The Honda CB500T Engine
 

AS you may well know the CB500T inherited its fundamental engine design from the CB450, launched on an unsuspecting world in 1965.
As such the 500T retained the novel torsion bar valve spring arrangement used in Honda's V12 Formula 1 racing engines.

A torsion bar is a straight length of spring wire which is subject only to twisting. Most coil springs are actually long torsion bars wound into a coil to conserve space. When a coil spring is compressed, the wire is twisted and it is the untwisting which causes the spring to extend again. Torsion bar valve springs are not subject to surge. The inertia of a high speed valve coil spring tends to resist the reversal of motion and acts against the twisting, leading to breakage. Most modern engines use dual springs stacked inside one another to avoid this.
Helmut Fath developed special very thick coil valve springs for his URS 4 cylinder engine to avoid surge.

   Valve gear schematic
       
 

THIS isometric view may help you to understand how the system works.
The torsion bar anchor has the torsion bar splined into it, and the anchor is bolted to the cylinder head. Thus this end of the torsion bar is fixed. The opposite end of the torsion bar is splined into the torsion bar sleeve which is able to rotate in the cylinder head. Splined onto the outside of the sleeve is the extension lever which operates the valve.
So effectively torsion bar, sleeve and lever are all one unit.
The cam follower pushes down on the valve which in turn, via the collets, deflects the lever and rotates the sleeve which twists the torsion bar around its fixed end in the anchor.

There you are, clear as mud!

   The arm which turns the sleeve which twists the torsion bar!
 

SOME people find it hard to imagine that the torsion bar will not snap. In fact they are very reliable, and a valve spring compressor is not required to strip the head.
However; I know a retired Honda mechanic who actually bought a new CB500T. He related a story to me about a 500T his shop sold which did in fact break a torsion bar.

 

On instructions from Honda, the engine was boxed up and shipped back to Japan without being stripped. A new engine was supplied for the customer.
This the only report I have ever heard of a torsion bar failure.
 

 

OFTEN maligned as the longest cam chain ever fitted to a bike, and the source of problems if not maintained correctly. As for being excessively long, what about the Kawasaki and Yamaha DOHC750 twins of the 70's?

As you can see there are lots of guide rollers plus the tensioner to stop things thrashing around.

The first sign of a loose chain is excessive mechanical noise, don't ignore this as the rollers get chewed up pretty quick and then the chain goes for the cylinder head. The result is lots of bits of plastic and aluminium swarf in the oil and these choke up the oil filter screen with disastrous results.

Incidentally, if you drop the cam chain into the bottom of the engine, don't despair. Because of the oil scraper tray under the crank. It can't actually drop far and can be fished out with a bit of wire. That is, assuming the barrels are off.

   Soure of many disparaging remarks
 

 

IN common with all previous Honda twins the CB500T used an 180° crank, as opposed to the traditional British 360° setup. Bear in mind that a 4-stroke cylinder fires once every 2 revolutions (720°) of the crank.

In a 360° twin both pistons move in step and reach top dead centre (TDC) etc together. One cylinder fires and then one revolution later (360°) the other does likewise. The problem with this setup is vibration, due to those two pistons flying up and down together (I'm not going to go into rocking couples and engine balancing etc here).

As you've probably guessed, in the 180° twin when one piston is at TDC the other is at bottom dead centre (BDC). Thus when one cylinder fires the other is 180° behind. Now 720-180=540, so the first cylinder does not fire again until 540° later.
This arrangement gives fairly smooth (its a parallel twin don't forget) running at higher revs and a more V-twin di-doom, di-doom sound. It does however need heavier flywheels to prevent stalling at tickover, and tends to encourage transmission snatch at low revs.

  

 

There are other options of course. Later Yamaha TDM and TRX models have cranks 270° out of step giving maximum intervals of 450° between firing. This was done to give the engine a bit more of that V-twin sound and some 'character.
Phil Irving, designer of the famous Vincent V-twin engine, postulated that (as I recall) 67.5° (or was it 76.5°) is the ideal crank setting for a parallel twin. I can't remember why, sorry!