Question about replacing 1973 Commando 850 fronk fork tubes

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DennisMo

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Jun 6, 2023
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I need to replace a fork tube and hope you can help me with the following!

1) Is it possible to replace a front fork tube by undoing the pinch bolt, loosening the top fork bolt a bit, whacking top fork bolt down with hammer to free fork tube, taking off front top fork tube bolt and then pulling fork tube down out through the bottom yoke? Then the reverse: put new fork tube in and tighten top bolt, pinch bolt etc

or 2) Do I also have to, FOR REASSEMBLY, undo the lower steering head nut, drive down the lower yoke so that the chrome headlight brackets can also be removed in order to put in new o-rings (three per side).

Or will the three o-rings just stay in place when I pull out fork tube in 1) above ?

Thanks
Dennis
 
Just did fork remove replace on my 74 850 an hour ago!
No I did not remove headlight and headlight brackets, they stayed in place just fine.

But you do need to disassemble the whole fork to replace the fork tube ("stanchion") as the lower end sits under the slider bushing and oil seal.
I've done this operation twice now and only once did one upper bracket o-ring come loose and need lots of fiddling to get in back in place when pushing fork tube up to top yoke.
 
Thanks. I already have the lower fork parts off the fork tube!

In case an o-ring comes loose on mine, how did you get your loose o-ring fiddled back in??
Flashlight and long screwdriver looking down into where the fork is being pressed up??
Many thanks
Dennis
 
O-ring was not in best shape, some pinch bends etc. Had a look at my o,ring kit but all thee were too thick. So I managed to reuse the old one. There is a recess in the upper yoke where o-ring is supposed to locate. Fiddling with a small scrw drive and my fingers needed to get it in that recess while pushing headlamp bracket down, away from upper yoke for clearance, then holding all in place. Twisting fork leg up into top yoke, watching that is clears past o-ring.
No lube other than a bit of grease on fork tube and some spillage of fork oil on its surface (I pre filled on bench before taking to bike).
 
DennisMo said:
I need to replace a fork tube and hope you can help me with the following!

1) Is it possible to replace a front fork tube by undoing the pinch bolt, loosening the top fork bolt a bit, whacking top fork bolt down with hammer to free fork tube, taking off front top fork tube bolt and then pulling fork tube down out through the bottom yoke? Then the reverse: put new fork tube in and tighten top bolt, pinch bolt etc

or 2) Do I also have to, FOR REASSEMBLY, undo the lower steering head nut, drive down the lower yoke so that the chrome headlight brackets can also be removed in order to put in new o-rings (three per side).

Or will the three o-rings just stay in place when I pull out fork tube in 1) above ?

Thanks
Dennis
Click to expand...
If you just want to replace the tubes, it is MUCH easier to:

1) Remove the wheel, fender, caliper
2) On one side, unscrew the bottom bolt and bushing lock ring and remove the slider from one side - a few downward jerks will remove it.
3) Undo the top nut. If you don't need to work on the damper, leave the damper rod alone
4) Loosen the pinch bolt on that side and pull out the fork. *IF* the o-Rings and fork ears were properly installed, they won't be disturbed.
5) Install the new fork tube and tighten the top nut.
6) Fully tighten the pinch bolt on that side.
7) Repeat for the other side.
8) Reinstall the sliders, best to use new internal fiber washers and to be very sure the sliders are very clean on the inside and the fiber washers are in their depression. I use a little grease on the washer bottom and slide them down a rod to get them in place.

If instead (I would) you want to do a complete job: https://www.gregmarsh.com/MC/Norton/FrontForks.aspx
 
I found today the new fiber washers for bottom of damper rod were a little undersized and would not fully seat on damper rod flange (fiber material was not flexible at all, very stiff). Took a little dremel work to fettle the washers before fully seating.
 
Thanks so much to both of you.

I took the right fork apart as it was making a click and found the lower steel bush loose.
Assembling with a new bush makes the fork stiff. I even rotated the brass upper bushing into
various positions but stiffness did not go away therefore I think the right fork tube is slightly bent.

So the project has grown and I have a new tube and I am hoping to not let the project grow any further
by having to undo the lower tube yoke....

I have two new fork tubes. Is it good practice to replace the left side fork tube that has no problems?
I could keep the good used left fork tube as a spare!

Thanks again

Dennis
 
Tornado said:
IMHO, would not worry about keeping old tube in service if not leaking or damaged.
Click to expand...
I agree, but if I go to the trouble to work on them it's all apart, new seals and o-Rings at a minimum and unless they seem perfect new bushings. I inspect the damper components and often change the cap. I also measure the springs and replace as needed. I recently redid a front end that had on spring a full inch shorter than the other!
 
DennisMo said:
Thanks so much to both of you.

I took the right fork apart as it was making a click and found the lower steel bush loose.
Assembling with a new bush makes the fork stiff. I even rotated the brass upper bushing into
various positions but stiffness did not go away therefore I think the right fork tube is slightly bent.

So the project has grown and I have a new tube and I am hoping to not let the project grow any further
by having to undo the lower tube yoke....

I have two new fork tubes. Is it good practice to replace the left side fork tube that has no problems?
I could keep the good used left fork tube as a spare!

Thanks again

Dennis
Click to expand...
is the new lower bush wrongly oversized??
 
DennisMo said:
Thanks. I already have the lower fork parts off the fork tube!

In case an o-ring comes loose on mine, how did you get your loose o-ring fiddled back in??
Flashlight and long screwdriver looking down into where the fork is being pressed up??
Many thanks
Dennis
Click to expand...
Dead easy and in which case best to remove the headlamp ear and start afresh, from memory -
1. Remove headlamp ear and remove all rings.
2. lightly grease the rings that sit in the recess of the bottom yoke.
3. slide assembled fork leg far enough through the lower yoke.
4. Gently cant the headlamp ear up into the upper yoke and swing the lower portion into place but hold it up just of the lower o rings.
5. Whilst holding the headlamp ear up slide the stanchion further through the lower yoke, leavin it about 1.5'' short from being home, let go of headlamp ear.
6. Locate the upper o ring between the headlamp ear and the upper yoke using a finger to seat it in place.
7. push the fork leg fully home into the taper of the upper yoke.
 
The new right fork lower metal bush is the correct size as I measured it with calipers.
Thanks for everyone's comments
Today I will put in the fork tubes
Dennis
 
If I might ask a related question, why would a fork leg that rolls straight on a marble tabletop bind only at certain points of relation to the fork slider? If I secure the slider, and rotate the fork leg and push it into the slider, it binds at certain points but otherwise moves freely.
 
Jdub said:
If I might ask a related question, why would a fork leg that rolls straight on a marble tabletop bind only at certain points of relation to the fork slider? If I secure the slider, and rotate the fork leg and push it into the slider, it binds at certain points but otherwise moves freely.
Click to expand...
I had that on a BSA, the slider was distorted when BSA welded a brakeplate lug to the slider. When the perfectly straight leg went past that point it got progressively stiffer to move and then jammed at the bottom. Could also be bushes not being concentric.
 
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