Author Topic: Post war pistons  (Read 2765 times)

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Offline Dougiethenoo

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Post war pistons
« on: 22 Mar 2016 at 19:54 »
I have read a few posts about the different piston types available, but could not draw a conclusion on pros and cons, especially related to oil burning.

I have 2 sets available to be used in Nikasil coated bores, both sets are in good condition and both +30 thou oversized.
One set is from Hepworth (PHC) and has oil holes drilled in the skirt below the oil ring.
The rings are much wider than other sets I have seen.

The other pistons are of unknown origin, have no oilways and rings that are about 2/3 the width.

Does anyone know if the holes in the skirt reduce oil burning?


Offline douglas1947

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Re: Post war pistons
« Reply #1 on: 23 Mar 2016 at 06:11 »
The holes in the skirt of the piston are for the "scraped "oil to leave the piston. It is for the stepped type of piston rings.
Slotted oil rings have the bores inside the ring slot.


Offline eddie

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Re: Post war pistons
« Reply #2 on: 23 Mar 2016 at 08:43 »
If we exclude the Mk1 T35 (which has the high dome pistons), original Douglas postwar pistons came in 3 types. Pistons for the Mk3-5 machines were solid skirt and had 2 compression rings (1/16" wide) above the gudgeon pin and a 3/32" stepped scraper at the bottom of the skirt. 'Plus' pistons looked identical but were made of 'Y' alloy and were much lighter. For the Dragonfly, Douglas changed to a split skirt design (presumably to cut down on mechanical noise and reduce the chance of pistons nipping up). Whilst this was probably true, these pistons did suffer from the crowns collapsing when worked hard - the usual cure being to revert back to using 'Mark' pistons.
  Back in the 1980's, original pistons were getting rather scarce, so the LDMCC had a couple of batches of pistons made by Peter Hepworth (PHC). These had the scraper rings located just below the gudgeon pin (a practice that we were ensured was better than the original set up). Experience showed that oil consumption varied dramatically from bike to bike. These pistons were then returned to be modified by drilling the oil drain holes below the scraper ring (which improved matters but was not a complete cure). The PHC pistons were also different in that they had wider metric width rings which didn't help the oil consumption as they would have taken longer to bed in.
  In the 1990's, JP pistons announced that they could make pistons for the postwar Douglas, and following good reports, the LDMCC started supplying them through the spares scheme. All was well until the early 2000's when the quality control seemed to drop off - rumour had it that the rings were being 'bought in' rather than made 'in house' - the pistons were OK, just the rings needed careful checking.
  Around 2010, following some adverse comments, the LDMCC looked elseware for pistons, and are now getting them made in the UK.

  So that is the saga of the pistons for postwar machines - there are, of course, pistons from other machines that can be made to fit (usually Honda), but the above is just about the complete list of pistons that were available via the factory or the LDMCC.


Offline Dougiethenoo

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Re: Post war pistons
« Reply #3 on: 24 Mar 2016 at 12:51 »
Thank you Eddie.
I have 2 sets (picture attached).
One is the PHC set, with oil holes drilled.
Rings are a little bit worn but the gap is only about 0.007" at the unworn part of the bore, which seems to conform to the rule of thumb of 3 thou per inch of piston diameter.

I found a brand on the other set - Specialoid.
They were in the bike during the most recent "campaign" which has resulted in 0.009" ovality on ones cylinder and the ring gaps are a little larger.

They appear to match the description of the standard Mk3-5 pistons, solid skirt, 1/16" compression rings and a thicker oil ring near the base. These pistons weigh 6.1 oz compared to 6.5 oz for the PHC items.

Since I am Nikaseal lining the bores to avoid +.040" over bore, I think I'll use the PHC version due to the lower ring wear. Cast iron rings are essentially sacrificial in a Nikaseal bore.

Ideally I'd like new rings but I have no idea where to get either the standard 1/16" wide at +0.030" bore, or the wider PHC rings.

If anyone knows of a source of either sets of rings I would appreciate pointers.

Offline dalgrae

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Re: Post war pistons
« Reply #4 on: 26 Mar 2016 at 08:34 »
For piston rings try Cox and Turner based in Somerset

Regards. Colin