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Yamaha SR500 in 1991 Honda VT250 Spada frame

By Matthew Jacomos fourellowe@hotmail.com

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I bought the bike in 1988, from the original owner. I liked it except it was slow and the pegs ground easily, so the mods started.

I had a tuner go over the engine, ported head, wiseco piston, balanced crank, heavy duty clutch springs, rod kit and a hotter cam. A 38mm roundslide Mikuni helped , but at the cost of the power jet from the stock carb.

As to the chassis, the pillion pegs were removed and replaced with Honda pegs , rearset. The bars were swapped for clip-ons and I motored around for a while.

Next on the agenda was doing something about the brakes. A second disc helped, but the combination of 19" "malleable iron" rims and two solid rotors gave me the sort of front end stability of a gyroscope! Anyway, 19" front ends belong on 70's bikes, so I hunted around the country to get suitable hubs.
I found a TZ rear hub to suit my requirements for a disc brake wire wheel, with a cush drive.

Singles need cush drive rear wheels to ease the shocks of the power pulses.

At that stage of development an alloy tank came onto the scene, and I took it onto the track in club level racing.
The faster circuits left me in the wake of the twins and 1000cc fours, there was only one other single racing in the state then.
I had fun and the bike never let me down, we even won a sub class!
(401-600cc improved production).

In 1995 I bought a CBR600 and the thumper sat patiently in the shed.
I always thought about making a special, but had heard many stories about SOS racers cracking their chassis from vibrations.

By 1997 I could not afford to loose any more points on my license (for speeding) and I sold the CBR.

1999 and I was looking at 250cc four stroke twins with envy. I really needed to ride!

The bike shop near my work got a Honda VT 250cc Spada as a trade, and I spent a couple lunches staring at it.

The Spada was released in 1989 in Australia. It had a unique cast aluminium frame and some beefy 37mm forks.

I liked it when it came out and more importantly, it looked like it would fit the SR engine.
After all, a v-twin is really only a wide single.
A cast frame would hopefully deal with the vibes better than an extruded frame. The bike is tiny too!
The spare parts dept guys said the Spada had a cush drive rear wheel. That sealed the deal.

I bought a rolling chassis and enlisted the lads to help check the clearances.
We balanced the engine on paint cans and stuff, blocked the bike up and it looked like it would "just" work.

Hanging the engine in the chassis on wires, the serious design work began.

Little things can trip you up. But this combination had a straight line between the rear axel, the swing arm pivot and the countershaft sprocket. The line up between the rear sprocket and front sprocket looked close. There was room above the head for the dry sump, where the v-twin's down draft carbs went. This would let me use the stock battery box and it's always easier to use as much stock stuff as possible.
The total cost of the dry sump was $2 for mild steel and $25 for a TT600 tank for fittings. The tank was fusion welded and the fitting were bronzed. Oil lines were a bone of contention. Spend hundreds on braided stainless, or would the trans cooler lines from an automotive supplier do. On the advice of a guy who put an SR engine in his DR Suzuki, I went for the cheaper option.

Months went by, metal was cut and welded. I was fortunate to have an associate who had previously mounted an RD400 into a TZR. We kept the "Spadaha" (as it became to be called)

All engine mounts are "triangulated", they cannot rotate or slip. Force is constrained in three dimensions for each mount. (I hope you get the idea).

Mild steel tube, steel bar, and alloy spacers were used to form the mounts.

Making individual mounts is fine, but when the engine was installed the clearances swiftly reduced. There is a window of opportunity to fit the engine, with mounts attached. Though the mounts all fitted in situ, the engine with mounts attached would not fit into the chassis.

I ended up having the lower mount cut by spark erosion, which is pretty neat. the "sectioned" mount was kept in line via a steel cylindrical spacer.

Even so, with thru bolts installed in the mounts that HAD to be on the motor, the resistance was fierce. This thing is one TIGHT piece of work....

After finishing the work, apon discovering this site, I emailed another Aussie who had put an XL into an RGV chassis. He commented that the "chassis cracking" anecdotes were not applicable to well engineered mounts. that is, the better the engineering on the conversion, the less chance of cracking the chassis. His engine was solid mounted too.

Much struggle and a slipped disc later, the donk was in. My associate firmly stated the engine would never ever come out!

Well, that is fine in theory.....

There is about 5mm clearance between the engine and the frame, you can't get the RH sidecover off while the engine is in the chassis, and it is remarkable that when you've got all the bolts in the months prior to putting the engine in the chassis (because of clearances) that there is such a tiny "window" to fit the thing in.

In fact I found out the hard way that it is easier to get the engine into the chassis with the head off!

You could not remove the head in the standard chassis, without removing the motor, and valve adjustment access is easier too.(until the dry sump goes in)

I had to replace a bent valve 18mths ago. I bent the valve in 1990 when I put a fresh set of rings in the engine (didn't need them) and put the wiseco in around the wrong way.

The SR has a much larger inlet valve than exhaust valve and the flycut on the piston interfered by 1.5mm, bending the valve slightly on start-up. A mate's father straightened the valve in the pits at Mallala and it ran fine for years.

Once I decided to do the special, I decided to rid myself of the nagging doubt of the once bent valve (didn't need to) , and replaced it.

Because I'd stuffed up before, I really concentrated on everything I did, but being (as me mates say) an "all thumbs bum", I either neglected to properly tension the cam gear locking bolt (1off) or neglected to loctite it. (no loctite recommended in the manual, and Yamaha now sells a lock tab washer for this job.)

Anyway , after 18mths conversion I started it up and it was not too bad in the vibes dept.

I'd previously had the crank balanced for 7,500rpm.

On the inaugural ride through the Adelaide Hills the vibes were so bad my feet tingled after 40km. after 50km it stopped.

Originally I thought it was the stator, as they have a tendency to go if the bike has sat for a while.

Mates thought the dry sump was the problem, as it does not incorporate a swirl pot type design.

Funnily enough they had seen the sump before start-up and not raised the issue then!

The static test on the stator went fine so I pulled the valve covers off and as I turned the crank, the cam didn't turn!

The cam drive gear lock bolt was loose, it had snapped it's locating pin and destroyed the spigot.

Bugga!

At least there was plenty of oil in the motor!

With the cam whipping back and fourth without the drive cog pin engaged no wonder my hands and feet were tingling!

That's when I found if you pull the head off the engine is dead easy to get into and out of the chassis!

This weekend I'm splitting the cases to get rid of the shrapnel from the cam drive pin and the cam spigot, under the watchful eye of my mechanic buddy. I've got a 2nd hand cam and all the parts on order to put it together again. Should be running in a fortnight.

the moral is "If it ain't broke, DON'T FIX IT!"

So, the debris trail went from the cam spigot, straight down into the sump. A steel mesh screen stopped most all of the bits from entering the engine, and the strip down was an anti-climax. But better to be reassured, than having a MAJOR catastrophe.

The bike is running again and , whilst I'm ultra cautious, I think I'm going to enjoy riding this puppy. The stock cam makes her much smoother, and easier to start. I've gone back to 16:39 gearing on a 520 chain.

Push starting is a bitch, but I'm getting my technique down (sort of). the only bitch is starting, then getting dressed for the ride. If you put jeans or leathers on, you can bet the bitch won't fire! conversely, once it starts the thing has a tendency to "walk" due to the vibrations of a thumper at idle.
I recall looking out at the bike walking backwards down the footpath as I struggle to get into my leathers! (now there's an image!)

It's not a race bike, it's not an everyday ride bike, but it is kinda interesting and I'm looking forward to some track days in the near future!