Reconditioning Postwar Crankshafts

[eddie]

Back in 1995, I was rebuilding my Dragonfly and had got to the point where I would have to fork out on a rebuilt crankshaft. Being less than excited by the options available in the UK, I decided to rebuild the crank myself - using new crankpins and INA needle roller bearings. The bike has now done in excess of 30,000 miles on the home-built crank with no signs of any problems. I have now rebuilt about 10 cranks by this method and all of them are performing well.
    Attached is a brief description of the method used - hopefully it will help others get round one of problems with the postwar models.
                            Eddie.

Postwar Crankshaft Rebuilding by Eddie in .pdf format HERE. Adobe Reader required.

[trevorp]

Read your article eddie and its amazing that all that skin, swearing,  testing,  building special tools, and all that goes with it, fits one one page of paper
A great piece of info and thanks for sharing it

[aggettd]

Eddie:

This may be a question that falls into the "stupid" category, but, if I understand the instructions correctly, since you don't mention drilling the new crankpins for oil delivery to the bearings, does that mean that you believe the bearings will be sufficiently lubricated by sump splash, or should I assume that I need to drill those too?

Thanks,

David Aggett
Canada

[eddie]

Hi Dave,
             On my rebuilt Postwar cranks, only the front crankpin is drilled to carry the oil from the front web and into the centre web - there are no drillings to feed the oil direct to the big ends. In the write up you will see that I use the Mark type centre web (even on the Dragonfly) to split the oil equally between the 2 big ends. I know that some people think that the side feed is an inferior method of lubricating the big end, but my theory is that it is better than having all the oil forced out of the front big end by centrifugal force (as happens when the end float on the front conrod gets too large), leaving precious little for the rear big end. Eddy Withers experienced the same problem with the works 90+ racers - he resorted to extra drillings in the crankpins and complicated bobbins in the crankpins to send all the oil to the centre web to be split equally and then fed through drillings direct to the big ends.
  The first crank I rebuilt went into my Dragonfly in 1996, and has now done in excess of 35,000 miles and is as good as the day it was fitted. There are about 25 of my cranks being used here in the UK - with no reports of any problems.
  Regards,
               Eddie.

[eddie]

Dave,
          Here is the sketch Eddy Withers produced to show his modification to the oiling on the works Pluses.

[Douglas52]

Hi Eddie
What oil pressure and rpm do these motors run to?
Cheers

[eddie]

Hi,
    I don't think an official figure has ever been given for the oil pressure on postwar Douglases, but from experience, 10psi seems to be about the normal on a warmed up engine. Volume seems to be more important than pressure! With the Dragonflies, even with the larger output pump, the lack of volume seems to be the reason most of the oil escapes from a worn front bigend under the centrifugal force created at high revs, leaving the rear bigend starved. The only plain bearing in the Douglas motor that is pressure fed is the front main bearing, for which Mr Douglas recommended 3 thou clearance (presumably to allow the oil to flow through it, and permit some flexing of the crank). The rest of the bearings are splash fed, so should also have enough clearance to allow the oil to get in and do it's job - even a slightly tight camshaft bush can result in a motor being less willing!
With regard to the revs these engines will run to: The low compression Comp engines had a 5,000 rpm ceiling, Mark engines went to 6,000rpm, and the Plus engines were good for 7,500rpm. Later works Plus motors were further tuned and were reputed to be capable of 11,000rpm - but nobody seems to know for how long!!!
   Regards,
                 Eddie.

[aggettd]

Eddie:

Thanks so much for your detailed response (as usual). I see I have my work cut out for me.

I'd have replied sooner but we just got power back after a 36 hour outage thanks to the remnants of Hurricane Arthur. Credit goes to our generator (Honda, not Douglas ) that the inconvenience wasn't too great.

Cheers,

David A.

[hvirtanen]

Hi,

do you remember, if you once sent one of your reconditioned cranks to Finland?

With my brother we once got one reconditioned crank from UK, but just now I can't remember the name of the man who did the job.
And the original instruction manual for installing it isn't with me here, where I'm writing this message. 

with best,


Hannu Virtanen

[eddie]

Hi Hannu,
               I don't think it is one of my cranks. I certainly haven't sent one overseas. Sorry I cant be of more help.
  Regards,
                Eddie.

[hvirtanen]

Hi Eddie,

after sending the message I already remembered that it wasn't you. It was a quite much earlier story than your reconditioning scheme seems to be. It happened in the beginning of 1970'ies.

The next time when I'll visit the place where that installation manual is nowadays, I'll check it. 

Actually just now I don't even remember, if that crank is nowadays in my Mark V, or if we put it in my brother's engine...

Your cranks look excellent.
Are you willing to do reconditioning works for overseas customers?

with best,

-hv