Nickel Thumper Curtis Honda 520 - Nick Caldwell

The important thing about our beloved sport of vintage and classic offroad riding and racing is that we have a remarkably diverse array of like-minded people who will do almost anything to get their daily fix of old dirt bikes. As well, the many different machines they use makes our chosen pastime so interesting and you can have certain riders who wouldn’t entertain anything other than their beloved two-stroke Japanese bike that they’ve raced and owned for the last twenty years, or on the other hand you maybe have owners who are fans of the great British fourstroke machines like BSA or CCM who are so loyal to the brand that it would be a cardinal sin for them to even think of entertaining any other make or model of motorcycle.

One such offroad fanatic is Scottish Twinshock Scramble Club Chairman Rick Caldwell, who it’s probably fair to say is not just a fan of Honda four-stroke motorcycles but he has an undeniable thirst for those big XR500 Honda motors and currently has quite a good collection of them in various stages of development scattered around his workshop floor.

Over the last twenty years or so Rick has put together many variations of these Honda XR500s to use as race bikes with his local Twinshock Scramble Club and he also has a good reputation as being the XR godfather when it comes to knowing his way around these big four-stroke machines.

Rick has built many variations of these big four-bangers in the past but always had a hankering to build an XR special one day using a custom, hand-built chassis and using one of his big Honda XR engines as its power source. When Rick found out that he could purchase such an item hand-made from Curtis Bikes

Curtis Honda 520

Rick Caldwell

Words and photos by Chris Montignani

All business at this end.

then he knew straight away that this particular chassis would be perfect for his next XR500 project. Although this wasn’t going to be another run of the mill Curtis bike, this was going to be a special kind of machine with many of Rick’s own personal touches.

Curtis Bikes was started by Brian Curtis back in 1972 when he was aged just thirtytwo years old. Prior to that, Brian was a highly successful motocross racer in his own right back in the 1960s before he then went to work and honed his frame building skills with none other than the great Don and Derek Rickman of Rickman Motorcycles.

Naturally, once Brian had gained the skills and knowledge of motorcycle chassis building it wasn’t long before he then started up his own motorcycle frame building company called Curtis Bikes. They currently make very high quality motorcycle and mountain bike chassis, and they also build complete machines for customers using those tried and tested Honda XR500 engines. Curtis can also supply you with a bespoke frame of your choice if

Even if you’re not a four-stroke fan, this Curtis could easily encourage you to change sides.

you wanted to put together your very own machine which is exactly the option pursued by VMX reader Rick Caldwell.

The chassis featured on these pages was actually built by Dennis Mapp, who works for Curtis Bikes, and this superb piece of art is all hand-made from the very best materials, These superbly brazed chassis are all made to the customer’s own requirements as Curtis never hold any stocks of frames at their premises in Barton Street, Somerset, in the UK, because they only build their projects to customer orders.

As this was a special project, Rick decided to have his Curtis chassis nickel plated but of course you can have yours done in whichever colour that takes your fancy. Suffice to say that this frame with its included swingarm and other bolt-ons (like the footrests) are all top quality and look fantastic decked out in their shiny nickel plating.

So Rick had his frame and next up was the motor which wasn’t a big problem because Rick had plenty of XR motors lying around

in his workshop. Because this was going to be a special build and as he wanted to have some upgrades done to the motor, a complete powerplant was then sent to Rod Spry Engineering to have the motor completely refurbished and rebuilt. Rod dismantled the engine to its sum of parts and fitted a midrange camshaft to the top end. The cylinder was also bored out to 520cc and the balancer shaft was removed to make the engine respond quicker to the throttle. To help cope with the increase in horsepower, a set of heavier clutch springs were also fitted to enable the transmission to put that upgraded grunt to effective use.

One of the other mods that was carried out on the motor was that every second cooling fin was machined from the barrel. Apparently this is a very common procedure now on these XR Honda engines and some say it’s done to reduce weight, while others say it’s done to improve the cooling and to stop mud and debris from jamming between the fins if it’s raced at a particularly muddy race event which could then lead to overheating. Although there is another school of thought here and that is that this procedure is purely cosmetic just to make the engine look aesthetically better when it sits in the chassis but any one of these explanations

has its own story to tell so make your own mind up on that particular issue.

Anyhow other than those aforementioned upgrades the motor was basically then left alone as Rick didn’t want too much interference on the engine and his emphasis was on keeping it more reliable than actually have it tuned for out and out power and speed. Rick did though have the motor’s ignition and clutch casings powder coated in black to add that little bit something different to its looks. To finish off the completed engine Rick fitted a 36mm Mikuni carburetor with a Twin Air filter and then it was all ready to stick into that beautiful Curtis frame.

The bike’s front suspension features a nice pair of Italian-made 45mm Marzocchi Magnum units and these were completely stripped down. New internals were fitted and they were re-valved. These Marzocchis also have plenty of scope for adjustments to their rebound and compression damping.

To keep the fork sliders clean and free from dust and moisture Rick also fitted a quality pair of rubber gaiters to the fork tubes. A clamp for the front brake anchor arm had to be fabricated as the left side Marzocchi fork leg didn’t have this fitted as a stock item.

The rear suspension turned out to be one of the more expensive parts of the machine

Header pipes have a wonderful hue.

In some ways it is just a collection of ‘bits and pieces’ but don’t they all come together wonderfully well in this Curtis.

with the fitment of Swedish-made Ohlins piggyback shocks, which as you know are not cheap but are just about as good as you can get for a Twinshock race bike.

To get the rolling chassis up and running a brand new set of gold anodized SM Pro alloy rims were purchased and these were laced up to a Husqvarna twin-leading shoe hub and brake on the front with a standard Honda XR magnesium hub used on the rear. Heavy-duty stainless steel spokes were used on both sets of wheels and the wheels were completed with a set of Michelin Cross Competition tyres.

Naturally a new fuel tank was going to be required for this special build and a fuel cell from a 1979 Honda CR250 Red Rocket was more or less a straight bolt-on fit to the Curtis frame. The tank was repainted in white and all of the graphics for the tank and sidepanels were supplied by Ian and Alan Reid at MXM Graphics in the UK. The bike’s seat was made up from a brand new seat base and high density foam insert and the customised seat cover and the Curtis logo were also supplied by the guys at MXM.

With regards to the bike’s sidepanels, they may have the look of being made from plastic, but those custom-made side panels are nicely sculpted from alloy. The Honda special’s

Main: The Curtis is a timeless design that still looks ‘right’ today 1. That is one helluva good looking XR500 engine! 2. If you look closely you can still see the Husky heritage in the chassis geometry. 3. Detailing everywhere is first class.

black front number plate and front mudguard are parts that Rick sourced from a KTM motocross bike although the rear mudguard is your standard Honda part that you find on the back of any stock XR500 Honda.

As for the controls, a sturdy set of modern Renthal handlebars were sourced and fitted along with a pair of Scott handlebar grips which we had to scrub clean during filming! Yes, the bike got blown over in the howling winds while we were snapping these pictures but thankfully no damage was done. A set of Apico clutch and front brake levers were bolted in place and an engine kill switch was also added.

Those lovely exhaust headers and tailpipes on the engine can’t be found in any shops or websites because those twin pipes were custom made for this machine by Rick’s friend Barry, who is a bit of a genius when it comes to manufacturing these kinds of bespoke items. The pipes are not straight through units as they do have baffles fitted inside, and no, they’ve not been borrowed from a Volkswagen Beetle as some people have (unkindly!) suggested.

So this entire Curtis bike build project took Rick quite some time to complete between all his work commitments but you have to admit it’s all been worth the wait. At the time of writing this feature, as far as I’m aware, Rick hasn’t had the chance as yet to give the bike its baptism on the track.

Having said that he’s already hinted that he may even sell the bike before its racing debut to fund another couple of similar bike building projects that he has on-going in his workshop, so if he does decide to sell it then somebody is going to get themselves a special bike indeed! But as you know I’ve said it before and at the risk of repeating myself will keep saying it - every time I come across projects such as this, you just know that if a bike looks right then it usually goes right. This fantastic looking Curtis Honda 520 certainly ticks all of those boxes in that respect and I can also tell you that I’ve heard the rip from that XR520 motor through those custommade exhausts and the sound is every bit as good as the looks!

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