Fork Slider Thread Damage

Even ones not cross threaded can get issues with aluminium to steel giving galvanic corrosion hence why I use stainless collars. I will measure the thread tomorrow using a thread gauge and try to mount a disc brake one in my lathe for a spin to see if the thread is concentric to the large OD if I get it in the chuck. On the disc brake one the oval section cannot be used unless you have a 4 jaw chuck. If it is then a possible repair is more likely.

The thread size is 1 15/16" OD by 20 t.p.i. It is possible to machine out the damaged thread and fit a screwed insert to bring the thread back to the std size or screw cut a bigger thread (say 2" OD) and make a custom collar.

Easy to mount the slider in the lathe and using the fixed steady to support the top end. The only thing is to make sure that the bottom of the slider is running true. That requires having a plug that will register in the spindle nose and locates the bottom damper bolt recess.   

... can gall and effectively seize in alloy, hence the reason for using anti-seize on things like Japanese bikes' exhaust studs.

Confirm 20 tpi and 1" 15/16 but the OD of the slider is close to being circular/concentric but does show signs of where the casting is low or has been polished low so 4 jaw chuck would be advisable unless any low spots can be avoided with a 3 jaw.

Richard,

I will be strung up as a bodger for this.

Have had success in the past restoring the female thread to 'good enough'. Not on a fork leg though.

By using your old seal holder, which will be steel and the thread undamaged.

Use the side edge of a flat file, 3 to 5 mm thick,  to cut a sharp edged groove through the thread and in direction of fork tube. Repeat 4 or 5 times round the thread.

Think of a normal thread tap with 4 or 5 narrow flutes.

With a bit of patience to get the seal holder started right, lots of grease, you can restore the thread in the slider bit by bit. Back out regularly to remove the aluminium cuttings from the grease filled slots. Two holes through the walls of the seal holder and a Tommy bar with one hand pulling, the other pushing, will give you best control and best chance of keeping the threads true to original.

If you are familiar with cutting angles, you can even slope the groove edges to cut quite well.

My use of this technique has restored female threads to 'good enough'

Or find that good local machine shop!!

Peter 

I have repaired a few large threads over the years by using a dremel cutting wheel to cut slots in a mating thread and then if possible using the lathe to keep everything at 90 degrees.

 not a bodge if done carefully. There's plenty of metal there.

Surely "emergency engineering"?

.. founder of the VMCC was a great enthusiast for the art of bodging - for example screw jacks on a (Triumph I think or may have been Norton) cylinder head bearing on the frame top tube to hold it all together. Not to be confused with botching.

Of course the term bodging comes from using the old pole lathes.

With the slider held by soft jaws in a vice with a strong light to illuminate things, you may be able to knock the damaged "crossed" parts of the thread somewhere back in line with a small chisel or drift then using valve grinding paste on the seal holder thread, slowly work it in going backward and forward carefully by hand making sure it is square. Saves cutting the seal holder thread up.

Best wishes, Al.

"Of course the term bodging comes from using the old pole lathes."

As I uderstand it, "bodgers" were a group of skilled woodworkers who made hand crafted articles of generally good quality and of a type that would be highly valued today as artisinal. But, with the advent of factory produced factory items, where a uniform, machine-made consistency was the word, bodger came to be used as a term of derision. Sad really.

Bearing in mind that well-used sliders are often worn internally where the steel bush runs and that no-one has come up with a cost-effective means to repair, and with new sliders available, is it worth hours of work or specialist costs to repair an old slider ?

A few years ago i had an email conversation with the late John Bould (of Lansdown fork conversion fame) about the sleeving of worn sliders and the problems of boring/reaming long deep holes. He had posted a photo of the fitment he had made, to hold the slider in his lathe to enable him to single point the bore to full depth, making the bore straight and then he put the reamer in to get the finish. New oversize bushes completed the job. All this work didn't come cheap. As Richard says, cheaper to replace.

I have a 500 in at the moment that has supposedly been 'restored'  Amongst all the other faults (and there were many)  the owner complained of juddering when the front brake was applied. No wonder, the headstock was loose and the forks could waggle back and forth when the brake lever was pulled. The fork top nuts were hand tight, in fact i undid one with my fingers! Looking down the stanchion i thought 'that looks dry' Lo and behold, no fork oil, both legs bone dry. One drain screw was a cut down 6mm cap screw, it did have a fibre washer under the head though!  

There was a simpler way to bore out the slider to a new size posted on this forum some years ago, it used the damper body retaining hole as the datum and a small 2 bladed self centralising cutter mounted on a rod to take the cut from top to bottom. Then you just needed oversize bushes to suit the new bore. I saved the pics but cannot find them.

A self centralising cutter will follow the bore it is going down, if the bore is not straight, i,e, worn, it will follow the worn bore and not be straight, round but not straight. That's why the bore has to be straightened first. John used a floating reamer to finish the bore. 

I'd worry about the strength of the slider with material bored out of it. Especially so if upgraded brakes were fitted. I think Richard's idea of getting new slider fitted would be far preferable, they aren't that expensive.

A lot of Johns work went to be used on race bikes and the fact that only enough metal was removed to eliminate the wear I wouldn't be worried about the strength of the slider.

I'm sure that the sliders can be recovered with skill and the necessary equipment. There is certainly plenty of 'meat' on the 850 sliders, at least...but the new price for a pinch bolt slider from Andover is £163 + VAT and it will accept standard bushes. If one has to pay an engineer to recover a casting, how many hours work can we anticipate for that price ?

Even then, there will be over-sized bushes required, and at every revision in the future. With regularly changed oil, I don't reckon on seeing a lot of slider wear. I replace the £10 bottom bushes occasionally as well as the damper caps if they're apart.

Anyone who thinks they can bore out a round slider is off their rocker, ever wonder why Norton went over to the oval slider. The round slider has quite thin wall section, the oval slider not much different in the wheel spindle direction, but a lot more in the for and aft direction. 

Richards advice is the best of the above, most probably the safest and cheapest as well.