Author Topic: Piston for mark3  (Read 2034 times)

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Offline Edin

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Piston for mark3
« on: 13 Dec 2014 at 19:05 »
Hej all
I am restoring/rebuilding a mark3 1947 - from bits. I need help and advise about new piston for the engine.
The cylinderdiameter is 60,8mm + a little wear.
In my store I have a pair of piston,- HEPLEX - 60,8+0,030" for Dragonfly - (brand-new) mark inside (11854 - 413 - 616820)
1. can these piston be used on mark3 ?
2. If yes,- what bore diameter should be used for this type of piston.
In foreward thank you,- and merry christmas to all members of the douglas net.
Regards Edin DK

Offline eddie

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Re: Piston for mark3
« Reply #1 on: 14 Dec 2014 at 08:26 »
Hi Edin,
            If your pistons are the split skirt type for the Dragonfly, they will fit a Mark engine but most owners have found them to be inferior to the original Mark pistons. When fitted to the Dragonfly and worked hard, the crowns have been known to start collapsing, resulting in the ring grooves taking on a wavy form and trapping the rings - with the result that the oil consumption becomes excessive. A lot of Dragonfly owners have reverted to the earlier Mark pistons to effect a cure.
  On postwar Douglas engines, the quoted bore size is the actual size of the bore - the clearance is allowed for on the piston - so standard size is 60.8mm (2.394") - so +.030" would be 61.55mm (2.424").
  Hope this is of some help,

  Regards,
                Eddie.

  P.S.    Have a good Christmas - but don't enjoy it too much!!!!

Offline Edin

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Re: Piston for mark3
« Reply #2 on: 14 Dec 2014 at 09:58 »
Hej Eddie
Thanks for reply,- just what I needed
Regards Edin DK