Author Topic: Inlet valve sizes  (Read 2220 times)

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Offline Pensioner73

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Inlet valve sizes
« on: 10 Aug 2017 at 15:27 »
Can anyone tell me the head size of the inlet valve of all Mark series, touring and sports, and Dragonfly.

Thanks.

Offline douglas1947

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Re: Inlet valve sizes
« Reply #1 on: 11 Aug 2017 at 10:52 »
I have measured an inlet valve MK4 engine 31,45mm dia; ex valve the same dia.

Michael

Offline eddie

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Re: Inlet valve sizes
« Reply #2 on: 11 Aug 2017 at 11:16 »
Mark 3 -5 machines (including the sports) and the Dragonfly used the same valves in both the inlet and exhaust, and as Michael has said, the heads are 31.45mm dia (1").

Regards,
               Eddie.

Offline Pensioner73

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Re: Inlet valve sizes
« Reply #3 on: 11 Aug 2017 at 13:19 »
Many thanks boys. Mildly surprised that the Sport didn't have a bigger valve though there isn't much room for a bigger valve in my Dragonfly head.
What about the Plus 90 valves?

Ian

Offline eddie

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Re: Inlet valve sizes
« Reply #4 on: 11 Aug 2017 at 16:54 »
The only difference on the Sports engine was the lift on the cams. The 'Plus' series engines had valves much the same size but with a different shaped head. They also had different stems, collets and spring caps to suit the triple springs.

  Eddie.

Offline Pensioner73

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Re: Inlet valve sizes
« Reply #5 on: 11 Aug 2017 at 19:29 »
Thanks Eddie. My next question would be, can I fit a Sports camshaft into a Dragonfly as everything seems to be interchangeable?
I think the only diffenrence I can think of is the width of the timing gears, wider in the Dragonfly.

Ian

Offline eddie

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Re: Inlet valve sizes
« Reply #6 on: 11 Aug 2017 at 20:12 »
The Sports cams will fit in any of the post '48 motors - the only difference between the sports cam and the touring cam is the diameter of the base circle - the sports cam being about 30 thou smaller - giving about 15 thou more lift. Oddly, the wider timing gears in the Dragonfly give more problems than the narrower Mark gears. I put that down to them being over-oiled, and generating a hydraulic lock between the wider teeth (which eventually causes tooth breakage). When I first got my Dragonfly, the timing gears would last about 8,000 miles before the cam idler would shed teeth. During the second engine rebuild, I drilled 2 drain holes (5/16"dia) in the front wall of the crankcase so that the cam gear teeth only just dipped into the oil (by the depth of the teeth) - this seems to have been successful as my engine has now done another 35,000 miles without any further shedding of teeth. Also, if you want better, reliable performance, try running with double the tappet clearance. Mark series engines (with steel pushrods) had 5 thou tappet clearance, so it makes sense to run Dragonflies (with alloy pushrods) with a larger clearance - say 6 thou instead of the specified 3 thou. I am convinced Douglas only specified 3 thou because of critical comment regarding the mechanical clatter from the earlier engines. Likewise, with the recommended SAE 140 oil in the gearbox - the gearbox runs fine on straight 40 grade oil.

  Eddie.

Offline Pensioner73

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Re: Inlet valve sizes
« Reply #7 on: 11 Aug 2017 at 21:57 »
Thanks Eddie. I owned a Dragonfly in the early sixties during my apprenticeship and I think I replaced the gear at least three times and sold it for a pittance with the problem and it doesn't seem to be around now. I have seen the posts about over oiling and subsequently drilled the holes in my current Dragonfly. Performance is dismal until about 50mph and I have just found out I have lost about .040" in cam lift so looking to replace. I am informed by a camshaft manufacturer familiar with Douglas cams that the Sport cam would fill in the power band from 2000-4500 revs which would seem to me more useful in todays traffic conditions than peaking with 17bhp at 5000rpm, so I am looking to fit Sport cams in my Dragonfly.

Offline douglas1947

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Re: Inlet valve sizes
« Reply #8 on: 12 Aug 2017 at 16:50 »
Hi,

it is allways interesting what details Eddie can tell!
I like to lern of these special things about Douglas bikes.

I have just looked to a new 90+ valve:
it is nearly the same size (32,5mm dia), but it has a special "tulip" shape. I thing it is sometimes used for "quick" engines, also by other makes.

Michael

Offline Pensioner73

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Re: Inlet valve sizes
« Reply #9 on: 12 Aug 2017 at 19:30 »
Thanks Michael, I think that squashes any thoughts of trying to fit a bigger inlet valve.

Ian

Offline eddie

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Re: Inlet valve sizes
« Reply #10 on: 12 Aug 2017 at 19:57 »
Ian,
       Be careful with trying to fit larger valves to a Dragonfly - they already suffer from cracks between the spark plug and exhaust valve seat. The heads tend to run hotter (probably due to the through bolts interrupting the air flow over the heads) and our modern unleaded fuel adds to the problem as it tends to burn hotter. I have found increasing the main jet by 10 and a slightly smaller cutaway slide improves top end power and pickup. Lowering the needle may then be necessary to get clean mid range running.

  Eddie.

Offline cjt

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Re: Inlet valve sizes
« Reply #11 on: 28 Apr 2020 at 04:59 »
Hi all, I am presently working on a 1948 Douglas Mk3 sports which came to me in pieces and lots of parts missing, this post is with regard to the valves as they were missing , can anyone help me with the overall length, head diameter and stem thickness of the inlet and exhaust valve for the Mk3 sports , also the inner and outer valve spring length.
I had bought a set of valves and springs for the Douglas stationary engine thinking that it would be the same, but they are shorter, so need to find the correct dimensions of the valves
If anyone has a spare set I would like to buy it, please do let me know.

 

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