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Discussion Starter · #1 ·
Hi All,

I noticed the other day that the left front fork is leaking some oil. Based on the Clymer manual this looks like a job I should be able to do myself without any specialized tools. I'm working on a 78 CB400.

The manual recommends to first drain and replace the oil in the forks before doing any work on them, but unfortunately on this model, that's not an option. The only way to drain the oil is to take the forks off the bike and disassemble them. If I'm doing that I might as well replace the 30 year old rubber pieces that are in there and likely deteriorated.

What suggestions do you have for doing this job?

What parts do you figure I'll need to replace?

The one's I'm thinking about are:

1. Dust seals
2. piston rings
3. Rubber bushing
4. Oil seal
5. Oil seal stop ring (prob not necessary, but I might as well order them while I'm placing the order)

see parts # 8, 10, 17, 21, 22 here

Thanks,
BaileyMan
 

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Fork seals are pretty straight forward and all you need is fork oil and fork seals.
Do not use ATF, use "FORK OIL"

This is the part number for 78 400T fork seals

OIL SEAL (33X46X11)
91255-413-881 (replaces 91255-369-000)

All you need is some fork oil and fork seals.
I use 10 wt fork oil.
 

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Sensei
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You will probably find that the dust seals (wiper caps) and the fork seals are all you will NEED to replace.....
The split plastic "rings" on the dampers are probably fine, but replacing them won't hurt.....
Replacing the spring steel retainer clips is probably overkill unless they are severely rusted up.......
Allow me to suggest new o-rings on the top caps (part#27)
 

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Discussion Starter · #4 ·
Thanks for the recommendations and the speedy replies!

After looking a little more at the Clymer manual it looks like I might need the "Fork Seal Driver Body" and "Fork Seal Driver Attachment" tools. What do y'all know about those? Can I get by without them?

Cheers,
BaileyMan
 

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66Sprint said:
You will probably find that the dust seals (wiper caps) and the fork seals are all you will NEED to replace.....
One thing I like about buying OEM Honda fork seals is when a dust cap is required, Honda includes it in the package with the new fork seal.
 

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BaileyMan said:
Thanks for the recommendations and the speedy replies!

After looking a little more at the Clymer manual it looks like I might need the "Fork Seal Driver Body" and "Fork Seal Driver Attachment" tools. What do y'all know about those? Can I get by without them?

Cheers,
BaileyMan

Yep. Fabricate a seal driver tool made of a piece of appropriately sized PVC pipe from Home Depot. $2.
 

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Correct, these forks have no drain bolts.
But you really do need to remove them and drain, you don't know how much is in there, and you need to start from scratch..
There's a huge allen bolt in the top - use the head of a bolt instead - jam two nuts on the bolt and use it's head to remove the allen bolt. No need to run out and buy a huge allen wrench that you'll only use for one task.....

I just looked through my plumbing junk and found a piece of plastic pipe with the right diameter to use as a seal driver.....
 

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Discussion Starter · #8 ·
Thanks guys! I'm looking forward to my next project...

I'll post more questions when they come up...

Cheers,
BaileyMan
 

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Discussion Starter · #9 ·
I replaced the oil seals and got the forks back together, and all seems mostly ok. I used 10w fork oil, and it seems to dive a little more than I'd like when breaking. Not horribly so, but enough that it makes me wonder how some 15w might feel. The other thing that I'm noticing now is that the forks don't always return back all the way on their own. I'm guessing this is the role of the spring. Is there anything else that plays a role in returning the shocks after they've been compressed? It feels like there's a little extra friction somewhere that's hanging things up a bit.

I put the recommended amount (140ml) of fork oil in as far as I know, but I'm totally willing to accept that perhaps I came up a little short.

Your thoughts?

Thanks,
BaileyMan
 

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Forks not returning sound like a condition called sticktion (sp?). Could be caused by the forks being slightly misaligned either in the triple trees or the fender. Try realigning.
As for dive that could be associated with either preloading, volume of oil, or wt of oil. When you put them back together did you get the big allen bolt completely seated in both forks? If not that could cause both the sticktion and dive condition. Otherwise consider the 15wt and double check volume.

Others?
 

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The forks not returning are a function of the springs and, as was said, stiction. However, with enough springs pressure or air pressure you can overcome the stiction but that's not what you want.

In the racing world we use a measurement called "static sag".

"Step 1: Extend the fork completely and measure from the wiper (the dust seal atop the slider) to the bottom of the lower triple clamp. This measurement is L1.

Step 2: Take the bike off the stand, and put the rider on board in riding position. Get and assistant to balance the bike from the rear, then push down on the front end and let it extend very slowly.

Where it stops, measure the distance between the wiper and the bottom of the triple clamp again. Do not bounce. This measurement is L2.

Step 3: Lift up on the front end and let it drop very slowly. Where it stops, measure again. Don't bounce. This measurement is L3. L2 and L3 are different due to stiction or drag in the seals and bushings, which can be particularly high for these old forks.

Step 4: Halfway between L2 and L3 is where the sag would be with no drag or stiction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true spring sag: static spring sag=L1 - [l2 + l3) / 2]."
- Race Tech web site.

On the new street bikes this number should be between 25 -30 mm. The proper way to adjust the sag is to replace the springs with heavier springs. You can also use preload spacers but be careful how much you pre load them since you can get coil bind on near full compression.

You can do this test 4 different time and sometimes get 4 different numbers. It used to drive us crazy until we figured out that you just need t need to use the averages as long as they weren't way out of wack. Also, this procedure is based on modern bikes. I don't know how theold time racers used to do it but I imagine it was quite similar.
 

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I forgot to mention, once you get the static sag set then you have to worry about rider sag. Rider sag is basically the same as static sag but with the weight of the rider on board. The rider sag is a function of the spring RATE.

So, to sum up, the static is primarilary by the spring length and preload while ride sag is determined by the size of the spring or the spring rate.

Once those two things are set then a good ride is based on damping rates which starts getting real complicated but on these old bikes it mostly a function of oil weight unless you want to get into drilling damping rods and such which I don't recommend.

Getting back to sticktion. Any little amount of pitting will cause a problem with stiction. On our race bike we micro polish the sliders and the new bikes all have Nitride coating on the fork tubes along with low friction bushing and tons of other stuff to try and deal with stiction. If you have the fork tube re-chromed then chances are good that the coating might be a little thicker than stock and that will cause more stiction in the fork legs and seals.

Good luck.
 

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Discussion Starter · #13 ·
Thanks for the info on stiction.

What's the recommendation? Take them apart, drain them, check for pitting (they looked ok when I checked before reassembling them, but my novice eye may have missed something), micropolish if needed, make sure there's enough oil in there?

Thanks,
BaileyMan
 

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BaileyMan said:
Thanks for the info on stiction.

What's the recommendation? Take them apart, drain them, check for pitting (they looked ok when I checked before reassembling them, but my novice eye may have missed something), micropolish if needed, make sure there's enough oil in there?

Thanks,
BaileyMan

I think your best bet is to take them completely apart and ensure everything is clean and you have the proper seals. Check to ensure there is no buildup on the fork leg where the fork tube slides against it. Is there a published spec for the fork spring length? If so, check to make sure they haven't collapsed over time. If not, compare the spring lengths against each other. Again, these are pretty old and were not that great when new. Magazines of the day complained constantly about stiction. I think all you can really do is make sure everything meets spec and soldier on.

Another thought might be to contact a company like Progressive and see if they have any ideas but the chances are they'll just want to sell you something.
 

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Discussion Starter · #15 ·
Coolio. Thanks Mike. Now all I have to do is somehow convince my better half that a good way to spend part of my spring break is working on my bike, when she would rather me spend time with her... :roll:

Cheers,
BaileyMan
 

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Get her involved...tell her you need her help....(so you both can ride it soon)....
 
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