Forked off!

Our man Mark finds out that his K2 is a bit of a bitsa. Does this mean he has a moun­tain to climb?

Classic Motorcycle Mechanics - - WORKSHOP -

Imust ad­mit I did buy the CB750 K2 on a whim without even see­ing it, though I did have a few not-very-good pho­tos to as­sess it. One thing I had no­ticed was that the front stay of the mud­guard was miss­ing, and pos­si­bly there was some­thing not quite right about the brake caliper.

When I had got the bike home it be­came clear. Not only was the stay miss­ing, there were no mount­ing holes for it and the caliper was def­i­nitely not right for a K2 (Photo 1). Then it all added up: the front-end is not from a CB750 but a later model, ei­ther an F or F1 from 1975 or 1976! So the mes­sage was clear: I would need to find the cor­rect forks and caliper for a K2. Ex­cept that I had al­ready de­cided I was not go­ing to be over-zeal­ous about orig­i­nal­ity and the later parts were ac­tu­ally an im­prove­ment over the orig­i­nal ones! One point that came up in con­tem­po­rary road tests for the pre-k3 mod­els was that the forks dis­played a high de­gree of so-called ‘stic­tion’ i.e. static fric­tion. This meant that the forks were re­luc­tant to re­spond to sud­den small move­ments such as cross­ing over the joints in roads made of con­crete slabs, then com­monly found in the USA. The re­sult was a rather an­noy­ing reg­u­lar jar­ring, and this was later cured by a re­design of the forks which we shall see. The caliper was also re­designed and we’ll take a look at that later on. So on with the work. The main is­sue was that the forks were com­pletely worn out (Photo 2) and leak­ing. The worn plat­ing would be an ob­vi­ous MOT fail­ure as it would not be able to seal prop­erly, so the forks needed to be dis­man­tled. The first step is to re­move the front wheel and straight away I could see that the chunky tyre which had been fit­ted (it looks more suitable for a trac­tor than a mo­tor­bike) was much too wide and the lumps at the edges were catch­ing on the mud­guard (Photo 3). Not a great choice of tyre, even though it was (it said) a Qual­i­fier. This later de­sign of Honda fork is very sim­ple and fol­low­ing the usual pro­ce­dure to ac­cess the forks (which we have cov­ered many times) the first step is usu­ally to drain the oil, but I do this later on as it is less trou­ble. It is easy enough to ex­tract the fork legs and take them to bits and then the oil can be poured out of the bot­tom cases (this is the Honda ter­mi­nol­ogy for the lower cast­ings). One side con­tained a small quan­tity of thick black oil in (Photo 4) and the other had a gloopy sub­stance sim­i­lar to what I dredge from the bot­tom of my pond (Photo 5). This would prob­a­bly ex­plain why the fork tubes looked like this (Photo 6). You can see that the hard chrome layer has dis­ap­peared, as has the nickel un­der­neath, and the steel tube it­self has started to

wear. Not sur­pris­ingly, the leg us­ing mud in­stead of oil was worse. I sent them off to AM Philpot’s for the usual treat­ment, i.e. check­ing, straight­en­ing if nec­es­sary, grind­ing off the old chrome, re-plat­ing and fin­ish grind­ing to size. Their turn­around time was six weeks, which was an im­prove­ment over the 12 weeks when I last en­quired. The seals are held in place by sim­ple wire clips, which were easy enough to ex­tract as the oil leaks had kept them rust free (Photo 7). I know other cor­re­spon­dents use a special tool to ex­tract the seals but I find a sim­ple small crow­bar or pry bar (Photo 8) works per­fectly well though a small piece of wood pre­vents the end of the lower case be­ing dam­aged. I know it sounds sac­ri­le­gious but even if it did get dam­aged this way that would prob­a­bly make no dif­fer­ence as it would have no ef­fect on the fork’s op­er­a­tion, nor would it be vis­i­ble un­der the boot. But why not use the bit of wood any­way? One rea­son why these forks are very much sim­pler than the orig­i­nal pre-k3 type is that they lack bush­ing. That does mean that the bot­tom cases will even­tu­ally wear out but for­tu­nately mine were ac­cept­able. I sup­pose con­ceiv­ably it would be pos­si­ble to hone them out over­sized and have the tubes re-plated and ground to a suitable over­size as a so­lu­tion. While the tubes were be­ing pro­cessed I ren­o­vated the cases and the eas­i­est way is to use a Scotch-brite wheel in a bench grinder which pro­duces a fin­ish rea­son­ably sim­i­lar to the orig­i­nal: they were not highly pol­ished when new, though they were lac­quered. I shall not use lac­quer as it is not durable and looks a mess af­ter a while, as we have all seen on old Ja­panese bikes I sus­pect. So a lit­tle pol­ish from time-to­time is the way for­ward for me. One of the cases would have been not too bad (Photo 9) had it not been scratched in places so it needed to be re­fin­ished, just like the other one which has been com­pleted in this photo. The fork legs were re­turned from Philpot’s af­ter the pre­dicted time and the re­sults were very pleas­ing. A quick check with the tubes in the cases showed that the wear in the cases was not ex­ces­sive as there was no de­tectable play, and that the tubes had been ground to the cor­rect size. Rather than us­ing the gen­uine Honda oil seals I went to my usual sup­plier, Sim­ply Bear­ings, who offer a very fast ser­vice at rea­son­able prices (about a ten­ner for a pair in­clud­ing VAT and de­liv­ery). You can choose be­tween dif­fer­ent types of seals, the dif­fer­ences be­ing in the seal lip de­sign and I went for the TC4 op­tion as that is clos­est to the orig­i­nal Honda de­sign. Fun­nily enough, the seals I took out were dif­fer­ent. Photo 10 shows a view of the top of each.

Re­assem­bly of these forks is pretty straight­for­ward, but let me point out one or two points: you can as­sem­ble each leg ei­ther by putting the seal in the case and then the tubes through the seals or by as­sem­bling with the tubes in place, then fit­ting the seals into the an­nu­lar gap be­tween. I choose the for­mer be­cause it is eas­ier and it is not nec­es­sary to use a special tool: a suitable sized socket works per­fectly (Photo 11). One thing I find rather cu­ri­ous is that the seal ap­pears to be not deep enough as if it is pressed down on to the step in the case, which is pre­sum­ably where it is meant to be, the top is nowhere near the groove for the cir­clip (Photo 12). It looks as though a spacer is meant to be in­serted be­tween the seal and the cir­clip, but no such part is spec­i­fied. The other point is that the de­sign is per­haps slightly im­per­fect as it is pos­si­ble to as­sem­ble the bits in the wrong or­der with them fit­ting to­gether per­fectly well. Nat­u­rally, if they are in the wrong places they will not work prop­erly. Look­ing at Photo 13, you can see that the lit­tle spring fits per­fectly on that alu­minium com­po­nent. The lat­ter is known as the oil lock piece and it sits right at the bot­tom of the fork case, its func­tion be­ing to help to stop oil leaks and to lo­cate the damper rod. So it is easy to con­clude that the lit­tle spring is sup­posed to act as a bump stop be­tween the bot­tom of the fork tube and the oil lock piece, if the fork ever be­came fully com­pressed. The clue though is in the name: this is the re­bound spring and its func­tion is quite the re­v­erse in that it cush­ions the con­tact be­tween the damper pis­ton and the fork tube if the fork were com­pletely fully ex­tended. So it needs to be as­sem­bled as shown in Photo 14. Fi­nally, if you do use my method to as­sem­ble the forks, you need to be aware that the oil lock piece will not fit through the oil seal (Photo 15), so the first op­er­a­tion is to drop it down to the bot­tom of the case. The damper rod seems to fit into it quite re­li­ably later on. At the bot­tom-end of the cases a socket screw car­ries a cop­per washer to stop the oil from es­cap­ing, which you will of course re­place with a new one. You will prob­a­bly get a more re­li­able seal by en­sur­ing that the old washer or one pre­vi­ous to that has been re­moved and dis­posed of rather than added to (Photo 15). Lastly, when you fit the axle holder right at the bot­tom (Photo 16) you will find that it fits per­fectly with no gaps one way round, but the other way round it leaves a gap on one side. As far as I am aware, the gap is cor­rect. I have al­ways un­der­stood that the idea is that the first nut without the gap is tight­ened, then the other one with the gap, and this en­sures that the axle is gripped tightly. Next time we shall look at the caliper and the steer­ing-head bear­ings.

1 Some­thing’s not quite right here...

7 Seals are held in by clips.

5 ...the other, this. Yuck!

2 Forks: worn and leaky!

8 Pry bar re­moves them.

9 Some ex­ter­nal scratches were a pain.

6 The tubes’ chrome layer has gone.

3 The mud­guard was catch­ing.

4 One fork con­tained this...

16 Mark says a gap here should be cor­rect...

14 But this is how it should fit.

13 Spring fits per­fectly in here.

15 Drop oil lock to the bot­tom of the case.

11 Special socket fits the seals.

10 Dif­fer­ent fork seals.

12 The seal doesn’t seem deep enough!

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.