Author Topic: D/Fly crankshafts  (Read 2068 times)

0 Members and 1 Guest are viewing this topic.

Offline norman

  • Member
  • *
  • Join Date: Aug 2009
  • Posts: 8
D/Fly crankshafts
« on: 07 Feb 2014 at 06:49 »
I have just rebuilt my third crankshaft but this time the alignment is way out, it has gone what I believe is to use a 'technical term' banana shaped. Any suggestions on how to solve this problem please.

Offline eddie

  • Master Member
  • ****
  • Join Date: Mar 2006
  • Posts: 1655
  • Location: Hampshire, UK
Re: D/Fly crankshafts
« Reply #1 on: 07 Feb 2014 at 08:39 »
Norman,
             If it is impossible to get it to run true, then I fear a total strip down will be needed. Before dismantling, check for any signs of a crack across the centre web oil holes. If the centre web is OK, then check that the front and rear webs are running true by clamping them to a lathe faceplate and 'clock up' the shafts. Bent front webs are not uncommon on Dragonfly cranks, as the extra overhang provides enough leverage to do damage where the shaft meets the web. If OK, then check that metal hasn't been sheared out of the eyes by poor fitting of the crankpins - if it has, the bores will need to be reground and oversize crankpins made. When making new crankpins, adhere to the instructions on the drawing and grind a 10 degree x 15 thou wide chamfer on the ends of the pins - then finish off by giving this chamfer a good polish - this, along with the application of some tallow, will help prevent the pins 'picking up' on reassembly. When reassembling the crank, make sure you keep the webs square to the pins. Finally, check that the webs are parallel to each other by using a gauging block and feelers (I find a spare centre web is good for this).
   If you have to go for a full crank rebuild again, do yourself a favour and fit a Mark type centre web and make the crankpins without the 5/64" holes that feed oil to the bigends. The oil will then be carried by the front crankpin to the centre web where it will be split equally between both bigends. I know side fed bigends are not ideal but they are better than the original Dragonfly set up where most of the oil gets thrown out of the front bigend by centrifugal force.

     Regards,
                  Eddie.

Offline norman

  • Member
  • *
  • Join Date: Aug 2009
  • Posts: 8
Re: D/Fly crankshafts
« Reply #2 on: 08 Feb 2014 at 05:51 »
Eddie

Thanks for the prompt responce, I was afraid that the answer would be to strip and rebuild it. I am doing it for a friend the original had cracked the center web, he had a new one made up before I could tell him to get it made like a mark one with the side feed holes. As you suggest I will clock the separate parts to check for bending, which thinking about it is possible as he was riding with the shaft broken!

Regards

Norm L