Dunelt and Barnstormers

[33d6]

I've been curious about the "supercharged"(note quotation marks) Dunelt for a long time. It's just another weirdo two-stroke but it seemed to last on the market a few years longer than most of the other oddities.

Their Model K had an extra attraction as it shares the same lightweight Sturmey Archer gearbox with my 1926 Matchless so not only an oddity but also a possible source of spares. Luckily those lightweight S/A boxes turn up fairly regularly so no longer a problem.

Anyway, the Barnstormers site in NZ, www.barnstormers.co.nz has just put up an illustrated copy of the Model K spare parts list so one can study the internal mysteries of a Dunelt style engine and work out why you don't see many restored examples. I don't think the one piece roller bearing crankshaft helps. Not many enthusiasts are willing to stump up to have a complete new crankshaft made when the big end needs renewal and as the top hat piston is utterly unique with no possibility of substituting a foreign make jobbie that just adds to the financial pain. Very few are willing to rebuild a 250cc two-stroke single when it will possibly cost as much as rebuilding a J.A.P. vee twin in a Brough. Strange about that.

Anyway it's been very quiet here lately and I thought others might be interested in having a nosy at how Dunelt did it.     

[john.k]

In the last days of Villiers ,Bernard Hooper  designed a V4 using the double diameter piston idea,and when NVT folded,he continued to manufacture the V4 for some years as an ultralight engine.

[cardan]

The stepped-piston Dunelt two strokes are seriously cool.

The crank arrangement is very Douglasy, except that, unlike Douglas, it looks like the the rollers were crowded rather than caged. Way too complicated really, but that's part of the charm.

I know of a couple in sheds in Adelaide, but they are both 250s. I reckon the 500 two stroke would be fun, but I don't think I've seen one, or even parts, in Australia. Some interesting technical challenges in rebuilding the motor.

Leon

[33d6]

I don’t know about other States but the majority of Dunelt imported imported in to Victoria were the later Sturmey Archer/Raleigh engined models with the odd Villiers powered beastie in the mix. I haven’t found many dating before 1928 or so at which point the 500 cc big singles were near the end of their production run. I’ve only found one or two dozen of the 250cc K series as shown in the Barnstormers spares list. Most had engine numbers up in the 14000’s which I gather was towards their end.
I don’t think making a new crankshaft would be that hard, merely expensive unless you had a rather good machine shop at your disposal. The piston is a different matter as the double diameter would make the average piston manufacturer think outside their normal routines. Very hazardous making people think. Rather expensive also. My 1938 Polson piston catalogue has the Dunelt K piston as the second most expensive motorcycle piston they offered. It was only topped by the 500cc OHC Excelsior Manxman piston and I wouldn’t think Polsons sold many of them.
I really fancy a Dunelt, but rebuilding a Villiers two stroke is a much simpler option.

[cardan]

I've often wondered whether a piston could be externally sleeved. Waste of effort on a normal piston but it might be worth thinking about for the bizarre stepped Dunelt piston.

Unfortunately my 1929 Dunelt brochure is badly water damaged.

I also have a fabulous little booklet that describes the attack on the TT Course on a 500cc Dunelt (Sturmey Archer ohv four stroke, rather than the earlier 500cc stepped-piston two stroke) in the snows of early 1930. 350 laps of the TT course - 13,199 miles - in 15 days 19 hours 4 minutes 45 seconds at an average speed (including all stops) of 34.8 mph. Not bad. The total vertical climb as 24x Mt Everest - 132 miles vertically up.

Leon

[33d6]

The original piston was not only double diameter but also bi-metallic with an iron/steel section cast in for the ring grooves. Every owner I’ve talked to has complained of separation issues and overheating when trying to resurrect an original piston.
I think for the effort involved I’d rather make new than try to resleeve.
I have no original documentation but have slowly collected the odd repro catalogue and photocopies of bits and bobs. I know one would be a monumental drama to restore but I find the weirdness near irresistible. Luckily the Villiers wallet makes the decision to stay away fairly easy.

[john.k]

Several methods of building up the OD of a piston......easiset will be electro tin plating....up to at least 010 thick.

[cardan]

Several methods of building up the OD of a piston......easiset will be electro tin plating....up to at least 010 thick.

I'd be interested in the details John. Is it a special process? How do we know the plating won't flake off?

I've rebuilt a MAG twin for a friend - bottom end is good, cams and followers like new, bores are almost unworn, but surprisingly the pistons are about 10 thou too small for the bores. Very weird... thus the interest in building up pistons.

Leon

[R]

How do we know the plating won't flake off?

By the quality of the plating, and (hopefully) previous trial-and-error ??
You ever see a tin can in the supermarket with the plating not adhering ?
Although piston duty plating may be a different matter ..

If this is a reliable form of recondition, could negate the need for many a rebore ?

[chaterlea25]

Hi Leon,
10 thou would not be unheard of if the pistons are alloy, cast iron would need to be tighter?
However I recently worked on a Triumph model H and found 16 thou clearance between piston and cylinder
The kickstart would still hold my full weight tough 85kilos

John

[cardan]

Yeah they're cast iron pistons, which like to have rather small clearances. The 72mm bores are worn 2.8 thou on one cylinder and 4.8 on the other - not enough for me to want to rebore an old engine. However the pistons must be made of something very soft as the clearance is 9 thou at the sides and 13 thou fore-and-aft. The motor was black and gungy throughout, no doubt from the combustion gases blowing past the pistons, and yet other than the pistons it was unworn. Weird. On a 72mm bore, I'd be shooting for 2-3 thou clearance for a cast iron piston.

Plating sounds attractive, but I fear damage if anything began to peel off. And with sharp edges at the ting grooves etc. there are plenty of things to go wrong? I'd love to hear if anyone has done it successfully.

Leon

[cardan]

The original piston was not only double diameter but also bi-metallic with an iron/steel section cast in for the ring grooves.

Hi 33d6,

Wow! All the more reason to be Dunelt-intrigued.

Here's my entry in the bizarre bi-metallic piston competition. There's a pair, never used, with alloy tops and ring grooves, and steel sides. Any ideas?

Leon

[33d6]

Oh yes. They are standard vintage Talbot.  Way back before they became Sunbeam Talbot in the Rootes group. Today they’re commonly known as “Roesch” Talbots after the designer, George’s Roesch.
I’m a firm admirer of George’s Roesch. He was Swiss and liked to combine his holidays with testing his cars so holidayed back in Switzerland thrashing the latest Talbot up and down Alpine passes.
That part of the world is Gods gift to we motorcyclists. The riding is fantastic.. unfortunately so far I’ve only managed to get there a couple of times but still hope to ride there again. If you haven’t eaten a bratwurst at the top of the Stelvio you can’t count yourself as a serious rider.
Sorry, went a bit off piste there but you get the idea.
Back to Dunelt. They were also one of the few to fit a Binks carb as O.E.M, and I suspect Binks did a special carb body for them. Yet another reason why we don’t see many restored two-stroke Dunelt.

[cardan]

Oh yes. They are standard vintage Talbot.

Genius. Appendix 1 of Motoring Entente (the story of Sunbeam, Talbot and Darracq) gives the 1930 16 h.p. Talbot as 67.5 x 95, 2040cc, fitted with "bi-metal pistons". Not sure that my two n.o.s. standard-bore bi-metal pistons will ever find a home... let me guess that restorers of 1930 16 h.p. Talbots, if there are any, don't care too much about the spec of their replacement pistons.

Leon

[33d6]

I think you’ll find the opposite, Leon.
Roesch Talbot are deadly serious motors. Deeply admired and deeply serious. It’s a bit odd to have just two pistons but there are enthusiasts out there who would be proud to own them.
If you want to know about Roesch Talbot I suggest you get yourself a copy of “Georges Roesch and the Invincible Talbots” written by Anthony Blight. This being the story of a breathtaking machine being an ordinary pushrod ohv with a single carburettor that slaughtered it’s opposition.
There are very few Roesch Talbot in Australia but I learnt about those two part pistons from a very proud owner.