classic motorcycle forum
Motorcycle Discussions => British Bikes => Topic started by: 33d6 on June 27, 2022, 03:30:01 AM
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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.
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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.
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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
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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.
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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
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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.
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Several methods of building up the OD of a piston......easiset will be electro tin plating....up to at least 010 thick.
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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
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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 ?
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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
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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
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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
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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.
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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
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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.
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Plenty of tin plated iron pistons ..you can be sure the technolgy works .71 and 92 series pistons are tin plated 007 thick.
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71 and 92 series pistons are tin plated 007 thick.
What are they when they are home ?
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In GM 71 and 92 series diesels ....probably in the bigger ones too,but I dont know for sure.
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I really fancy a Dunelt...
Hi 33d6,
I thought of you today when a friend said he was going to part with his Dunelt - a late 1920s 250 very complete and running when it was put way, like the one in my brochure above.
Leon
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Sigh.
Unfortunately, as I wrote a little later, the 33D6 wallet makes my decisions for me. I’d still like a Dunelt but as time goes on even restorable Villiers parts are getting thin on the ground and getting expensive which pushes a prospective Dunelt even further away. Frustrating isn’t it.
I’m getting more and more interested in the likes of the plated pistons that JohnK tells us about. Resurrecting ancient fixed head two- stroke cylinders is becoming a pressing problem. Most I find are wildly overbored and barely usable. They’re a stinker to fit with a liner and when you do fix it what do you do to keep down future wear? Various plating techniques offer hope. Nikasil lined cylinders? Plated pistons?
I should say post war Villiers stuff presents no problems but proper vintage and post vintage engines are a different kettle of fish.
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It's rather nice, in a slightly-bizarre two-stroke way!
Not sure what model it is, but I was surprised to see beaded edge rims. So maybe 1928? Unfortunately the thing is hanging about 10ft off the ground, so at the moment it's a bit hard to get numbers.
Leon
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Seems to be the Sports "K" - the tank divides at the rear to sit over the top tube, and the gear change lever is mounted on the gearbox.
Can't be earlier than 1928, or later than 1930.
Leon
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Yes, they came in a variety of guises from basic bones to sporting to de luxe touring. That one is even more desirable as it appears to still have the special Dunelt B&B carb and air cleaner arrangement.
With a one piece crankshaft, a double diameter piston, a unique carb AND a unique to Dunelt Pilgrim pump Dunelt really went out of their way to make life hard for the mug restorer.
That’s why I look and sigh but my wallet goes into spasms at the very thought. You have to be really really keen or daft as a toothbrush to take on a Model K Dunelt.
Dunelt really wanted to march to their own drum beat and they succeeded brilliantly.
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See I think I can weaken you. Pretty sure the carb is a rather special Binks affair. Very nice. I can see the thing going down the road nicely in a faux duel with a period Super Sport Villiers-engined James...
Pretty certain it's a 1928 Sport, but there weren't big differences between the models. The price mentioned probably wouldn't buy a restored BSA Bantam. Tempted yet?
Leon
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Yes, Binks.
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You’ve carefully avoided the photo of the other side showing the hole where the missing Dunelts own Pilgrim pump should be.
I must admit it would add a bit of style to any vintage run wouldn’t it.
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Seems to have a very unusual feature for an Oz bike.......the sprocket nut has never been attacked with a hammer and chisel.
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And that clutch doesn’t look at all Sturmey Archer. As I’ve mentioned earlier the Model K Dunelt uses the same Sturmey Archer gearbox as my 1926 Matchless so is a familiar friend but that clutch is a stranger.
Can we get a photo of the other side Leon?
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No oil pump you say? Just looking for reasons not to buy it!
The oil pump is there, chain driven from the crank. The tin cover is not, but because it's the non-electric model it would have been smaller than the triangular affair in the brochure illustration.
It's very hard to get to the other side because it's hanging against the balustrade of a mezzanine, very high up. The photos are from the camera poked in at arm's length.
Leon
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Clutch is explained in the 1928 catalogue: "GEAR BOX Sturmey-Archer 3-speed with specially designed 2-plate clutch..."
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Interesting to read about the combined oil filled and sight glass - apparently there is a bowl and drip feed under a glass filler lid, which somehow swings out of the way when it's time to fill the oil tank.
Tempting to say the rear carrier is missing, but on p33 of the parts list there is a set of mudguard stays "if required". So maybe the stays are original. Back stand is missing. Love the Dunelt-specific flanged hubs with 4 7/8" drums.
Quirky little bike, even before we get to Professor Supercharge's stepped piston.
Leon
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I don’t know what that gearbox is but it certainly isn’t what I expected to see. I expected to see an FW (Featherweight) Sturmey Archer which that is not.
There’s nothing wrong with the FW box except the ratios are so spread out.
It’s not just a wide ratio box, it is ultra wide. Top is normal as it always is but bottom is so low you barely have time to get your feet on the rests before you have to grab middle. It’s so low you could haul wrecked Russian tanks off the battlefield. Middle gear then has an impossible job between the two. Perhaps the box now fitted gives a better spread.
That said it’s not too hard to find an FW box. Few sellers understand what they have. They look pathetic compared to other SA boxes.
Still, as you’ve shown Leon, this bike has all the impossible to find pieces so it’s a straightforward restoration. Not a twenty year hunt for hens teeth.
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That's weird - pity I can't get a look at the right side of the bike while it's hanging where it is. The idea of getting it down is a bit daunting.
My guess is that piston/bore condition would be a big factor in the restoration.
Sadly the owner's health dictates that the bike must go, so it is available. Best to go to a Dunelt-appreciating home. Yell if you're interested.
Leon
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I wish I could make an offer Leon but I’ve already got more restorations on the go than I can keep up with plus finding the room to put it is tricky to say the least.
Of course should I have an unexpected windfall from a friendly billionaire things could change but billionaires don’t knock on my door very often.
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I'm curious what a "fully restored bantam" would/should go for ?
I've seen them between $2k and $22k, although I doubt either of those numbers are typical !!
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The gearbox mystery deepens.
I was going through my Dunelt literature last night which includes a repro 1928 catalogue. This shows the Sturmey Archer FW box throughout but down the back it has a page devoted to the accessory Miller lighting system with a thumbnail picture showing how the set looks when fitted.
It’s a small and inadequate photo but appears to show the gearbox as being the same as that fitted to the Adelaide bike, not an FW Sturmey Archer. Did the 250cc K series Dunelt change gearboxes during its production run?
The next few days will be spent ploughing through old magazines looking for Dunelt stuff.
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I'm curious what a "fully restored bantam" would/should go for ?
I've seen them between $2k and $22k, although I doubt either of those numbers are typical !!
I would have thought $A4-5k? https://www.gumtree.com.au/s-motorcycles-scooters/bsa+bantam/k0c18322r10
Re the gearbox mystery, I'll try to get the thing down to ground level to get a better look. If you don't hear from me again, you'll know what happened!
(I turned pages of 1928 mags, and Dunelt literature, but all the gearboxes I could see were the "standard" FW Sturmey Archer, with the change lever either on the box or up high at the front for the fancy models. Mmm...)
Leon
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Don't know about in the colonies but in the UK prices for all bikes have slipped. A lot.
Where a good DBD34 could've made maybe £15/16K you'd get £9k on a good day. A7/10's were fetching £7-8k some years back but they'd struggle to achieve £5k now.
It's a harsh reality that those who like codger bikes are dying out, and their grieving families are dumping the bike collections onto an already flooded market, hence the price crash.
I reckon £2k would buy a top-notch Bantam now.
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£2k is not that far short of A$4000
The pooond starling is holding up quite strongly ?
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Just before easter ,a relative was selling a 1970s restored ,but now slightly rusty BSA B31 plunger ......complete with speedo...........anyhoo ,started at $10k ,eventually sold for $4200
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Didn't sell it on Facebook Marketplace, did he. ?
On that, you can see the original asking price and the new asking price.
I've watched in fascination as some stratospheric opening prices have been whittled down to sensible levels.
Euphoria replaced by stark reality, as it were.
Or desperation ....
Back to Dunelts.
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Haven’t found a different gearbox yet but have solved the “special Dunelt clutch” bit. Apparently Dunelt must have felt the sheer power of their engine would overwhelm the standard FW clutch so demanded an upgrade. Anyway, to meet their concerns Sturmey Archer fitted a BS clutch to the FW box. So no great drama, all standard Sturmey Archer parts but honour satisfied.
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The tiddler clutches are often clever use of pressed metal with the sprocket as the only solid part ..........by bending the tangs on one plate through slots in the sprocket and using the bent tangs to engage the other plate ............... clever when made ,an awful mess when severely bodged by the last desperate owner.
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I’ve got a little more information on the “special clutch “ arrangement. It’s not special at all. The Sturmey Archer FW blurb advises on which clutch to fit for what engine capacity. The basic for up to 200cc, then one for for 250-350cc and finally a heavy duty cum de luxe version if you’re serious. The K Dunelt features exactly what Sturmey Archer recommend for a 250.
They also fitted an extended kick start shaft and offset kickstart lever. All standard SA parts used on various makes.
Its surprising what detail you can find if you really ploughing through old spare part manuals but it drivesyou bonkers doing so.
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I got a slightly better look at the right hand side of the bike. The gearbox seems to be Albion? Certainly the lever has ALBION on it, but there are two drilled holes (one to mount the rod that goes to the gear change on the front of the box) which are drilled into the writing, so I wonder if the whole gear change lever and gate is a bit cobbled up. Maybe Albion CJ (2 speed +ks), EJ (3 speed + ks) fromt he late 1920s or Midget from the early 1930s?
Maybe the original box failed early in the bike's life, and this one has been adapted to suit. It looks like it's been there for a long time.
Leon
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And here's the advert mentioned by 33d6, showing a gearbox that is not Sturmey Archer FW (fitted to most model Ks). I suspect it's also not the box in the Adelaide bike.
Any idea what it is? The bike is flat tank, so maybe 1926-7.
Leon
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Hmmmm ?
http://www.sturmey-archerheritage.com/images/photos/pic-301-s.jpg (http://www.sturmey-archerheritage.com/images/photos/pic-301-s.jpg)
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It’s frustrating isn’t it. There are overtones of FW but not enough. Could it be a preproduction jobbie or a prototype FW. There’s not enough detail to tell.
The current Albion should be easy enough to identify from its makers number. Famous last words I know. Supposedly Albion only made two speeders up until 1926 when their first three speeder appeared. Could you locate the makers alphanumeric id number for us Leon? It’d be a great help.
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No numbers, because the blipping thing is still high in the air. If I were doing anything with the bike, I'd just leave the gearbox as is, unless it was no good in which case a Sturmey Archer FW could go back in.
I'm pretty sure the gearbox in the Miller electrics illustration is also Albion, something like this one https://auctionet.com/en/3007648-albion-gearbox-england-1920-30s . If so, funny that it appears in an advert telling us that all Dunelts use Sturmey Archer 3-speed gearboxes.
Leon