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HOW2: Overhaul the front forks / Replace fork Seals

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PostPosted: 01:55 - 11 Jun 2012    Post subject: HOW2: Overhaul the front forks / Replace fork Seals Reply with quote

This is NOT a definitive guide, nor is it a substitute for the PROPER Haynes / Clymar manual for the bike in question.

Bike featured is a 1986 Honda CB125TD-C. But principle pretty much of a much for most bikes.

First of all, the front wheel and mudguard need to be removed. If you are ONLY doing the forks, you don't have to take everything off the front end, but you will need to completely remove the fork legs, so when the wheel comes out, the brake calliper will have to come off, as will the speeds-drive. If the brake isn't to be touched, you don't have to take it off the hose, but it is good practice NOT to leave it swinging on the brake pipe, and support it on some wire or string or something from the bottom yoke!

Before removing the fork from the yokes, its best to undo the fork cap, because while its clamped in the yoke, the stanchion cant twist.

The clamps on the fork yoke can then be undone and the forks slid out and off the bike. You can remove and store the fork caps and springs if you like, but as Snowie models for us, this is the basic starting point!

The oil, (if there is any left in the fork) can be poured out into a suitable receptacle, and then you can start dismantling the fork. The two halves of the fork, the chrome stanchion and the lower leg or slider are normally only held together by one bolt known as the 'Damper-Bolt' as it holds the damper plunger in the stanchion to the slider. Its usually a cap-head allen bolt, cunningly hidden in a recess in the bottom of the fork slider, often underneath the axle clamp. A long allen-key or allen socket is normally needed to reach it.

As Snowie shows, you can often get enough 'reach' by turning a regular allen-key around and using the long end to get at the bolt, but it can be a bit tight to then turn, so a little added leverage might need to be applied. The bolt should NOT be too tight though, so be careful, you don't want to round out the bolt head! Worth poking it out a bit with something long and pointy to make sure you can get the allen-key all the way into the head, and then to 'tap' the allen key into the bolt head to make sure it is well engaged.

With the Damper bolt undone, the fork slider should slide straight off the stanchion.

Remove the damper bolt from the end of the slider, and you can screw it back into the bottom of the damper rod cap, to give you something to grip, and allow you to pull the damper rod out of the bottom of the stanchion, so you can remove the damper rod 'stop' or spacer.... the aluminium bit on the end!

Thats the one! Damper rod can then be pushed through and dropped out of the top of the stanchion.

If the damper rod doesn't drop out under its own weight, or with the encouragement of a light tap, it can, as Snowie shows, be pushed out with a length of rod. These bits can now all be cleaned and inspected, or set aside, while you strip the seals out of the fork slider.

On the top of the slider is the 'dust seal'. This should just lever out with a suitable lever, like the tyre iron shown here. I WONT say it can be done with an old screw-driver. Such abuses of the humble screw-driver are an insult to my engineering sensibilities, and the pointy end can easily scratch or damage the soft aluminium! It should NOT be done!

Beneath the upper dust seal, is the clip that secures the main fork-seal in place. Designs vary and many forks have proper circlips in this location that need proper circlip pliers to remove. Some have simple C-Clips that are a right pain and demand a lot of swearing and something pointy to dig out. The Honda CB125TD has a C-Clip, but one considerately endowed with two recesses to make it easier to get something in to pop it out. Proper circlip pliers would have been a good idea..... Here Snowie uses.... I cant bear it! a SCREWDRIVER! Its NOT good mechanics you know?!

Circlip removed, the main seal can be prised out. Here Snowie uses a tyre iron. NOT a SCREWDRIVER! The seals CAN be rather stiff to get out, and the trick if working with a lever is to try and get it to come out as square and level as possible, working around the seal prising it up just a little at a time. There are special seal removal tools available, but they are not always that reliable. some forks don't have enough recess behind the seal to be able to grip it and some seal removal tools just cant apply enough force to get the seals out. The humble and old fashioned lever (or I suppose if you HAVE to be a heathen, the 'old screwdriver'!) used with care, so you don't ding, scratch or damage the fork slider, and worked gently around to bring the seal out square, is about 99% reliable, to get most seals out.

See, even a weedy grirlie can manage it!

So, seal removed, you can see the slider bush beneath.

The entire fork leg disassembled, you simply repeat the process on the other leg, then you can clean and inspect all the bits before trying to put them back together.

If you want to paint or polish the sliders, this is where you take the opportunity. If painting though, be sure to mask off the inside so no paint gets onto the slider bush or seal seats, or where it could interfere with the operation of the damper mechanism.

After cleaning, the stanchion, will need to be polished. If it is 'slightly' rusted or pitted, this NEED NOT make the stanchion scrap.

The main function of the chrome on the fork stanchion is to give a smooth low friction surface for the slider bush and fork seal to run on. As long as the stanchion isn't significantly worn, and there are not major dents or scores or other surface irregularities, a small amount of 'pitting' can be tolerated.

It IS technically an MOT failure though, because by strict interpretation of the MOT rules, it is a suspension defect. However, the 'defect' is essentially that the rougher than normal surface will make the fork seals wear more rapidly than they would normally. Provided the surface is smooth enough that the fork seal WILL seal, and doesn't allow fluid out so that damping is impaired, it should not effect the operation of the fork. AND if a fork gaiter is fitted between the fork slider and the fork yoke, covering the swept area of the stanchion, an MOT inspector cannot 'see' the pitting, and is not allowed to remove the gaiter to inspect, and cannot therefore fail the fork on this criteria.

YES, it is a 'dodge'. But a legitimate one. And provided you know what you are about, and have prepared the stanchion as well as you can, removing as much rust as possible without damaging the remaining chrome, and haven't actually got huge areas of exposed metal or jagged edges of flaky chrome, there shouldn't be any major reason that the fork should give any major problem.

BUT, you DO need to remember its not a 100% fix and forget cure. The seals WILL wear out more rapidly than if they were running on a good chrome stanchion. Consequently IF you do use this 'gaiter-dodge', I advise that you periodically lift the gaiter to check the stanchion condition, to make sure it hasn't deteriorated, and to check that the seal hasn't started weeping.

I also advice that the seal be replaced at regular intervals on a 'precautionary' basis to avoid actual seal failure. How frequently precautionary replacements ought to be done, I cannot say. Depends on the bike, the way its ridden, where its ridden, and how far its ridden. But as a rough reckoner, once a year, for each MOT is probably a start! On high mile, hard worked bikes, it may be worth doing with every major service, if that's more frequently.

So, stripped, cleaned, polished, new seals, gaiters & oil procured, forks can be re-assembled

The CB125 has a spacer under the fork seal, above the slider bush, which needs to be fitted, the correct way up, before the fork seal.

Then the fork seal can be pressed into place, ensuring you have it the right way up, before its tapped into its rebate.

then driven home, using a suitably sized socket as a dirft, driven with soft hammer.

With the seal 'home' the retaining circlip can be re fitted. Then the damper mechanism reassembled into the bottom of the fork stancion.

Dropping the damper tube into the stanchion is the easy bit...

Getting it out the bottom can be a bit more tricky! Here I have a length of M8 'unithread' rod (available at most good hardware or DIY stores) which is the same thread as the damper retaining bolt, but a LOT longer...

so I can push the damper rod down, then from the other end, fish in and screw into the bottom of the damper rod, and draw it out the bottom, thusly....

Fitting the aluminium bottom stop, should stop it falling back through, while assembling to the slider.

Bottom Bolt can then be fitted, remembering copper sealing washer, and tightened to hold everything together, and the damping oil in.

Damping oil can then be measured out, and the fork leg filled with oil.

Fork Spring can then be inserted into the stanchion, and the end cap fitted.

Often its difficult to apply enough preload to the spring, and grip the stanchion to tighten the fork end cap on, and it may be easier to slide the stanchion back onto the fork joke and nip the pinch bolt to hold it steady, or to leave the spring, oil and cap until the fork has been fitted to the yokes.

All that remains is to slide gaiter or dust cap over the stanchion and onto the slider, and that's the leg fully rebuilt and ready to fit anyway.

Both legs done, gaitered up, and back on the bike, yoke pinch bolts torque up, ready for the front wheel to be refitted.
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

Last edited by Teflon-Mike on 23:15 - 23 Nov 2012; edited 1 time in total
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PostPosted: 03:05 - 11 Jun 2012    Post subject: Reply with quote

Good post, bookmarked.
1998 CG125 (Sold) CB500 [+ Clio 172]
Usually_Wrong: I tend to fail more than pass. [Speaking about Theory Practice]
Usually_Wrong: im on unemployment at the moment
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