Unique Triumph desert sled: Pre-unit motor, unit chassis

Ian Williets has become something of a Meriden Triumph specialist in recent years. The Wiltshire-based former solo and sidecar racer got into the classics when a friend asked him to fettle the engine for a Suzuki GT750 restoration project he was working on. And once that was completed, the same friend asked him to rebuild a unit Triumph motor.

Working on the British engine was something completely new to him but once he’d finished the project, he couldn’t wait to buy his own Triumph TR6, which was then stripped, rebuilt and ridden. 

“I really enjoyed working on the Triumphs and suddenly I found an interest in old British bikes that I’d never had before. After I’d quit racing I rode enduros and had a Honda road bike to tear around on. But not only did I enjoy working on the Triumphs, I loved riding the TR6. I never realised you could enjoy riding a bike so much without having to ride it flat out everywhere. It’s just so relaxing building and riding them.”

Since then, Ian’s built countless Meriden Triumphs. Initially it was purely ground-up restoration work on pre-unit as well as unit Triumphs. He helped Ace Classics with their 500cc twin they race at Goodwood. He built choppers, bobbers, modified stockers to give them a different style – and then, post Covid, got the bug for building street scramblers in the style of the early American desert sled race bikes – ie stripped down to the bare bones dual purpose bikes. 

“It’s just not so expensive to create a ‘desert-sled’ style compared to a full-on restoration, plus I like the look of them and enjoy riding across the hills with a few mates on similar bikes,” said Ian.

After building a straightforward TR6 street scrambler, Ian got the idea to create something a bit different – a pre-unit 650 in a unit construction Triumph chassis. From the timing side the motor looks like a squashed-up pre-unit, but on the other side, there’s a unit construction chain case. We asked him how the build came about…..      

Superbike: You’ve built an impressive special with a pre-unit 650cc T110 Triumph engine, shoehorned into a unit construction frame. What prompted you to even try such a thing?

Ian W: There was a bunch of us chatting one day about our various projects and I said I’d like to fit pre-unit Triumph engine into a unit Triumph frame. Somebody said, ‘that can’t be done,’ which was a bit like a red rag to a bull. Within two weeks I’d done the initial work to fit the pre-unit engine into the unit frame.

But why would you want to do it in the first place? 

I like to do different things – and to prove people wrong!

Were you thinking there might be advantages of doing it?

Not really. There weren’t any real advantages that came to mind when I started the project. It was just meant to be a bit of fun to prove it was possible. But the bike turned out to be a lot lighter as well…… It just knocked weight off it here, there and everywhere with the different things I’ve done……

How can a pre-unit engine be lighter than a unit engine?

I’m not sure it is. We couldn’t work it out because when we weighed the engines we discovered the pre-unit, with gearbox and clutch, weighs 54kg while the unit only weighs 42kg. But the whole bike turned out to be lighter in the end than my 650 scrambler. 

Is the overall weight of the bike lighter because you changed your suspension quite radically, and you’ve cut down on things like the electrical components?

Well, I’ve done away with the magneto and fitted electronic ignition. It has got different forks and brakes, different wheels, different shocks, plastic mudguards – as well as a ton of other different a bits and pieces. So yes, that’s probably the reason.

Okay, so let’s go back to the engine.  What exactly have you done to get it fit in? What was the process?

I had to shorten the overall length of the engine/gearbox assembly. I had to move the gearbox as close to the engine as possible.  I made engine plates to mount the engine and the gearbox together and then drop the whole lot into the frame. I fitted a different chaincase from a unit-construction engine which means fitting a chain tensioner from a unit engine (as fitted to 650 and 500 unit twins), rather than adjusting the primary chain by moving the gearbox back or forth.  I’ve done away with the magneto and put electronic ignition on the end of the crank, which also meant I was able to cut the timing chest down. Ignition is Electrex World, exactly the same as the unit on the 650 scrambler I built before. I had to modify it to make it fit because them come on a different backing plate to what would suit this engine. 

What are the tech specs of the engine internals? 

It’s got a lightweight crank, T140 750 conrods, 650 pistons. I’ve created a short-stroke motor.  It’s got 3134 cams. The cylinder head and valve gear are all standard. 

So with a shorter stroke, it revs more freely. Don’t you need more grunt for dirt?

Maybe, but this project is purely for me to experiment with different ideas. It does rev a little harder but if something doesn’t work, then I can always go back and trying something else. It’s not about using this bike in serious competition. It’s about building something that very different to try different ideas. A bike I can have fun on a few times a year, riding with a bunch of mates. But not racing. Anyway, I like to experiment. Have you ever seen a Triumph like this before? You won’t have. It is, as it says exactly on the tank, number 01, the only one. 

How much shorter is the engine compared to a stock pre-unit Triumph?

I haven’t measured it. I just moved the gearbox as close to the back of the engine as it would if I had to file a little bit of here and a little bit of there.  The gearbox cover was shaved – well, carved – to get it as tight as possible to the crankcases and still be able to remove the gearbox casing screws. 

What frame mods have you made to be able to fit the engine into the unit-construction frame?

I made my own engine brackets but the only modification to the frame is welding an engine mounting bracket to the frame to allow me to bolt the back of the engine to the frame. The front engine plates of the unit-construction 1970 model are different. On earlier models the brackets were welded to the frame and you fit the engine casing between the bracket. On the later models, like this frame, you can take the bracket off the frame.

What about the suspension components?

The forks are off a big-capacity Yamaha – an XT600 maybe? I’m not 100% sure.  I had to shorten them, by around 60mm because they were too long. Kawasaki motocross wheels front and rear. Disc is off a Kawasaki. Marzzochi yokes, modified. The forks are 36mm stanchions but measure 35mm in the yokes. The standard forks were 36, waisted to 35mm, then 36mm at the tops. But then I cut the top part of the forks off because they were too long, so, where they bolt into the top yoke, they are 35mm.

How did you determine fork length?

I’ve got Marzocchis on my other scrambler so I copied the length from them. I’ve used 13” gas shocks from In Motion. 

Where did you get the air cleaner – it looks like the kind of thing they used to run on the desert sleds back in the day?

I make my own air filters. Got the same style on my more standard 650 Triumph. Air filter is off a new Triumph triple with an airbox of my own design. Oil tank is off a pre-unit. I made my own seat. The base was originally a Triumph seat, cut down shorter to suit and recovered. Brake lever is different too because of the hydraulic discs. I made the rear brake lever. I just can’t wait to get out and ride it now.