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General => Douglas Motorcycles - General Discussion => Topic started by: cbranni on 23 Jun 2020 at 14:57

Title: Question about my recently rebuilt Dragonfly
Post by: cbranni on 23 Jun 2020 at 14:57
Hi to everyone,

Due to restrictions in Wales I have been unable to run my Dragonfly since it's rebuild, but I have today ridden it for about eight miles and it is a joy, it's smooth, handles fine and I really like it and want to use it but! there is always a but, I am leaving a plumes of blue smoke behind me  :(

The bores were standard size although worn, but I honed them and bought set of rings from Cox and Turner but as I thought it didn't fix it, so I have read up about fitting Honda CB250N pistons and rings and compared sizes but I have a few questions.

1. What is the Honda part number? is it 13101-442-004 this is for a std 62mm piston
2. The Honda piston is 5mm greater in height than the Douglas, any issues there?
3. I shall have to make a pair of small end bushes so could somebody tell me the correct spec material to use
4. The small end to top of piston seems ok , unless anyone knows different.
5. If I fail to find 62mm pistons how bigger can I go before the cylinder spigot gets too weak.

Thank you for reading this
Kind regards Colin
Title: My Dragonfly
Post by: cbranni on 23 Jun 2020 at 15:07
Sorry forgot to add a photo

Colin
Title: Re: Question about my recently rebuilt Dragonfly
Post by: AndyMorgan on 23 Jun 2020 at 17:25
I have just rebuilt my Dragonfly with 62.5 oversize 250N pistons, Although I haven't ridden the bike, it's still in bits, the engine runs fine and recently cured the smoking barrel by fitting the correct rings. I had the small end bushings made from Phosphor Bronze PB104 (CuSn8) and had an extra set made if you need them let me know by PM. Once fitted you need to drill through the oil way and then ream back to 15mm.

Cheers

Andy
Title: Re: Question about my recently rebuilt Dragonfly
Post by: cbranni on 23 Jun 2020 at 17:51
Thanks for the reply Andy, can you remember the Honda part number please for the piston set and did you use genuine  Honda parts or were they pattern.

Kind regards Colin
Title: Re: Question about my recently rebuilt Dragonfly
Post by: AndyMorgan on 23 Jun 2020 at 18:25
Pistons were AE Hepolite
The box says

" REF. 20355  +0.0197" +0.5mm
Suitable for BMW (with "Honda 250 S/D" penciled in)
Original bore 2.4409" /62mm
DB07 119"

Cheers
Andy

Title: Re: Question about my recently rebuilt Dragonfly
Post by: AndyMorgan on 27 Aug 2020 at 16:12
When I fitted Honda pistons to mine I made an extra pair of Phosphor Bronze (PB104  CuSn8) small end bushings as it was just as easy to make 4 as 2  and just in case. I have since rebuilt the engine and it all seems fine so these are now surplus and you are welcome to them for a modest contribution to my Dragonfly restoration fund.  The bushings can be pressed in cold and once fitted you will need to drill through the opening in the small end and then open it up to a slot with a Dremmel or similar to form an oil way into the bushing and then ream to 15.0 mm for the Honda Gudgeon pin


I have only one servisable Dragonfly piston +.060" and one standard which is a bit of a mess which are for sale on Ebay
The Honda Pistons I used  were Hepolite ref 20355 which were 62mm +0.5 oversize (my barrels were very rusty when I got them)
The box label says suitable for BMW which is scribbled out and Honda 250 S/Dr (Super Dream)

Hope this helps
Cheers

Andy
Title: Re: Question about my recently rebuilt Dragonfly
Post by: otaio26 on 28 Aug 2020 at 11:21
FW Thornton Ltd (01952 252892) can supply std and o/size Pistons.Made by JP in Australia they are solid skirt not split like the original Hepolites and completely eliminated the smoking twin barrels! Delivery next day and not necessary to replace small end bushes.Mine starts first kick,no trace of smoke cold or hot and no noisier than the original Hepolites I first fitted that smoked very badly,even after a thorough running in.Alternative to Honda piston and retain the recommended bore sizes with JP.
Title: Re: Question about my recently rebuilt Dragonfly
Post by: cbranni on 09 Sep 2020 at 12:51
Just an update on my Dragonfly cylinder re bore, I decided to go the Honda piston route and bought a pair of 62dia standard pistons complete with rings, gudgeon pins and circlips, my cylinders were on a very worn standard bore 60.8 so asked the re-bore guy to machine them to suit the pistons. He rang me to say he hadn't reached size yet and the lowest horizontal hole in the cooling fins had broken through in to the cylinder, please see photos.................

I now have to find another cylinder on standard bore to match the un-bored one, but what I can't understand is how others have bored these cylinders larger than I have without issues, I welcome any suggestions to help me overcome these issues.

Kind regards Colin



Title: Re: Question about my recently rebuilt Dragonfly
Post by: eddie on 09 Sep 2020 at 15:40
Colin,
          To me, that hole looks too round to have been cast in - I think it has been drilled, and the drill has probably deflected due to a depression in the casting (making it too close to the bore). If it were my barrel, I would make an attempt to reclaim it. That hole is well down the barrel and must be pretty close to the extent of travel of the rings, so probably won't affect the operation of the engine too seriously if blocked up. I would be tempted to tap a thread in the hole and screw in a good fitting stud turned from cast iron and coated in high temperature Loctite. Once the Loctite has cured, it should be possible to continue with the boring.
    Hope this helps,
                               Regards,
                                               Eddie.
Title: Re: Question about my recently rebuilt Dragonfly
Post by: cbranni on 10 Sep 2020 at 08:32
Thanks Eddie for your reply and I do tend to agree with you, I have a number of options in my head at the moment so I shall mull them around and pick a solution.

Kind regards Colin
Title: Re: Question about my recently rebuilt Dragonfly
Post by: cbranni on 28 Nov 2020 at 13:54
Just an update on progress, I found another pair of cylinders as my originals were beyond repair and had them bored to suit standard Honda pistons I had bought and fitted them yesterday and seems well, engine fired up second kick and ran perfectly, no smoke or nasty noises I am well pleased, thanks to Eddie and Andy on here for their help and advice and a special thank you to Roy Staunton who went the extra mile to help me.

Cheers Colin