Author Topic: 90+ oil pumps and timed breather  (Read 661 times)

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Offline roger h

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90+ oil pumps and timed breather
« on: 23 Apr 2018 at 10:33 »
I've been reading about the developments in the engines for the 90+ model.
There seems to have been a couple of changes in the oil pump delivery rate, the fitting of "aluminium blocks" to prevent oil starvation to the big ends when cornering? - plus and there is talk of a "timed crankcase breather" which was worth an additional 2 bhp when fitted.
Does anyone have details of these changes, or are they all Dixon/Withers secrets?   - which were the various oil pumps? - where and how were they fitted? - what do the aluminium blocks look like? - what does the timed breather look like? - are there any drawings, where was it fitted and how was it timed??
Thanks,
Roger

Offline eddie

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Re: 90+ oil pumps and timed breather
« Reply #1 on: 23 Apr 2018 at 12:49 »
Hi Roger,
              I am of the opinion that oil delivery is not the problem with 'Plus' cranks - the main problem is where it goes once it has reached the crank. Back in the late 20's when Douglas produced the Dirt Track models, they recognised that the oil needed to reach the centre of the crank before it was divided to supply the 2 big ends. They continued with this oil circuitry up to the advent of the 'Plus' machines - even the generator engines and the Mark series cranks fed the oil to the centre web before splitting it between the 2 big ends which were fed from one side (not ideal!). On 'Plus' and Dragonfly engines, the crankpins were larger in diameter and drilled for direct oiling - this provided better oiling to the front big end but tended to starve the rear big end if there was too much endfloat on the front conrod, thus allowing the majority of the oil to be flung out of the front big end under centrifugal force. Eddy Withers recognised this problem and produced a more complicated oiling diagram that split the oil in the centre web but still fed it through holes in the crankpins (I'll see if I can find the Eddy Withers drawing).
  Those riders who raced the Pluses in the 50's found that in the longer races (TT, etc.) the engines seldom lasted till the end of the race, and it was often down to big end failure or piston seizures.
  In the case of the Dragonfly cranks, I have rebuilt about 30 cranks using INA needle roller bearings, and haven't heard of anyone suffering further problems. In the case of my own Dragonfly, I reverted to the older Mark type oiling (side feed and oil split in the centre web) - my crank has now done about 30,000 miles and at the last check, was as good as the day I first fitted it.
  With regard to higher output pumps - George Easton told me that when he was developing his 500cc Mark based racer, he fitted a 'Plus' oil pump but had to revert to an ordinary Mark pump as the engine was overoiling and the rings couldn't cope!

  Hope some of this helps,
    Regards,
                  Eddie.

Offline eddie

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Re: 90+ oil pumps and timed breather
« Reply #2 on: 23 Apr 2018 at 16:57 »
Hi Roger,
              As promised, attached are the working drawing for the Eddy Withers crankpins and the scanned image of the Eddy Withers crank with the modified oiling and crankpin plugs - apologies for the poor quality of crank drawing, it was scanned from a very well thumbed original!

  Hope they are of some help,
     Regards,
                   Eddie.

Offline roger h

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Re: 90+ oil pumps and timed breather
« Reply #3 on: 24 Apr 2018 at 09:17 »
Thanks very much for this Eddie, very good and prompt reply!
Do you have any info on the timed breather please, or could you point me at someone else?
Thanks again,
Roger

Offline eddie

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Re: 90+ oil pumps and timed breather
« Reply #4 on: 24 Apr 2018 at 10:09 »
I'm sorry Roger, I don't have any detailed info regarding the timed breather - other than it was a disc with 2 windows that ran against the back of the revcounter drive adaptor plate, and was driven by a tongue that engaged with a slotted nut that held the magneto gear - an odd arrangement as it's timing would be affected by the amount the nut was tightened. I think I would be inclined to fit studs in the threaded extractor holes and make a drive plate to suit (especially as the rotation would be trying to undo the magneto nut!).
    When you get around to timing the breather, you will probably find that it works best with some bizarre static setting. I once checked out the breather timing on my DBD34 Gold Star, and the breather opened with the piston rising! Evidently, it's all to do with the pressure waves set up in the crankcases and their frequency at racing speeds - at 6,000+rpm the piston is on it's way back up by the time the pressure wave it created on the way down reaches the top of the timing chest (and the breather port).

  Regards,
                Eddie.