Sunbeam S7 (1947) Review: Staff Bikes

Date 24.1.2013

Presented by
  • Motorcycle Trader

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1947 Sunbeam S7

For those of you who came in late, I’m the sometimes proud owner of a 1947 Sunbeam S7. It’s a great example of post-war optimism in Britain, even if the sausage didn’t quite live up to the sizzle.

The 500cc tandem twin was fairly innovative for its day, with the overhead cam being a particularly unusual feature. However, it produces a very modest 18.4kW (25hp), which ain’t a lot to shift a fairly heavy machine.

Those who know their Sunbeams respond with, “Oh, you have the early one,” in hushed tones, suggesting you have their deepest sympathies. With good reason. While the most collectable of this series (there were three models: the S7, S7 Deluxe, and the S8) it’s also the least reliable, prompting one publication to cheerfully note that an owner will need “deep pockets”.

Sadly, they’ve turned out to be right. Shortly after I’d gone to considerable trouble and expense to replace the wiring, rebuild the generator and distributor, plus replace the engine mounts, it threw a leg out of bed. Yep, a con-rod snapped, broke out of the crankcases and went sailing off to freedom in a paddock not far from the picturesque Victorian hamlet of Maldon.

The first replacement engine I bought was stolen from the back of a pick-up in Florida, thanks partly to the vendor’s undeniable urge to “wet a line” and leave the truck unattended.

Engine two (or at least the core of one) was tracked down via the generous assistance of Colin Parish, a motor engineer and Sunbeam nut living in Wangaratta, Vic. He assembled a ‘new’ crank/rod/piston set using a mixture of my bits, his spares and new parts. So far, so good.

Months down the track, I’m finally getting stuck in. The generator, clutch and gearbox had to be liberated from the deceased powerplant, before we could start on the new – so that’s what has been occupying my limited attention in Shed Guido.

Now if it was a Japanese powerplant from the last few decades, I’d be a whole lot more comfortable with this, as I’ve worked on several and have a reasonable understanding of their architecture. However, there are times when Winston (as I’ve named it) has me flummoxed.

The Lucas generator is a good example. Looks simple enough, but how about we refer to the three print and several electronic workshop resources I have on hand? Most were written when the bike was current and therefore assume a fairly high degree of familiarity with 1940s engines. That’s not a lot of help when you’re standing in your shed, 65 years later.

So I undo the screws for the alloy cover, only to discover the latter is firmly wedged in place. Is there another fastener hidden away somewhere? Could there be something I’m missing? All my resources offer is the sage advice, “remove the generator”. Cheers. With a stick of gelignite, perhaps? Thanks for nothing.

As a last resort I attack it with a rubber mallet and it eventually gives up.

Getting the cast-iron (!) casing and the brushes off is no great problem. Then I have to deal with the core. Again, the workshop manuals remain tight-lipped. Is it a left or right thread, and will I find out only when I’ve stripped it? Of course it’s a left thread. In fact the Sunbeam has a few traps when it comes to threads. Most are conventional right-hand direction, but a few are the opposite, just to keep you on your toes.

Oh, and while I’m on the subject of threads and bolts, did I mention I had to buy a whole new toolkit for this job? Though I already had a good range of metric and SAE devices, the Sunbeam requires Whitworth. I was whining to Phil Pilgrim (of Union Jack Motorcycles) about this one day, bemoaning the fact that no tool shop in the country stocked this style of tool. He casually reached under his counter and held up a socket set in one hand, plus a set of spanners in the other. “Which one do you want?” he asked. I took the lot.

Back in the shed, the gearbox was suspiciously easy to remove – just four bolts – though the clutch is putting up a better fight. One of the six nuts holding the spring plate in place has gone AWOL. That’s because I didn’t bother to hold the plate against the spring tension, so the final nut took off into orbit the moment the thread released.

The centre clutch hub nut is requiring a worrying amount of force to dislodge. I think a heat source (a flame-thrower has a certain amount of appeal) is going to be required.

Just about the only thing that has so far prevented me from throwing it over the back fence is that, every now and then, I get some glimmer of kindness. After the generator experience, I put an appeal out on the web asking for advice on the clutch nut. Almost instantly, from the other side of the world, came a reply from the owner of Stewart Engineering, which sells Sunbeam bits. He didn’t try to flog me anything, but did provide the info I needed.

Speaking of Stewarts, it’s worth a look at www.stewartengineering.co.uk, even if you’re not a Sunbeam victim. It’s a great example how a successful cottage industry can be built up around a surprisingly small customer base, particularly if it has an international reach.

As you read this, I’m probably back in the shed, shouting little pearls of wisdom such as “WTF!?” at regular intervals. I may even be tackling some reassembly. Wish me luck…