Author Topic: Stroking a Mark or Dragonfly Crankshaft  (Read 1460 times)

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Offline Paul Taliesin

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Stroking a Mark or Dragonfly Crankshaft
« on: 19 Nov 2017 at 21:07 »
I have a Dragonfly project and looking to make it into a special. At most you can rebore the cylinders up to about 62.5mm at best. This gives 368cc
Ideally I would like to increase capacity by stroking the crank my current crank needs a rebuild and have spoken to Alpha Bearings about rebuilding it.
Have done some work on 4 stroke singles in past and came across eccentric crank-pins which allowed an increase in stroke by 3-4 mm by having a special eccentric crankpin made up. How strong are the standard cranks I see the Dragonfly has stepped crankpins so should be a strongish crank to take the greater stress of the mods. There is enough room inside the crankcases to go to around 66mm stroke.
I know this route maybe a bit costly


Offline eddie

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #1 on: 20 Nov 2017 at 07:24 »
Paul,
        The weak point in the Dragonfly crank is the centre web. It is drilled right through (about " dia) for oil. Even a thou or so increase on the interference fit will crack the web across the oil hole. When I rebuilt my crank, I went back to the earlier Mark type oiling (because the centre web is stronger and it distributes the oil more evenly between the 2 bigends). With the Dragonfly oiling being direct through the pin, an excess of end float on the front rod allows most of the oil to escape, thus starving the rear bigend (Eddie Withers found this out in the early 50's on the Plus models - often engines wouldn't last a whole race!). Also eccentric pins are not a good idea unless they are a taper fit in the webs and nutted (along with a key) to prevent them creeping round.
  Be careful who you get to recon your crank - even the work of some of the better known names leaves a lot to be desired - I have photographic proof!

Regards,
              Eddie.

P.S. It might be a good idea to just build the bike in it's standard form to find out it's other weaknesses - in my opinion, it is almost the perfectly designed bike - the engine nearly makes it accelerate and the brakes nearly slow it down again!
« Last Edit: 20 Nov 2017 at 07:32 by eddie »

Offline Paul Taliesin

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #2 on: 22 Nov 2017 at 16:31 »
Thanks Eddie

The Eccentric crank pins we used previously were always welded into place to ensure the crank pin does not creep.
It maybe an ideas for me to get a new centre web made up out of EN36 steel possibly a full circle for greater flywheel effect and strength. I have a good friend who works at an engineering machine shop in the local University who maybe persuaded to do the relevant machining.
On that note Eddie a few more CC will certainly help with get up and go.
Have you had an experience with the Sports Cams used on Mark IV models is this worth tracking down for use in my Dragonfly?

Offline eddie

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #3 on: 22 Nov 2017 at 17:04 »
Paul,
        I would suggest that you need a higher tensile steel than EN36 for the centre web. EN36 is a case hardening alloy steel that is not particularly tough in it's natural state. Welding in the crankpins will probably make the crank un-rebuildable in the future. Also, I have known standard cranks with concentric pins to twist even though they had been welded.
 With regard to the Sports cams - many years ago, I checked out the cam form on both sports and standard cams - they are both the same except that the sports cam has a base circle that is about 50 thou smaller. This gives 25 thou more lift as well as a longer opening. The extra 25 thou lift is, of course, further multiplied by the arms of the rockers being of different lengths.

Regards,
             Eddie.

Offline Paul Taliesin

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #4 on: 22 Nov 2017 at 21:18 »
Eddie,
         EN36B seems to be widely used on a lot of pressed up crankshafts even for racing application such as G50, Manx etc why would I need to go to EN40 for the nitriding hardening? 
Surely EN 36 with the proper hardening treatment it will be strong enough for the centre web.

On that note any idea what grade the original Douglas Dragonfly web is made of?

Also any idea what the rod lengths are centre to centre they seem to be around 120-125mm do you know if Dick in spares stocks replacements?
« Last Edit: 22 Nov 2017 at 21:29 by Paul Taliesin »

Offline Doug

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #5 on: 23 Nov 2017 at 00:57 »
Case hardening steels are by their nature, a compromise. There are stronger steels available better able to resist stretch, which is the limiting factor in the postwar crank for gripping the crank pin. Even the factory did not trust their cranks for more than one pressing. Factory reconditioned units were honed +0.004" to make the hole nice and circular again and oversized crank pins made. They were then etched +004.

Now I do not know if the postwar crank webs and throws were case hardened; never checked. I do know the prewar one were not particularly hard, certainly not over and above the core hardness heat treatment, so at that time they were not too worried about the side of the conrod eye boring into the face of the web. This is with the built up cranks, which generally had a separate crank pin race. Where the crank was in one piece, like the DT, the face of the center web was case hardened at the same time the crank pin surface was. The opposing surface on the removable counterweight was not case hardened. Mind you, if the cage (where used) was skewed, it bored into the center web no matter what. Never the easily replaceable counterweight, oh not never!

So if case hardening is not needed, one of the through-hardening steels would give you a much greater tensile strength. A G50 and a Manx bottom end has a lot more metal around the crank pin to resist stretch. I am not sure why they would use a case hardening steel for that as don't they have removable hardened thrust washers anyway? It might be because that material it is readily available in the large disks needed. Sometime is it just marketing. "Made out of EN36", which any vintage motorcyclist is likely to have heard of at one time or another. They don't use the BS 970; 1983 designation, do they?

The only thing I can think of that having a high surface hardness will aid (beside resisting side wear from the conrod) is resisting galling when pressing up. And surface finish has a large part to play in that too. If so, they you only need a very thin surface hardness. In that situation, nitriding is an advantage. You get a hard surface, no distortion, and no alteration to the core properties. And you can use it on through hardening steels that you cannot otherwise case hardened, like SAE 4340 (EN24). The core strength of 4340 is way above EN36. And if 4340 is not strong enough you can alway use 300m (good luck affording it!) 4340 and nitriding is being used here for a lot of custom and short production run gears, supplanting the more traditional use of 8620 or 9310 with case hardening. Though, if you need a deep case for maximum wear or dynamic surface rigidity (heavy tooth contact loading or for bearing rollers), then a through hardening steel with a superficial skin hardness from nitriding is not sufficient. But a press up joint is a static load.

Just some food for thought.

-Doug


Offline Paul Taliesin

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Re: Stroking a Mark or Dragonfly Crankshaft
« Reply #6 on: 23 Nov 2017 at 15:34 »
Excellent info Doug all add up to the bigger picture. I think it may end up being a new crankshaft in the end!!
Here is what I was thinking of for the new centre web similar to some BMW there's more meat on the web to take the loads more oval in shape than the original Douglas Web.

 

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