134, are fully compatible with both engine families.

Critical Warning: The visually similar AMSOIL EaOM135 fits the threads but must never be used on Twin Cam or M8 engines because it does not contain the required Anti-Drain Back Valve.

Table Of Contents

Technical Comparison

Twin Cam vs. Milwaukee-Eight

Harley-Davidson did not change the filter mounting interface when moving from the Twin Cam engine to the Milwaukee-Eight. As a result, the oil filters are functionally interchangeable.

Specification Twin Cam (1999–2017) Milwaukee-Eight (2017–Present)
Thread Size
3/4-16 UNF
3/4-16 UNF
Gasket ID/OD
~2.4” / ~2.8”
~2.4” / ~2.8”
Internal Valves
Bypass + ADBV
Bypass + ADBV
OEM Filter
63731-99A
63731-99A / 62700296
AMSOIL Equivalent
EaOM134
EaOM134

Verdict: These filters are interchangeable because they share identical mounting threads, sealing geometry, and internal valve requirements.

5-Micron vs. 20-Micron

Filtration Requirements

Harley’s Modern Requirement

With the introduction of the Twin Cam 88, Harley adopted a 5-micron nominal filtration standard to protect:

  • Hydraulic chain tensioners
  • Hydraulic lifters with tight lash tolerances
  • Piston cooling jets

These components require finer filtration than older Evolution engines.

Synthetic Media Performance

High-efficiency motorcycle filters use synthetic media designed to capture microscopic contaminants and maintain flow under demanding conditions. Their efficiency is optimized for Big Twin lubrication systems and meets or exceeds Harley’s filtration expectations.

Internal Valves

Bypass Valve & Anti-Drain Back Valve (ADBV)

A common misconception is that Harley filters rely solely on engine-mounted bypass valves. Technical data confirms otherwise.

Bypass Valve (11–17 PSI)

Both the Twin Cam and Milwaukee-Eight require the filter to contain an internal bypass valve. If the media becomes restricted, the bypass valve opens to maintain oil flow.

  • Spec: 11 to 17 psi.
  • Purpose: Maintains oil flow if the filter media becomes restricted or during cold starts.

Anti-Drain Back Valve (ADBV)

Because Big Twin filters are mounted horizontally, they must include a silicone ADBV. This valve prevents oil from draining out of the filter after shutdown.

  • Purpose: Prevents oil from draining out after shutdown, protecting against dry starts.
  • Material: Silicone is used to remain flexible at peak operating temperatures.

Without an ADBV:

  • The filter drains empty
  • Oil pressure is delayed at startup
  • Hydraulic lifters and tensioners suffer accelerated wear

This is why only certain filters are approved for Big Twin use.

The Critical Difference

AMSOIL EaOM134 vs. EaOM135

Although the EaOM134 and EaOM135 share the same thread size (3/4-16) and height (3.40 inches), they are engineered for different engines.

EaOM134 / EaOM134C — Big Twin Filter

  • Thread: 3/4-16
  • Bypass Valve: Yes (11–17 psi)
  • ADBV: Yes (Required for Twin Cam & M8)
 

EaOM135 / EaOM135C — Evo/Sportster Filter

  • Thread: 3/4-16
  • Bypass Valve: Yes (11–17 psi)
  • ADBV: No
  • Applications: Evolution Big Twin (1984–1999) & Evo Sportsters (1986–2022)

⚠️ Why This Matters

Using an EaOM135 on a Twin Cam or M8 allows the filter to drain empty after shutdown because it lacks the Anti-Drain Back Valve. This causes dangerous dry starts. EaOM135 must never be used on Twin Cam or Milwaukee-Eight engines.

Sportster & Revolution Max Compatibility

Harley Sportster models have separate fitment rules.

Evo Sportsters (1986–2022)

  • Designed to use the EaOM135 (No ADBV)
  • Their oiling system differs from the Big Twin and does not require the ADBV.

Revolution Max (Sportster S / Nightster)

  • Thread: Metric M20 x 1.5
  • AMSOIL Part: EaOM103
  • Compatibility: Cannot use any Big Twin (3/4-16) filter.
Feature EaOM134 EaOM135
Thread Size
3/4-16 UNF
3/4-16 UNF
Height
~3.40”
~3.40”
Diameter
~2.97”
~2.97”
Gasket ID/OD
~2.40” / ~2.78”
~2.40” / ~2.78”
Bypass Valve
Yes (11–17 psi)
Yes (11–17 psi)
ADBV
Yes
No
Engine Family
Big Twin
Evo & Evo Sportster
Frequently Asked Questions

FAQ

Can I use the AMSOIL EaOM135 on my Twin Cam or M8?

No. It lacks the required Anti-Drain Back Valve.

Did Harley change the filter for the M8?

Harley introduced a longer-capacity filter (#62700296), but the original Big Twin spec (63731-99A) is still fully approved.

Does the EaOM135 include a bypass valve?

It includes a bypass relief valve rated at 11 to 17 psi

What efficiency do these filters provide?

AMSOIL EaOM filters are 99% efficient at 20 microns

Why do some charts list separate filters for Twin Cam and M8?

Marketing segmentation—specifications remain the same.

Can I use an automotive filter if the threads match?

Not recommended. Automotive filters may lack the proper bypass valve setting or ADBV.

Source Data:

This article references verified specifications from the AMSOIL Motorcycle Oil Filter Data Bulletin (G2235), which includes bypass valve ranges, Anti-Drain Back Valve usage, gasket dimensions, and application differences between EaOM134 and EaOM135.

Download the official AMSOIL Filter Specification PDF (G2235)

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