Big end, endfloat, Mk crank. Rumble, rumble, rumble........

[Alex Hall]

Good morning  all,

After begining to hear a light rumble in the Mk 3 Sport, I decided to open up the engine.

It was a sad state of affairs to be honest, with many things being buggered with in the past.
Namely, the top 2 lugs on the left side barrel were broken at some  time in their life, before being brazed back on, one of which is currently loose. Yes, there is distortion in the bore but not  enough to be impinging the 40 thou piston too much.
Secondly, one of the threads in the fly wheel for its removal was no more.....In careful desperation I bored 2 8mm holes in the flat perimiter and attached a puller to that, which worked. I shall thread two 8mm grub screws in to these holes when the fly wheel is in use to restore some of the removed mass.
Thirdly, the timing gear nut was literally hammered on!, it was very much askew, with hardly any thread left on the crank and wa sheld in place with thread lock and a peened over crank! Only yesterday a dear friend of mine gently built up the threaded portion with weld and have since ground it down to 9/16 and threaded it to 9/16 unf.
Crank ran trueish in the lathe, and a couple of taps of a heavy mallet brought the mains closer to true.
The bronze bush behind the nut was visibly turning in the housing, but fortunately, the wear was not bad enough that some bearing lock will not attend to, naturally the steel locating pin in the crank case was worn. I will attempt to remove and replace this today.

However the cause of the rumble was not visible, I and other more experienced folk could not feel or see any wear in the big ends. The only discrepancy that was found was that the left conrod could be inclined more on the journal and there was substantially more endfloat clearance than on the right conrod big end. Could this be it? Can the endfloat be closed up at all? What should the endfloat be?

Certainly looking forward to hearing your thoughts chaps, and feel free to set me straight on any repairs I made which are incorrect.

Cheers,
Al

[eddie]

Al,
     Firstly, it is very risky to try to re-align a Douglas crank without first removing the plugs in the crankpins. With these removed, you should then be able to insert test bars into the crankpins and they should align with the corresponding holes in the other crank web. Without using this method, it is possible to get the main shafts to line up but the whole assembly may still be out of balance. Let me explain! If you look at the front of the crank (with the centre web horizontal), the front web could be rotated slightly in, say, a clockwise direction - resulting in the mainshaft being high but only displaced a thou or so sideways. If the rear web  is similarly out of place in an anticlockwise direction, the whole assembly will appear to be running fairly true, but will be out of balance due to the centre web being offset from the centre of rotation.
    Loose front main bearing bushes are not unheard of, but are unusual in machines up to and including the Mark 3 Sports. strangely, the resultant wear seems to take place on the outside of the bush rather than in the bore of the crankcase, so replacement of the bush should not cause too many problems. If you have a new bush made, do not attempt to reduce the running clearance to obtain a quieter motor - the clearance should be 3 thou, otherwise you risk the chance of it nipping up on the crank and turning in the cases (resulting in another strip down).
    Moving on to the endfloat on the conrods - originally it would have been in the region of 5 - 10 thou, although this is not critical on the Mark series. Providing the crank has not been modified during it's life, it will have plain parallel pins, so it is possible to press it up a little more to restore the correct endfloat (only do this if you are confident of your abilities!).
    Now down to the 'rumble'! It could have been due to the bush being loose in the cases, or more likely, due to the timing pinion not being pinched by the butchered nut on the crankshaft. If the pinion was able to move, the key would be likely to get hammered, resulting in the knock/rumble increasing quite rapidly.
    Lastly, the broken lugs on the barrels. With the cylinders sticking out, any 'coming together' with the road or a roadside obstruction can cause damage to the heads and barrels, but what is not so obvious, is the damage it does to the crankcase. Whilst you have the engine stripped down, I would reccommend bolting the crankcases together and getting an engineering machine shop to check the surface for the barrels is still flat - if not, get them to skim it to a minimum cleanup. This is the only option - just replacing the barrels will only result in more damaged barrels.
   Hope this helps sort out some of the problems - get back to me if you need more advice.

     Regards,
                 Eddie.

[Alex Hall]

Once again Sir,

Many thanks for the insightful reply. The delay in writing this is due to burning the midnight oil.

As suggested I avoided tweaking the crank other than closing up the gap. In a lathe it ran slightly out of true but nothing too major. Checked the side faces of the crank case and they re quite square thankfully, this was also the ideal opportunity to fit new BSF studs

Re brazed the lugs and faced the barrel base. Shimmed the crank end float to about 4 thou. Crank case is now in frame and currently refitting the pistons, barrels and heads. Hope to have some life in the bike this evening or tomorrow morning.

However in the reassembly came across some other evils in the bikes history............... looks like the idler gear spigot housing has been re welded back to the casing at some time in the distant past. There are the marks where the gear dug into the housing as it tried to turn and twist. My impression is that the magneto must have siezed and upon kick starting...hell broke loose.

Eddie, in the long term I will consider re stripping the engine when I rebuild the bike, to spray the frame etc. any suggestions who to send it to for a proper job?

Its been a long drawn out process getting XVS 123 ship shape again, but we re reaching the point of cosmetics only. Which is something I can live with.

Once again Eddie and all the folk who posted their ordeals on the forum, many thanks.
Al