* Before we start let me first say that the following process involves using Oxalic Acid. Therefore please wear protective gloves, goggles or glasses and do not breathe any vapour in. Best practice is to do this in a well ventilated space or outside.
* Another note is that this feature is referring to the NSR250 MC21 SE/SP & MC28 SE fork top adjuster (and NC30 late type). Oxalic acid is used in some forms of aluminium anodising but I do not know what the effect that Oxalic acid will have on the blue anodised adjuster on the SP models, if you decide to give it a go that must be at your own risk.
The brass rebound damping adjuster on my SE fork top was tightly stuck in place and I wanted to remove it to service the top and get it working again.
After removing the fork top assembly from the damper rod, I wound the preload adjuster out from the top nut and put the top nut to one side.
The slot in the top of the rebound adjuster was already damaged so I guess this adjuster had been stuck in the preload component for a good while.
I used all the usual go-to methods for removal. Heat, pen oil, more heat & pen oil and lots of swearing but all that happened is the brass top got more and more damaged.
So I did some research and I found this document in the maritime world:
“...if conditions are right you could experience galvanic corrosion. this is an electrical effect experienced when dissimilar metals are brought close together in a conducting liquid. For example, immerse brass and aluminum in seawater and electrons move from the aluminum to the brass. This can be a problem in boats where brass fittings are close to or even in contact with aluminum”
Galvanic corrosion between brass and aluminium triggered by salted water made sense to me, we have a lot of road salt on our roads during winter and early spring and I set about finding out how to treat this corrosion.
I found that Oxalic Acid is one of the easier treatments to use. Oxalic acid can also be used to remove light layers of corrosion (rust) from ferrous metals as well. (I have seen some online info showing people using Oxalic Acid to clean out light corrosion from the inside fuel tanks).
Before going any further I cleaned up the brass adjuster with a file to remove any burrs that might damage the preload adjuster when removing it.
I also removed the preload o-ring, the little spring and ball component and put them both aside safely.
I mixed up a small scoop (20ml) of acid crystals with some warm water (100ml), it dissolves easier in warm water and the heat helps the fluid get into the effected area of the adjuster. I used just enough warm water to cover the whole component. Use a glass or plastic container, NOT a metal container. I prefer plastic.
If you are using a larger container obviously you need to increase the recipe mix.
I let it soak for around 4 hours and using gloves I tried to give it a turn, I got a 1/4 turn in response and dropped it back into the mix for another 4 hours or so and…
Hey Presto!
After 8 hours soak time the adjuster wound out of the preload component quite easily by turning it clockwise…you screw the adjuster down through the preload component until it drops out of the bottom.
I quickly rinsed both components in lots of clean water to neutralise the acid action.
On inspection the adjuster thread is in perfect condition. It seems the corrosion occurs above the small o-ring at the top 30mm or so. Between the outside edge of the top of the adjuster needle and the top inside edge of the preload component.
The road salt solution sits on top of the adjuster seal and is just trapped in there until it dries out and causes havoc.
I could have had the brass needles repaired or maybe replicated by an engineering company but the honest truth is that the OEM needle adjuster are not that good to start with.
Many owners replace the whole fork top component with Tyga ones which offer much more adjustment as the separate components are not available OEM from Honda.
I found some replacement needles being advertised by Rick Oliver who is an NC30 specialist, after a phone call or two we agreed that the later type NC30 needle fits within the MC21 SE fork top so I purchased two as replacement parts.
The Rick Oliver needles are manufactured out of stainless steel and have a redesigned 18 click adjustment taper on the needle providing a performance upgrade. They also have a 6mm hex head to assist the screwdriver slot.
Installation is basically the reverse of the disassembly procedure, lightly coat the components in fork oil before assembly and remember to just be careful with the installation of the fork top. Follow theses tips:
Wind the needle all the way out fully anti-clockwise and wind the preload adjuster fully out anti-clockwise to the weakest setting. On the damper rod make sure the lock nut is all the way down to the bottom of the thread.
Turn the adjuster needle 18 clicks inwards then gently screw the complete fork top component onto the damper rod until the needle touches the damper rod enough to close the hole but only just finger tight.
Wind out the adjuster needle 3 clicks but keep the component in the same position.
Now do the lock nut up against the fork top to the required torque 1.8 - 2.2 kg-m.
Double check your new rebound adjuster, you should have a nice smooth 18 clicks from open to closed.
Because you have already completely wound out the preload adjuster you can go ahead and screw the fork top cap component into the fork leg and torque it up to the correct setting of 1.5 - 3.0 kg-m.
If you do not wind out the preload adjuster before tightening up the cap you may get a false torque reading due to the preload of the spring.
Before you mention it I know the adjuster is longer at the top, this is intentional so you can adjust it easier...and I like the look.
So that is it, all done. A nice performance upgrade without breaking the bank!
A big thanks to Rick Oliver for his patience and support.
Last edited by Max on Sun Jun 14, 2020 5:18 am; edited 1 time in total
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