Introduction: 1999 Marzocchi Bomber Mountain Bike Fork Rebuild
I was given a Wicked Fat Chance with a blown out Marzocchi Bomber fork. If you're not familiar with it, it was a somewhat high end fork when it came out. It offers adjustable dampening through changing the air pressure.When I got it, the fork wasn't holding air pressure and would go flat pretty quickly. Also fork oil was slowly leaking out of the fork into the hub and spokes of the wheel. Rather than getting a new one I decided to resurrect it to it's former glory. I hope these instructions will give you confidence to rebuild your own fork whether it is this one or another one. You'll be saving money and you can also have an understanding of the components in your bike. The build took me over a month since I didn't have all the equipment together. Hopefully this will be detailed enough to save you time and money.
Step 1: Ready Your Workplace
Attached are the official instructions from the Marzocchi website. In it are directions using specialty tools they want you to buy and a exploded schematic of the fork. The instructions pretty confusing and useless. The only thing worth looking at is the schematic.
Another good resource are the official instructions from the seal replacement manufacturer. It's for a different model but almost all of it is applicable to this fork. Unfortunately I found the site after I installed it since their site is so confusing so I didn't get to try their tips and tricks. You can find the page here. http://www.enduroforkseals.com/id3.html (it was working when I wrote this)
The items you will need or are highly recommended
Large Crescent Wrench
Compressed air blow gun and adaptor
Flat head screwdriver
Replacement Seals ( I found some online from Enduro)
Grease (I used Buzzy's Slick Honey)
Step 2: Remove the Fork From the Frame
Unscrew the handlebar portion. Then take a big crescent wrench to the nut holding the fork in. Once out, the fork should slide right out. Be careful to keep the bearings from dropping out. Also note the order of the bearings and washers so you don't have a problem during re-assembly
Step 3: Take Apart the Fork
Start by undoing the socket head cap screws on the "arch" with an allen wrench. The "arch" provides stability to the fork, keeping both sides parallel and preventing them from rotating. It also holds up the brake cable on my bike. Next loosen the screws holding the "stanchion tubes" just enough to slide the tubes out. The "stanchion tubes" are the shiny rods that slide up and down the fork bushings. Mine were in surprisingly good shape after years of wear and neglect. Remove the stanchion tubes. They should easily slide out.
Step 4: Removing the Cap
I placed the "slider" (black outside tube housing the seal and stanchion tubes) in my shop vise with some paper towels to protect it from getting teeth marks from the jaws. I also oriented it so that the stem that holds the brake components rested on the vise. Doing so allowed me to push down on the stanchion/ slider assembly without everything dropping through the vise. Next I screwed on top of the schrader valve an adapter I got from a kit in Harbor Freight. The purpose of doing so is to allow me to push the "cap" into the stanchion tube without damaging the schrader valve, revealing the "stop ring" (an internal c-clip). The proper tool to use would be a "Stanchion cap puller". I had no intention of buying specialty tools for an outdated bike component. Taking a hammer I gave the adapter and valve a hit sending it and the "cap" into the cylinder. A ridiculous amount of force isn't necessary but there's enough room so you don't have to be overly careful either. After whacking it, I was able to pick the "stop ring" out with a corkscrew tool on a multipurpose knife since I didn't have a pick at the time. The "cap" was still stuck in there good so I clamped it again in the shop vise, putting the brake stem on the far side, and proceeded to pull at the schrader valve with the blow gun attachment. Make sure an oil pan is underneath as the oil will start flowing out. The "cap" popped right out and I moved the "slider" to the pan, leaning it upside down to drain completely. I repeated the same steps for the other stanchion/slider assembly.
Step 5: Removing the Stanchion Tubes
Take a 17 mm socket to the inside of the bottom of the slider to remove the "foot screw". I didn't have a deep socket so mine required an extension at this step. With the stanchion tubes out of the way you can now access the seals and take them out.
Step 6: Removing the Old Seals
The upper seal is called the "dust seal" and is supposed to be removed with a screwdriver by prying between the seal and the aluminum slider. It looked like it would be quite damaging to the aluminum slider so I didn't do that. Instead, I started pulling at the "dust seal" with vise grips because the vise grip's jaws are shaped in a way to pinch the seal. I eventually got it out but it might be easier to pry it out side side at a time with a flat head screwdriver like the X-FLY 80 instructions of Enduro Fork Seals in the link I posted in the Introduction. Once the dust seal is out, you will find another "stop ring". I undid the stop ring with a screwdriver which revealed the "oil seal". I managed to yank those out with a needle nose vise grip. It wasn't the easiest task so I would definitely give Enduro Fork Seals' method a shot. They wrap the flat head at the lever point with electrical tape for protection and pry out the oil seal from underneath.
Step 7: Cleaning the Components
Underneath the oil seal you will find a washer. Removing that, you'll find the "pilot bushing". I pulled all the parts out, threw it in a tub of water with degreaser and took the opportunity to scrub down the other parts with dried on fork oil and dirt.
Step 8: Inspecting the Pumping Rod
This step is to check the "seal ring". I don't think it's necessary but I took the pumping rod apart because I was this far in anyways. The pumping rod is plastic and has an hollow aluminum cylinder called a "foot buffer" press-fitted on the end to keep it from coming out. I clamped the plastic rod and twisted off the "foot buffer" , wrapping it first to prevent leaving teeth marks. Once out, I pushed the pumping rod through with a flat head screwdriver and the pumping rod, with the "seal ring" and "rebound spring" on it popped out. I think the "rebound spring" is to prevent the pumping rod assembly from bottoming out. The pumping rod has a hole through the side which allows oil through at a certain rate to provide damping. Seeing that nothing was wrong with the seal, I put everything back together.
Step 9: Installing the New Seals
In the pack of replacement seals, the "dust seals" are blue which is a different shape and color than the old one. I'm not sure if it's better than the stock one but I would say that it looks more sleek. I then dryed off the slider components and reassembled them. After lining the "oil seal" up, I found a big socket that fit inside the slider and hammered it with a mallet to press the seal in place. The instructions say to use their special tool, the "seal press", which I'm sure is a waste of money. After putting the "oil seal" stop ring back on, I lined up the dust seal and also hammered that in with a socket. The Enduro Fork Seals instructions recommend putting the seals in a freezer before installing them. I didn't do that but I would recommend trying it. The effect of temperature on rubber's expansion is much more than metal so I can see the step saving some effort.
Step 10: Re-Installing the Stanchion Tubes
With the dust seal nicely in place, liberally greased the inside of the dust seal. Then slide the stanchion tubes in place, ensuring it is in the correct direction. Note how cleanly it wiped a layer of grease off the stanchion tube. Put the "foot screw" back on this time with a torque wrench. The manual says to torque to 9 N-m which 6.6 ft-lbs. I didn't have a good torque wrench so I tried my best to hit 10 N-m. To check I held a ratchet to it and it felt snug to my hand so I didn't torque it any further. I doubt the torque wrench is necessary but if you're paranoid about locking it in place well and worried that you'll strip it, a weak torque wrench would be a worthwhile expense.
Step 11: Adding New Fork Oil
I used motorcycle fork oil so I wasn't able to find 7.5 weight. To achieve that I mixed equal parts of 5 and 10 weight oil. It was poured in using the funnel of the bottle. The level should be 1.57 inches from the top of the stanchion tube. I tried my best to get 1.57 inches with a ruler and marked a line on my screwdriver with some masking tape. I poured some in, stuck the screwdriver in, checked to see if it needed more, and kept on doing so until it was just right. While doing so pump the stanchion tube to its full range of motion several times to ensure the oil is squeezed into every part of the cylinder.
Step 12: Re-Assembling the Rest
Put the components back in reverse order. Take care to line up everything and put every component back in.
Step 13: Enjoy Your New Ride
Here is a picture of it back on the frame without air. Once I filled it with air it felt stiff yet it was able to dampen any impact. The final result was well worth the effort.