Front Forks - advice on repair

[rwbeard]

Good morning,

Having acquired an almost completed RGS replica machine 3 years or so ago, I have worked my way through the bike getting it to near perfect running order. The machine had been put together by previous owner starting in 2007, and has virtually all new parts including a £5k+ SRM engine rebuild!

I have never been happy with the front forks, they felt stiff, and there was always a roughness to their action. Also there was significant vibration on braking, even though the brake drum was perfectly set up and had been checked for distortion and found to be OK.

So decided to strip down forks (having invested in the right tools some time ago), and was very easy indeed. Fork seals were hardened, and leaking. But it soon became evident why the fork action was rough when I removed the bushes for inspection. The lower bush that slides within the bottom fork leg is worn, and the inside of the fork legs are not smooth. I have included photos. There was also slight play on the right hand bush to its stanchion, and I suspect that this may be the main constructing factor to the vibration through the forks when braking. Bearing in mind what I found, I suspect that the original fork components were not particularly well matched.

Considering the bike has probably only done 150 miles since being rebuilt and recommissioning, I am surprised at the wear, and I know the bushes and stanchions were new as I have the original receipts.

Whilst it would be a simple matter just to fit new bushes, I am worried about the potential damage to the inner surface of the fork legs on the longevity of the bushes. Can the insides of the fork legs be gently honed to a smooth finish? Or would it be best to buy a new set of fork legs? If the latter, does anyone know the best source to purchase from as well as the respective part numbers.

Much appreciate your help

Thanks

Bob

[edboy]

hi, its easy to resolve your problem but you may not want to copy my old shop method.
i fit the new bottom bush on the stantion , done up tight, then attatch to a bottom yoke put in a bench vice. i add loads of grinding paste to the bottom bush and slider and twist on the slider[ if its not tight bush is wrong] . continue back and forward along the bush until it doesnt bind.adding a little oil also helps.
removing all the old grinding paste is a challenge which is why i m loathe to post this method. but i use it.

[chaterlea25]

Hi rwbeard,
Those legs look bad, secondhand  replacements are needed as new legs are very rare and expensive if you find them
Usually if theres only slight damage they can be lightly honed
but  any metal removed adds to play in the assembly

Edboy
if grinding paste is used it will embed in the bronze bush and continue its grinding once the fork is assembled

John

[Joolstacho]

The sliders typically get 'belled out' where the lower bush slides. Nasty, - new bushes won't help if this is the case. Should be possible for a machine shop to replace the tube section shouldn't it? Anyone ever done this? (Mind you it would probably cost a leg  ).

[BSA_54A10]

No you don't put a sleeve in there.
You get the leg line bored and then make up some custom bushes the same as you do with any worn out bush.

[Joolstacho]

Ruddy'ell who said put a sleeve in there!!!
If you're referring to my post I said "replace the tube section", not sleeve it. There's precious little wall thickness for line boring down there as it is, you'd be weakening a very critical structural component, I certainly wouldn't trust it on our roads around here! Surely easier and MUCH better to machine off the top and bottom lugs and fit new tubing to them?

[edboy]

an amazing amount of work suggested for an easy repair. with grinding paste you are only honing the fork bore to the bush. all old grinding paste does has to be removed but i cant agree with greybeard [sorry greybeard ] about it remaining in the bush after it has been properly cleaned. however my suggested method is shop and not really for a beginner which is why i was loathe to post it. but the choice rests with the owner.

[bsa-bill]

an amazing amount of work suggested for an easy repair. with grinding paste you are only honing the fork bore to the bush. all old grinding paste does has to be removed but i cant agree with greybeard [sorry greybeard ] about it remaining in the bush

I've been guilty of this (if guilt is there to be found) - the logical safeguard is to use a sacrificial bush

[nimrod650]

you tube BSA FORK BUSHES a guy rebuilding a65 worn forks interesting he also takes on work

[Greybeard]

.. I cant agree with greybeard [sorry greybeard ] about it remaining in the bush...

Not me guv!

Mind you, I agree that bronze bushing is very likely to hold particles in its porous structure.

[edboy]

sorry greybeard, no it wasn t you who originally suggested old grinding paste remained in the bush but now i see your firmly in the leave camp. what is this country coming to.

[Greybeard]

...greybeard...
...i see your firmly in the leave camp...

Only with regard to grinding paste on bronze bushes.

[RichardL]

Really, how bad would it be if some microscopic particles of grinding paste remain embedded in the fork bushes after a thorough scrubbing? The forks are not reciprocating at thousands of times per minute, like a piston, wherein warnings of cleanup with sandpaper may be appropriate. The gobs of paste used in the first place to actually get the job done will be gone. The microscopic particles will not be leaving the forks to contaminate other surfaces, like rod journals. Others will have more experienced opinions, but it sounds like much ado about nothing to me.

Richard L.

[muskrat]

G'day Richard.
You'd be surprised how many times the slider moves up and down just riding around the block. You'd also be surprised at how much grit will be inbeded in the bush even after a good scrubbing. I myself would not do it.
 I'd give the sliders a quick hit with a cylinder hone then machine new bushes to suit.
cheers

[chaterlea25]

Hi All,
A new bush should be a good fit in  the leg,
adding grinding paste as suggested will lap away the bronze and some of the leg as well,
this will lead to a loose(r)  fit
It may be of use when theres a ding in the leg, but will wear an equal amount from the opposite
side ofthe leg !!!! 
I would not use the bush again, as the embedded particles will continue to lap away at the fork leg

Theres a tool called a "David Brown" reamer which with the correct blades could be used to ream a worn leg circular (provided you could remove the taper plug at the bottom)
A  "Sunnen" hone would do the same job, but are a megabuck item
Then special oversize bushes would need to be made to suit

Over the years I have made two different tools to overcome light scores and also to remove a ding in an otherwise good leg
The first is a hone of sorts, a steel plug thats a good sliding fit in the leg has two slots milled lengthways
for about half its length, these are sized to accept square section hone stones
cross drilled holes in the plug slots take some springs to push the stones outwards
the length of the hone stones extends past the end of the tool to work on the fork leg walls around the taper plug at the bottom.
The opposit end of the plug is drilled and tapped to accept a piece of 1/2 round steel, so the hone can be spun with an electric drill


The second tool was made from an old stanchion and bushes,
I cut away a piece of the bottom bush and leg and silver soldered in a piece of tool steel
this was then ground back and a cutting edge formed until it was a sliding fit in the undamaged part of the leg, the stanchion was rotated slowly in the lathe and the leg was fed inwards until the tool steel cut away at the high spot
It took several attempts to get the cut just right and the new leg and bush to slide nicely 

I have heard of "modern" stanchions and proper dampers being fitted into period forks
Has anyone more information on such a mod????

John