All the Things I Wish I Knew Before I Ever Serviced a Fork

“It is my belief that the best possible sets of instructions on how-to-do-it are those written by people who have just mastered the art themselves and still have all the problems they conquered firmly in mind. If they wait a few years the tricks they discovered have become such a habit with them that they are prone to forget, when writing about it, to throw them in. The result is a very learned dissertation which is of value to no one but another ‘expert.’ This is true in any trade you wish to name—gunsmithing, engraving, car repair, rock-hounding—you name it!” —Bob Brownell, 1969

That stuck with me from the moment I first read it. Modifying motorcycles is fun. It’s also an area of life where experience and knowledge go hand in hand, even when theory (the service manual) and practice (whatever mongrel parts are in front of you) do not.

I’m hoping you’ll follow along over the coming weeks as we address a typical fork glow-up on a conventional Harley-Davidson front end. Whether you’re attacking a 35mm, a 39mm, a 49mm… they’re all really similar, and most of the concepts presented are applicable broadly across right-side-up forks of Japanese manufacture.

In preparing to write these upcoming pieces, I tried to remember how my first fork service went decades ago, and I laugh now at how minor hiccups really slowed me down. Just like Ol’ Bob predicted long before I ever picked up a wrench, I’ve found that as I document how I do things, I often forget to explain why I do things a certain way. 

This guide is my attempt at seeing if I can’t rectify that for the next waves of chopper pilots.

The hydraulic fork has limited applications

If you’re just replacing tubes on a stock bike, this section is of little import to you. However, if you are raking your frame and extending your neck, it is vitally important that you understand that hydraulic front ends have an upper practical limit with regard to the performance envelope. Once the frame rake starts getting too aggressive, the telescoping fork stops trying to telescope and eventually try to flex. Being tubular, rather than solid, the tubes resist this bending rather well, meaning the fork as a whole doesn’t really suspend very well.

Examine the really long bikes you see—there’s almost always a springer on there. It has been my experience that 37 degrees is about where telescoping forks really start not working as designed. It’s not a magic number; 36 is no walk in the park and 38 isn't rigid, but that’s where performance noticeably degrades. I’d say 6 over on a mildly raked frame is about the upper limit of what you can get away with insofar as using a conventional hydraulic fork. Yes, we sell 8 over tubes at TC Bros. That’s touching the edge of what might work. I wouldn’t even think about it unless you’re cool with adding a little rake and having heavier steering.

I mention this because if you are in the planning stage of a build, this should be top-of-mind for you.

You need a good spot to work

Indoors is best. (See the last tip in the article for more on that.) Most of you will probably try to attempt this work on a bike jack. The paragraph after this is some editorializing on that.

A bike jack is kind of horrible to work on. (I’m talking about those freestanding units, not the little platform jobbies designed to be used in conjunction with a lift; those are great.) I recognize that’s the most likely tool at hand for a lot of DIYers, but they are tippy and badly behaved and I never trust them. The cost of this job goes way up if you wind up replacing a tank, fender, and paint. If you are able, suspend the bike from above (A truss in the barn and ratchet straps work pretty well for this) or get the whole bike up onto some wood blocks with the help of a few buddies. 

We don’t sell this stuff at TC Bros. so I have no dog in the fight. I’ve just been doing this for a long time and I still hate working off a jack and you won’t like it any more than I do, I bet.

You will need some specialized—but not specialty—tools

Nothing you’ll use is particularly exotic, but there are a few things that are not DIY-at-home-guy tools that really make life easier.

• Large sockets: Different fork caps from different aftermarket companies came in different sizes; but inch-plus sockets aren’t in every toolbox. Yes, you can get away with an adjustable wrench or something. 

• If you don’t have a good array, at least measure what you have and buy that one. Can you get by with an adjustable? Sure, I guess. Until it slips and you mar something. Just buy the right tool; it makes life easier.

• Long hex bits: To spin the damper bolts out, a long hex bit is best. Short ones don’t fit and hex keys don’t give you enough leverage. And they’re not compatible with the next tool.

• Impact wrench: If you look at your manual, you’ll see that the damper is held fast to the floor of the fork lower with a bolt. Trying to loosen them by hand is a huge pain and doubly so if the fork spring has been removed. Don’t reach for any tool other than an impact with the long hex bit or you’ll spend tons of time on this instead of ten seconds. As I reread this piece before publishing, I decided this little sub-nugget is probably the single most valuable tip in this article.. I hope you agree.

• Vise with soft jaws: Don’t fight these silly legs on a bench. Clamp ‘em down in the vise and make sure you have soft jaws as the aluminum lowers are very soft and coated tubes are very easy to nick.

• PVC pipe: PVC is affordable and very handy for driving seals home. There are lots of things you can use, including a proper seal driver. Just have something in mind before you start.

• Measuring cup: You need to measure your fork oil accurately. Your girl will be mad if you filch one from the kitchen; buy one for the garage.

The triple tree is a tool, too

Crack your fork caps loose before you remove the fork legs from the trees; it keeps them from spinning. If you think about it, the triple is almost a custom built fixture to hold the tube. Even the vise and its jaws won’t get you as much surface area contact, and the best part about the trees is they hold the fork tubes exactly where the finish is hidden, a trick your vise can’t manage. On the off chance the tube spins, the damage won’t be visible—but I’ve never spun one in the trees.

Fork oil is a tuning element

Fork oil is often measured in cubic centimeters, ounces, or distance from tube top, depending on the bike. Fork oil also comes in a few weights. Both of these factors, volume and viscosity, can be used to tune a suspension. Damping rod forks as found in most of the common Harley front ends have fixed orifices, so the amount and thickness of the fork oil you use will affect fork behavior. Be prepared to try a few permutations if you’re trying to upgrade performance on the cheap.

Now is the time to deal with the appearance of those lowers

Showa’s final finish on lowers for Harley-Davidson beginning in the 39mm era was often a coat of clear paint—over aluminum. What I’ve seen is stones from the roadway nick the paint, then moisture enters and works its way under the remaining clear, generally manifesting itself as white, black, or grey corrosion that creates very unsightly lower fork legs. The practice carried on, so 49mm lowers are the same. Yes, there were black and chrome lowers, but the bulk of production was clear coating over a brushed finish.

It’s not hurting anything to leave them skeezy-lookin’, but it’s unsightly, and if you’re buying fork tubes, especially coated ones in black or gold, the whole idea is to look good, right? Figure out how you want to spruce up the lowers before you start ripping things apart. I’ve seen paint and powder used to pretty good effect, and sandblasting also leaves a nice finish. I like natural aluminum, personally. I think vapor-honed look best, and the cheaper cousin to that is a finish applied with oil and Scotchbrite pads; I’m partial to those methods because they allow the aluminum to darken and patina naturally and can be spot-repaired on the bike, something an applied finish can’t manage. (That’s how we got into this mess!)

This may take time if you are sending the work out, so plan on the bike being down for a bit, which is why I recommended working inside. If you’re gonna just let ‘em fly or rattle-can them, that’s less important and you can probably be done in an afternoon, but having a plan for a spendier appearance is helpful; there’s no need to figure that out after the bike’s ripped apart.

And that’s about all. I’ll break some of this down a little more fully over the course of the suspension overhaul series, but even if you don’t follow that, there might be a gem of wisdom in here that will help you to succeed where I have failed before.