Author Topic: D/Fly Cam gears  (Read 2290 times)

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Offline norman

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D/Fly Cam gears
« on: 28 Nov 2010 at 03:42 »
Are there any new theories as to why Dragonfly cam gears shed their teeth, mine used to do this regularly in the early days, (1960s) but has just started the annoying habit again. Has anyone ever tried fitting a timing chain?

Offline Chris

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Re: D/Fly Cam gears
« Reply #1 on: 28 Nov 2010 at 07:23 »
Hi Norman
      In 35 years of Dragonfly ownership I have never heard of this problem. The only gear I have experienced having extreme wear is the idler pinion which being smaller than the cam gears and being in contact with three other gears is only to be expected. The only reasons I can attribute to teeth shedding of the cam gears are bad wear of the cam shaft bushes permitting angular loading of the straight cut teeth of the gears and/or faulty heat treatment of the gears themselves. I have never seen a timing chain conversion but knowing some people's inventiveness I would be surprised if it has not been attempted at some time. Chris.

Offline eddie

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Re: D/Fly Cam gears
« Reply #2 on: 28 Nov 2010 at 08:07 »
Hi Norman,
                  My theory on this problem is that the gears are being overoiled, and that the oil trapped between the teeth strains the teeth to the point that they eventually break.
       This may sound a bit far fetched, but take a careful look at the situation. On the Mark series engines, the same gears (but 1/8" narrower) gave good service. The increase in width of the Dragonfly gears should have improved their performance - but the reverse is the case. Also, on the Dragonfly you will find there is a large cast in drain port to let the excess oil return to the crankcase - but there is only one - the oil the other side of the tunnel for the oil filter is trapped to a depth that can cause the overoiling.
       I bought my Dragonfly in 1973 (in bits) and put it on the road in 1976. Just like a lot of other 'Flys, it shed it's teeth about every 5,000 miles (usually from the cam idler). At the time of the bikes second rebuild (1996), I decided to improve the oil drainage from the timing chest (having discussed the problem with other LDMCC members). This modification just requires 2 holes 5/16" dia to be drilled in the back wall of the timing chest - one each side of the tunnel for the oil filter, at a level which leaves the teeth of the cam gears just skimming the surface of the retained oil. Since doing this mod, my Dragonfly engine has covered another 30,000+ miles without any further problems. (I had the engine apart earlier this year and the gears still look good enough for another 30,000).
      As I say it may sound a bit far fetched, but hydraulics can be a complicated subject, and where speed is involved, liquids can put up almost as much resistance as solids. Take even a thin liquid such as water as an example - absolutely harmless when applied gently - but put that same water in a swimming pool and do a belly-flop from the diving board - its resistance to moving can, and often does, cause serious injury. Obviously, speed is the factor which governs the liquid's resistance to change shape, so the harder you rev your engine, the greater the problem becomes - those who cruise around gently don't seem to suffer this problem.
      This thinking may be a bit too lateral for some, but it has worked when applied to my Dragonfly.
      Regarding the conversion to a timing chain - there was a Dragonfly at this year's Annual Rally that was reported to have such a conversion, but I don't have any further information as to how it was achieved or to the longevity of it's operation. It certainly didn't sound much different to the standard setup when it was running. From experience of the post war engines, a chain conversion on the Mark series would be relatively simple (because the mag drive is outboard of the cam drive, the chain would only have to connect the crank to the 2 camshafts), but on the Dragonfly, the chain would have to be looped back to obtain enough contact with the crankshaft sprocket and also drive the distributor.
  
                                   Regards,
                                              Eddie.

Offline Bynorthsea

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Re: D/Fly Cam gears
« Reply #3 on: 13 Oct 2018 at 10:19 »
Hi Eddie just came across this post, as I am just about to rebuild it could be a useful mod. Reading your description  I have marked up the position on one side, is this as you suggest? There is a web either side of the filter housing the hole would emerge directly underneath. There is a lot more space on the outer edge could they be placed there?

Offline eddie

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Re: D/Fly Cam gears
« Reply #4 on: 15 Oct 2018 at 07:58 »
Hi,
     The height of the hole needs to be such that it retains just enough oil for the teeth on the cam wheel to just dip into it (by about the depth of the tooth). you need to be careful how you drill it, as it needs to emerge on the other side just below the divider between the crankcase proper and the sump. On my crankcases I drilled a second hole to drain oil from the other camwheel (in my opinion, the cast in hole is high enough to retain too much oil on that side also). The holes I drilled were 5/16" dia.

Regards,
               Eddie.

P.S.  Having had a second look at the photo you posted, it looks as if your engine might benefit from having a shim inserted between the rear main bearing and the clutch release - it looks as if all the clutch thrust has been taken by the oil retaining plate bearing on the face of the front main bearing. I normally assemble the crank and cases so that there is about 5-8 thou clearance between the front of the crank and the rear face of the front main bearing, then insert a shim behind the rear main bearing so that the clutch thrust is taken directly against the clutch release. This set up allows the crank to spin more freely and running temperature doesn't affect the clutch adjustment.
« Last Edit: 15 Oct 2018 at 08:11 by eddie »

 

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