Author Topic: Douglas Dragonfly excessive oil pressure !!!  (Read 2405 times)

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Offline cahallett

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Douglas Dragonfly excessive oil pressure !!!
« on: 16 Aug 2012 at 09:42 »
I am getting pressure in the crankcase following a rebore and new pistons.
There is no smoke from the exhausts but removing the oil filler with the engine running produces enough pressure to blow oil droplets 2 feet into the air.
If the oil filler bung is in place then the pressure comes out of the grommet protecting the wires from the alternator.
Also the engine is very noisey mechanically with the noise appearing to be coming from the camshafts. New followers & guides were fitted. The valve clearance is .003" as the book.
Engine fires first kick and idles nicely with the exhaust burble drowned by this noise.
any ideas please?????

Many thanks
Chris

Offline eddie

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Re: Douglas Dragonfly excessive oil pressure !!!
« Reply #1 on: 16 Aug 2012 at 11:46 »
Chris,
        Oil droplets being pumped out of the open filler spout is to be expected. Both pistons rise and fall together, so produce a pumping action within the crankcase - this is used to good effect to send oil spray up the pushrod tunnels to lubricate the valve gear. If your new pistons have had very little use, the rings may not yet be fully bedded in - resulting in excess crankcase pressure. With the filler cap replaced and the engine ticking over, the engine should breathe through the breather port in the bottom front of the timing cover. If there is no breathing at the port, then it may be blocked, causing the excess pressure to exit via the generator grommet.
     Regarding the 'noisy' engine - postwar Douglas engines are not known for their mechanical quietness, and is probably the reason Mr Douglas decided to reduce the tappet clearance to .003" for the Dragonfly - with the alloy pushrods, it should have had larger clearances than the earlier Mark series engines (I have always run my 'Fly with .006" clearance, and have adopted the theory that 'it is the name on the tank that makes it rattle!!!'). Having said that, if your engine is excessively noisy, it may indicate the onset of problems with the timing gears (Dragonflies do have an appetite for timing gears). If you do have problems with the timing gears, whilst you have the cover off, check that the alternator rotor is not loose - the nut may be tight but the rotor is built on a hexagonal centre and the alloy body of the rotor often works loose on the hexagon. A larger diameter spacer between the rotor and the crankshaft timing gear, along with a large diameter washer behind the retaining nut will trap the actual body rather than just the centre boss. A loose rotor usually manifests itself as a deep rumble from the front of the engine - most noticeable just above tickover.
  Hope some of this helps,
      Regards,
                  Eddie.

Offline cahallett

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Re: Douglas Dragonfly excessive oil pressure !!!
« Reply #2 on: 20 Aug 2012 at 12:15 »
Thanks for the reply Eddie,
It does breath at the front so perhaps it is just the rebored cylinders, new pistons and rings.
Still unbelieveably noisy though so there is something a miss inside the engine.
Best Regards
Chris