Author Topic: 2nd gear on Mk 4  (Read 7347 times)

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Offline Alex Hall

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2nd gear on Mk 4
« on: 30 Jul 2008 at 01:18 »
Good evening all. Having difficulties with second gear. It engages well but when applying load or increasing throttle it seems to jump out and judders away, only to re engage when I ease up on the throttle. Tried the bike a couple of times and have covered some 50 miles, so all working very well and enjoying the ride. Dog teeth? Detente spring? Or worn selector fork? Would these bits be visibly worn or damaged  if I open up side cover? Repairable or replacing? Cheers from Malta

Offline Doug

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Re: 2nd gear on Mk 4
« Reply #1 on: 30 Jul 2008 at 02:19 »
Alex,

Uh-oh. Pretty nasty stuff. My Mark had the same problem initially. It was a combination of a worn sector fork not shifting second gear all the way into mesh, and worn dogs (probably acerbated by the proceeding) on the gear but primarily on the layshaft, jumping in and out of mesh.

I welded up the dogs on the gear with Stellite, but got a new old stock layshaft from club spares (the last one they had.) The shifter fork had most of the wear on the peg (noticeable flats) that rode in the sector cam, though the slots in the cam hardly had any wear. I tried welding these up and re-machining, but difficulty holding them in the lathe to turn; ended up with an accident and the fork coming adrift and getting broken. I got a replacement good used fork somewhere (probably an advert in the NCR or maybe from Graeme Brown), but can not recall exactly where now (it was twenty years ago.) I think now were I to do it again I would not try to turn the peg round in the lathe, but set it up in the milling machine in a vice where I could grab on to it better, then use a rotary table to spin the part and an a mill a new cylindrical surface. I think the reason I did not try to weld the dogs on the layshaft was that there was quite a lot of wear on the shaft where the gears revolved so it was not worth the trouble. I did make a drawing of the layshaft in case I ever needed to make another.

-Doug

« Last Edit: 30 Jul 2008 at 13:18 by Doug »

Offline Alex Hall

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Re: 2nd gear on Mk 4
« Reply #2 on: 30 Jul 2008 at 09:50 »
Doug, Thanks for that gut wrenching news!! gulp. At least it is repairable. Did the stellite hold up? Any chance of emailing the sketch you made of the layshaft? I understand the peg sticking out of the selector fork which engages the quadrant plate, this peg has to be perfectly round? As I understand it, it has to be a good close fit in the plate.  Cheers, Al

Offline Chris

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Re: 2nd gear on Mk 4
« Reply #3 on: 30 Jul 2008 at 11:59 »
Hi Alex
   The pegs on the selector forks are a definite weakness in the design of the postwar gearbox which in many respects is built like a battleship being at least twice the size of most motorcycle gearboxes. There have been several proposals in the past for repairing these which always wear with two flats permitting more and more free play of the forks and allowing less than full engagement of the dogs on the gears. One solution was written up in The LDMCC New Conrod magazine some years ago proposing removal of the peg following a drill jig having been made and used to ensure that a hole is left in the fork body that is central to the original peg. A new peg is then machined and silver soldered into the hole. I was not happy with this proposal as the thickness of the body of the fork is reduced at this point and there does not appear to be sufficient thickness there to ensure a strong joint with the risk of the peg breaking off. Another common solution has been that followed by Doug. i.e. building up the pegs by welding with Stellite or other suitable material. This will work but care is certainly needed in holding and centralising the peg for subsequent machining.
   The solution I have adopted with success and not too much difficulty still involves setting up the fork in a four jaw chuck to machine the peg (with suitable packing pieces) but permits easier centralisation as it done with the original worn peg which still retains its original diameter over a good percentage of its circumference. When the peg is running true about 1mm depth can be machined from the peg which in the majority of cases will leave the peg circular with the cut just touching the bottom of the flats.
     Using 5/16" diameter precision ground silver steel, a sleeve can then be machined to be a good fit on the peg. This can be soft soldered or even fitted with loctite to restore the peg to its original diameter. If there is wear in the cam tracks, you can use 8mm silver steel which being a few thou over 5/16" gives scope for making the pegs an even better fit.
     Another problem with these gearboxes is that in some cases it is very difficult, due to varying tolerances in their construction, to achieve equal depth of dog engagement in all gear positions. A tip I was given by an experienced Douglas enthusiast is to make up dummy bearings in brass or aluminium. These should be an easy slide fit in the housings and on the shafts so that the gear assemblies can be readily assembled and disassembled. With the selector cam plate engaging the pegs, (disengaged from the cover plate), the depth of engagement of the dogs in each gear position can then be observed and steps taken with shims or different thickness spacers on the main or lay shaft to rectify any unacceptable discrepancies.  A slightly negative angle ground on the faces of well worn and radiused dogs will also help to avoid dogs disengaging themselves or trying to push apart when selected.
Chris.

Offline Alex Hall

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Re: 2nd gear on Mk 4
« Reply #4 on: 30 Jul 2008 at 15:38 »
Chris, thank you very much for that. At least the bits are repairable, fiddly, but can be done. Can you please clarify what you mean by silver steel. Sounds like I ll have to start by making a jig to mount the fork first! Thanks

Offline Doug

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Re: 2nd gear on Mk 4
« Reply #5 on: 30 Jul 2008 at 15:57 »
Silver Steel, also known as Drill Rod, is a through hardening tool steel. Available in oil, water, or air hardening, depending on the properties desired (wear resistance, hot hardness, shock resistance, distortion, etc.) To achieve these properties the material needs to be heated to a transition temperature (when the part ceases to be magnetic is a good indicator) and quenched, followed by a thermal temper.

It is also popular because it is readily available centerless ground to a fine surface finish and a tight diametral tolerance. It machines poorly compared to mild steel.

It is a through hardening steel, so should never be used in application where great strength or shock resistance is needed as it tend to be too brittle for these applications. An often quoted example of parts not to make from Tool Steel are fork spindles and wheel axles. Part requiring a hard surface and a tough core should be case-hardened.

As a sleeve on the shift fork post, it should be fine. It will probably not achieve anything like full hardness if silver brazed on, unless it is of the air hardening variety. Of course then the rest of the fork will have been annealed so room temperature curing Loctite is probably the best solution.

If you chose welding, add a little material at a time to avoid overheating the rest of the fork and drawing the hardness out of fork surfaces as they are originally case-hardened where they contact the gears. I used a small TIG torch to minimize the heat affected zone.

-Doug

P.S. I will scan that drawing and email tonight.

Offline Chris

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Re: 2nd gear on Mk 4
« Reply #6 on: 30 Jul 2008 at 16:46 »
Hi Alex
   I was just about to respond when Doug beat me to it. Silver Steel is a high carbon tool steel to BS 1407 and typically contains 1% carbon, 0.3% silicon, 0.4% chromium and 0.35% Manganese. It is usually available from any good tool shop in 12" lengths and is not expensive. As Doug says, it is fully heat treatable (770 to 790 degrees Celsius and quench in well agitated water or 10% brine to harden followed by lengthy low temperature tempering). However, for the application I have described it should not be heat treated as the heat treatment of the fork would be damaged. Soft soldering should not affect the heat treatment as the melting temperature of soft solder is well below the transition temperatures of the silver steel or the fork. Loctite is certainly the easier solution.
Chris.

Offline eddie

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Re: 2nd gear on Mk 4
« Reply #7 on: 01 Aug 2008 at 12:16 »
Alex,
        In your original posting, you say that second gear engages well. If this is the case, check out the shock absorber on the gearbox sprocket - it may be riding over the top of the lobes - make sure the centre nut is fully tightened against the end plate and spacer. If it still rides over the top, a thin washer against the end of the sliding cam will restrict the travel, and effect a temporary/permanent cure.
                               Regards,
                                       Eddie

Offline Alex Hall

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Re: 2nd gear on Mk 4
« Reply #8 on: 04 Aug 2008 at 19:33 »
Hi Eddie,
Thanks for that will have a look and report back. Hoping for the best! Cheers

Offline eddie

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Re: 2nd gear on Mk 4
« Reply #9 on: 23 Dec 2015 at 15:50 »
Alex,
        If you have your gearbox in pieces, check that the screws in the figure '8' plate that retains the bearings are tight. With these screws loose, the shafts can creep forward, making the engagement of 2nd gear a bit shallow. At this point, it is a good idea to do a dummy build just to check that all gears engage by approximately the same amount. If 2nd is a bit shallow but 3rd is deeper, there may be some advantage in grinding a small amount from the layshaft where it abuts the rear bearing - this will move the layshaft back and increase the mesh on 2nd. When finally rebuilding the 'box, shim up the front of the layshaft so that the front bearing takes the thrust rather than relying on the figure '8' plate.  Similarly, if top gear is a bit shallow, a shim behind the sleeve gear bearing will give more engagement - unfortunately, this requires complete dismantling of the sleeve gear assembly to insert the shim at the back of the bearing housing (and a similar thickness shim will be required behind the square bearing retaining plate, as the outer sleeve of the bearing will now protrude a little).
 Unless the selector pins are very worn, they don't have much effect on the engagement of the gears - there is usually a narrow unworn area between the 2 worn flats, and this is the bit that registers in the camplate when a gear is selected - the worn areas only contact the camplate during changes from one gear to another. If you do find it necessary to have the pins welded up, make sure they are allowed to cool naturally - quenching will result in them being glass hard and snapping off in use.
  Hope this helps,
      Regards,
                   Eddie.