Busted 4CS preload cap - Compression


C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Well, this was a disappointing riding day. My son and I went riding and we came back to the truck after about 15 minutes to find the whole front of the bike covered with fork oil. It turns out the compression adjuster knob just popped off of the left fork, and fork oil was leaking out! No way to cap it off, so we drove the two hours home with it continuing to leak. Fork oil sprayed over the whole bike, and all over the bed of the truck! What a friggin' mess!

Never heard of this happening before, but I think this video described what may have happened. Same thing, just on the compression side instead of rebound side. Internal pressure just built up and shot the center of the cap clean off the bike:

Not even sure how to go about fixing this. It doesn't look like you can order replacement preload adjuster caps for 4CS that I can find, and the problem would still exist anyway. Don't think I have the right tools or experience to to rebuild the forks anyway, even if I had the parts. While I wouldn't mind swapping out for a AER 48, I don't have the correct axle, spacers or triple clamps. I think I would need all three to swap for AER 48, correct?

Anyone have a cheap pair of 4CS forks they'd like to get rid of? My son actually likes his 4CS on his MX better than my AER 48's on my MXR. But then again, he only weighs 130 lbs.

Any other suggestions?

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Philip

Administrator
Staff member
Location
Ann Arbor, MI
Weird.

To swap for an AER48, you would need the forks with fork guards, the axle, and the spacers. Expensive, and then you will have an unbalanced air-forked bike that your son won't like as much as he liked the old one.

I would just call Adam at Kreft, or any other KTM suspension shop, and ask if they could fix the leaky damper rod and get you a new compression adjuster. The whole procedure is not any more complex than replacing the oil seals, so it should be under $200 in labor, plus some small parts, plus shipping. Good luck!
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Bummer about the fork cap.

Are you bleeding the forks from time to time while riding?
It is not a regular practice, no. Never had a need to before. Of course, this was only 15 minutes into riding, so I certainly wouldn’t have done it that soon anyway.

I have the Motion Pro micro bleeders on my MXR, but not on my son’s MX.
 

F451

Well-known member
Location
Seattle
It is not a regular practice, no. Never had a need to before. Of course, this was only 15 minutes into riding, so I certainly wouldn’t have done it that soon anyway.

I have the Motion Pro micro bleeders on my MXR, but not on my son’s MX.

Once repaired, you may consider bleeding the forks regularly, I have quick bleeders and usually tap them before I head out on a ride, and then during a break mid ride, just depends on how long I'm riding. The quick bleeders are handy, but can be tricky, they are easy to strip out the threads on install, and can also be snapped off. I've never had any issues with them myself, but something to be aware of if you go that route. Or just keep a screw driver handy to bleed the forks, even a ride in the back on the truck in the sun can warm up the forks enough to increase the air pressure in them a bit.
 

Philip

Administrator
Staff member
Location
Ann Arbor, MI
This type of failure probably took thousands of psi of hydraulic pressure built up in a wrong chamber. I don't think bleeding 2-3 psi of air pressure would have prevented this.
 

F451

Well-known member
Location
Seattle
This type of failure probably took thousands of psi of hydraulic pressure built up in a wrong chamber. I don't think bleeding 2-3 psi of air pressure would have prevented this.

Its not just for preventing seal failure, releasing the air pressure insures your forks are working properly, resulting in a more comfortable ride, better handling, etc. Its why the forks have the air pressure relief screws in the first place. Not using the bleeders certainly didn't help in this situation.

But hey, if people don't want to use them for whatever reason, don't know what to say about that.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Its not just for preventing seal failure, releasing the air pressure insures your forks are working properly, resulting in a more comfortable ride, better handling, etc.
I’ve used much the same reasoning with my wife when I let one rip under the covers. She didn’t really buy it either.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Help! Okay guys. This fork oil leak might have some other repercussions that I hadn’t anticipated. I got another set of forks that I am going to put on, so that isn’t the issue.

I went to clean up the bike, and I noted that oil has pretty much coated every surface on the front and left of the Alta. When I went to wipe off the battery, it appears the bottom seal on the front and left of the battery is pretty much “melted”. As I wipe it, it is coming off as black goo. The goop on the rag in the pic is the exterior of the gasket material.

Soooo… am I screwed or what? Can I seal that seam with some kind of tape, or will I need to remove the battery and replace that seal? Not actually sure what that seal is made out of.

Ideas? Any advice at this point would be appreciated.

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rayivers

Well-known member
Location
CT, USA
My battery's covered with a non-hardening soft black sealant there too - mine would easily scrape off with a fingernail. If it were me, I'd just clean the area and re-apply a similar sealant.
 

Philip

Administrator
Staff member
Location
Ann Arbor, MI
That's not a big deal, @C5tor. You can flip the bike over like a bicycle, remove the subframe, and then either run a bead of RTV around the bottom seal, or remove the bottom cover and install a new black goop seal. I have an Amazon link to it in the Battery Repair sub-forum. No need to remove the entire battery.
 

C5tor

Chief Comedic Instigator
Location
San Ramon, CA
Got the new 4CS fork installed, new brake pads, and the bike cleaned up. Also laid a bead of RTV around the bottom lid on the battery. Crossing fingers that everything is working okay now. We'll see.

Anyone need a spare 4CS left fork? Cheap!
 

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