Author Topic: mk5 pistons  (Read 4166 times)

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Offline dalgrae

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mk5 pistons
« on: 21 Oct 2014 at 15:33 »
I have been trying to sort out the o/s cylinder smoke problem on idle with my mk5 so as my other post regarding guide clearances I have now begun to look at the o/s piston in detail, after trying club spares who also offered some very good advice I was told that my piston was not a standard part as both the top two rings should be the same width, I have been talking to my local motorcycle engineering company who has been doing all sorts of bikes for many years ,after giving him the casting numbers inside the piston skirt it turns out I have Dragonfly pistons fitted ,so my question is what are the implications of this regarding timing/performance or carburation or anything else you can think of 

Regards  Colin

Offline eddie

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Re: mk5 pistons
« Reply #1 on: 21 Oct 2014 at 18:41 »
Colin,
          If you have Dragonfly pistons, they will be split skirt pistons with slots beneath the ring land. The rings will be 1/16" wide top ring, 3/32" wide stepped compression ring, and a 5/32" wide slotted scraper. There are known problems with these pistons - if the engine is worked hard for prolonged periods, the crown of the piston starts to collapse into the slot, resulting in the grooves for the rings taking on a wavy form - thus tending to trap the rings. However, they have the same crown design and compression ratio as the Mark type pistons, so will not require any other changes to carb settings, etc.
  With regard to the O/S cylinder smoking on tickover, I would double check that the carbs are correctly balanced - the poor balance may be resulting in that cylinder not doing enough work to keep the oil back. Try easing back the cable adjustment on the LH carb so that the RH cylinder does a little more work and see if that improves matters.

   Regards,
                Eddie.

Offline dalgrae

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Re: mk5 pistons
« Reply #2 on: 22 Oct 2014 at 07:01 »
Thanks Eddie I have tried to set up the new carbs as best I can and am fairly happy with them,the o/s bore has several vertical scratches in them which you can feel with your nail although there is no sign of wear , also the valve guides are worn so I have ordered new parts for the head from club spares and also after reading your forum comment about Honda pistons I have 2 new sets of on there way to me

Regards  Colin

Offline dalgrae

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Re: mk5 pistons
« Reply #3 on: 22 Oct 2014 at 07:08 »
Eddie    please I do not suppose by any chance you have a couple of small end bush adaptors to suit the Honda piston pins ,or can you advise me please what grade of material do I need to buy to make some.

Regards  Colin

Offline eddie

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Re: mk5 pistons
« Reply #4 on: 22 Oct 2014 at 08:46 »
Colin,
         As your bores are standard size, I wouldn't resort to using Honda pistons at this stage. Standard size Honda Pistons are +.048" compared to Douglas pistons. so your bores will then be at their maximum size. I would suggest you get the bores 'glaze busted' to assist the bedding in of new rings. Build the engine on a cheap oil for running in (modern oils lubricate too well, and the bores are likely to get glazed again before running in has been completed). If you are concerned about the ring gaps, get a set of +.020 rings and gap them to suit.

  Regards,
               Eddie.

Offline dalgrae

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Re: mk5 pistons
« Reply #5 on: 22 Oct 2014 at 09:30 »
Thanks again Eddie the problem is no one has any rings of any size to suit my pistons I have tried everyone that I know

Regards  Colin

Offline Ian

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Re: mk5 pistons
« Reply #6 on: 22 Oct 2014 at 23:00 »
One "left field" thing to check - with the Mark 4 we bought it was blowing smoke - and had glazed the bores. Rehoned - but a problem I noticed were that the compression rings fitted were the ones with an inner chamfer - and they were upside down. Corrected and smoke mostly went away

Offline dalgrae

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Re: mk5 pistons
« Reply #7 on: 23 Oct 2014 at 17:52 »
Thanks for the advice and at last I have managed to track down some plus .020 rings to suit the Dragonfly pistons so on Monday as you advised me Eddie I am going to my local engineering company for a deglaze and ring gap to suit.

Regards  Colin

Offline dalgrae

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Re: mk5 pistons
« Reply #8 on: 01 Nov 2014 at 15:12 »
As I previously said in a  post about my MK5  having Dragonfly pistons when I look at the piston which is a Hepolite of the correct number I am puzzled that the oil control ring groove does not have any holes drilled into the bottom of it on the piston to allow any trapped oil to be released into the underside of the piston every other piston I have ever seen has these holes ,the only way any trapped oil can escape I assume is if one of the slots in the ring lines up with the thin slot that runs from the piston bottom to 3/4 the way up the piston side ,when I stripped out the cylinder parts there was a lot of oil present on top of the piston and the bore ,any advice would be most welcome please

regards  Colin

 

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