If your forks are leaking, or just don’t seem as responsive as they used to, it may be time for a fork rebuild. Many riders are scared of the thought of cracking open their front forks. It seems as though they believe what’s inside is some combination of rocket science and voodoo. In reality, it’s simpler than you’d think.
Most sportster models will have the same 39mm front end setup found on a majority of Dyna models, so the supplies are relatively cheap and can be used again later if you wind up buying a dyna. The only other OEM fork you’ll find on a Harley is a 41mm, found mostly on wide glide and touring models. If you’re unsure, use a caliper to measure your fork tubes.
Fork Rebuild Tools Needed:
- 39mm Fork Seal Kit (like this)
- 39mm Fork Seal Driver (motionpro)
- Fork Oil Level Tool (Like this one)
- Hex and Torx Sockets
- Various Wrenches
- 3/8″ Ratchet or Impact Driver
- 1 3/8 or 35mm Socket (for the fork caps, I used this)
- Torque Wrench (~5-80 ft lb, like this one)
- Motorcycle Lift or Jack
Supplies Needed:
- Service Manual – The factory service manual is one of the best tools you can possibly own.
- Fork Oil in Correct Weight* (See Note)
Note:
Harley Davidson uses their “H-D Type E” fork oil, and recommends it for all sportster forks, as far as I’m aware. All HD fork oils are petroleum based, non-synthetic oils. The specific weights of their oils seem to be a highly guarded secret, but it tends to fall somewhere between most other manufacturers’ 10w and 15w oils, as shown in the next section.
Fork Oil Viscosity AKA Fork Oil Weight
Viscosity is a fluid’s resistance to flow. In the case of fork oil, it’s viscosity is labeled as a weight (5w, 10w, 15w, etc.) but the base oil itself plus the additives in the oil to control foaming, oxidization, and other undesirable properties make each brand of fork oil’s actual kinematic viscosity (cSt) a little different, even when the advertised weight is the same:
| Fork Oil Brand |
Nominal Viscosity |
Viscosity cSt @ 40C (104F) |
Viscosity cSt @ 100C (212F) |
Viscosity Index |
Color |
|---|---|---|---|---|---|
| Harley Davidson | Type E (??) | 41.14 | 7.72 | 160 | |
| Maxima | 5wt | 16.2 | 4.41 | 202 | Clear |
| 10wt | 37.4 | 7.72 | 182 | Clear | |
| 15wt | 47.1 | 8.49 | 151 | Clear | |
| 20wt | 71.1 | 11.28 | 151 | Clear | |
| Belray | 5wt | 17.4 | 4.2 | ||
| 7wt | 29.3 | 5.5 | |||
| 10wt | 36 | 6.6 | |||
| 15wt | 52.9 | 8 | Green | ||
| 20wt | 77.1 | 10 | |||
| 30wt | 115.3 | 13.1 | |||
| Motul (syn) | 5wt | 18.1 | 4 | 119 | Amber |
| 10wt | 36.2 | 6.1 | 115 | ||
| 15wt | 57.2 | 8.4 | 119 | ||
| 20wt | 79.5 | 10.1 | 106 |
As you can see in the above chart, Harley Type E fork oil tends to test somewhere in between other brands’ 10 and 15 weight fork oils. Heavier oil will increase the effective damping, in other words the forks will move up and down a little slower and feel stiffer. Lighter weight oil will do the opposite, allowing the forks to move up and down faster and feel softer. The former is more popular for performance-oriented riders, the latter for more relaxed cruising/touring types.
Start near the factory oil weight (10w or 15w) and see how you like it, then maybe try a step up or down later.
The Fork Rebuild Process:
The video above is from Biltwell, and covers the process pretty well.
One Tip: Make a note of what direction the fork seals are oriented when they come out. One side has a single groove, the other side has 2 grooves. You’ll want to make sure the new seals go in with the grooves pointing in the same direction as the old ones were.
A fork rebuild is also a good time to paint or powder coat your fork lowers, replace brake pads & rotors, or remove the reflectors, if you’ve ever had a desire to do so.
Want more Sportster content? Check out my article on the Sportster Grenade Plate!
