Author Topic: Dragonfly cylinder heads  (Read 401 times)

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Offline cbranni

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Dragonfly cylinder heads
« on: 11 Aug 2019 at 20:13 »
Hello to all,
Both heads on my recently bought 1957 Dragonfly have cracks in, so I am looking to buy a pair heads in good condition, if anyone has any for sale please reply.
Many thanks
Colin


Offline eddie

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Re: Dragonfly cylinder heads
« Reply #1 on: 12 Aug 2019 at 06:24 »
Hi Colin,
              If your bike still runs OK, don't get over concerned about the cracks. My Dragonfly has a crack in the LH cylinder head (from the plug hole to the exhaust port) - it has been like it for at least the last 20,000 miles without getting much worse. In my opinion, the Dragonfly heads suffer from cracking due to a hot spot being generated around the spark plug due to the shielding from the through studs retaining the heads. Also, bear in mind that we now run on lead free fuel that generates more heat than was originally expected - to help keep the heat down, it pays to go up about 10 on the mainjet size.

  Regards,
                 Eddie.

Offline cbranni

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Re: Dragonfly cylinder heads
« Reply #2 on: 12 Aug 2019 at 07:23 »
Thanks for the informative reply Eddie, the bike was a non-runner when I bought it so I stripped and rebuilt the engine/box, I had a mate weld the heads and I machined them back, I feel happier now knowing yours have run with cracked heads but would like to a pair of un-cracked heads just incase.

Kind regards Colin

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Offline cbranni

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Re: Dragonfly cylinder heads
« Reply #3 on: 04 Sep 2019 at 07:53 »
Just to bounce my advert back the top of the list and jog some people's memories, I am still looking for a pair of heads but I have welded and machined back the cracks as much as I dare, the engine is now assembled and fitted in frame.

Kind regards Colin
« Last Edit: 04 Sep 2019 at 07:59 by cbranni »

Offline cbranni

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Re: Dragonfly cylinder heads
« Reply #4 on: 18 Sep 2019 at 07:49 »
Just an update on my search for cylinder heads, the owners club have found a pair for me, thanks to all who helped.

Colin

Offline eddie

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Re: Dragonfly cylinder heads
« Reply #5 on: 18 Sep 2019 at 09:07 »
As an update on repairing cast iron cylinder heads - I only ever found one company (sadly, no longer in existence) that could do a reliable repair. They would 'V' out any cracks, then bring the cylinder head up to red heat, and use a special oxy-acetylene torch with a powder feed attachment to fill in the V'd out cracks. The cylinder heads would then be left to cool down slowly, with the oven, over a period of about 48 hours. The machined faces would then be cleaned up, rendering the whole repair invisible. Back then (about 40 years ago), the process was expensive (about 100 per head) but we never suffered any more cracked heads (that was on Foden 2 stroke diesels that had a worse reputation for cracking than the Dragonfly heads).
   I am of the opinion that most castings are pre-stressed because the surface solidifies first and the core then tries to shrink as it cools, but can't. It is these areas of stress that fail first, especially with repeated heating and cooling.
   It would be an interesting experiment to stress relieve a new Dragonfly head by bringing it up to red heat and allowing it to cool very slowly - then see if that still suffered the dreaded cracks!

   Regards,
                   Eddie.