BIANCHI 125 REBUILD 78
Stu Thomson bought a 1960 Bernina 125 from an Italian collector, and set about its restoration. This month he explores inside the engine…
Stu Thomson bought a 1960 Bernina 125 from an Italian collector, and set about its restoration. This month he explores inside the engine…
Parts for the Bianchi were quite difficult to source as the machine seems to be fairly rare – even though quite a number were made and imported into the UK after 1960. It may have been a popular model in the day but there do not seem to be many around now… unless they are all squirreled away in Italian and UK sheds!
As the Bernina was an official UK model, that meant I could find a BMS manual in the National Motorcycle Museum shop. So If I wanted to junk the rusty rims and spokes and get new 18” alloys with stainless spokes, the spoke lengths were conveniently listed. At least, they were for the rear; the front would be slightly different as the brake drum was a different size.
This manual showed that the differences were not just with the wheels and front brake, but also with the front forks. The UK models had internal springs and the Italian version has springs outside the stanchions. The headstock is a little different, with a cast alloy cover for the British version. It also showed a speedo where mine had none and not even the drive for one. A speedo wasn’t a requirement in Italy for a 125. If I could find the parts I would fit the drive and speedo.
The engine has some interesting and unusual features. It uses a dry clutch like some Ducatis. The inlet shaft to the gearbox also doubles as the camshaft for the valve operation and the points cam / advance mechanism. This is quite clever: it saves a shaft and a gear, but it means that this camshaft and the pushrods are at the rear of the cylinder and quite high in the gearbox, situated either side of the inlet from the carburettor. This was very unusual for designs of that time. Most bikes had the pushrods at the side of the cylinder, driven from a camshaft geared to the crank.
It’s also unusual for an Italian machine
because the kickstart is on the right – like British bikes – on the same side as the gearchange, and the drive chain is on the left. The whole mixture of assemblies is quite complex, especially considering the clearance required for the exhaust pipe, silencer and crossover brake linkage.
This design also means that the input shaft into the gearbox rotates at exactly half crank speed. But that the clutch is mounted on the output shaft from the gearbox, cantilevered out in open air. So the clutch operates at the same rotational speed as the rear drive sprocket. I am still trying to work out whether this is a good thing or not – a clutch normally operates at gearbox input speed, not output speed.
It’s also possible to turn the engine over while in gear if you disconnect the clutch. Trials riding practice, I think!
I also found that the rear drive sprocket is welded onto the back of the clutch drum and not splined onto a shaft with a nut. No sprocket nuts to come undone here! But complex when the sprocket wears out. Bianchi certainly wanted to be different with this bike.
The motor was seized solid, which meant a complete strip and rebuild. The piston itself had seized. It came free quite easily with a bit of releasing oil and heat, but a rebore and new piston would be needed.
My first job was to mark up and remove the drive / timing pinion to the gearbox: easy. The second job was remove the double pinion from the crank: grrrr! This was really difficult to remove from the taper. I made a special extractor on my milling machine from 10mm bar and promptly bent it. I made another, slightly thicker, and applied some heat and releasing oil. Eventually it came off with a bang – apparently other people have had the similar problems with theirs.
Under here the oil is pumped from the sump to a bush on the primary drive side of the crankshaft, which has a drilling to take the oil to the plain bush big end. It’s a very similar feed in principle to some of the unit Triumphs, except there is a big ball bearing as well. Is this really only a 125? After feeding the crank big end and this bearing, the remaining oil is pumped up around one of the cylinder studs to the cylinder head rocker supports. Any other oil is pumped out of the bush to the main bearing of the crank and the primary drive.
There are some great aspects of this design. The oil from the cylinder head comes back down the pushrod tunnels and straight onto the top of the gearbox pinions and camshaft via a cast-in deflector plate. That may be why the gearbox is in a very good state. The primary gears are in a chamber separate to the kickstart and gearchange shafts, sealed by a gasket. So once the oil from the main bearing overflow flows into the chamber, it rises until the bearing of the primary shaft, where it flows back through into the gearbox. This effectively means that the primary side runs in a permanent oil bath which is
gradually replenished.
Due to the location of the camshaft, the pushrods are at the rear of the cylinder. With the centrally-placed valves, the spark plug is situated between the valves at the front, unfortunately directly in the line of debris from the front wheel. On the positive side, it’s well ventilated.
Inside the engine all the wearing items needed to be replaced: bearings, big and small ends, seals and bushes – and the pushrod return springs which seemed rather tired. As with many old four-strokes, the engine oil was contaminated. It was around 50% water, but corrosion to the gears and crank was minimal in spite of this. Maybe because the gearbox is positioned very high in the case; with a deep sump the water had sunk to the bottom leaving the oil above it. When I got the Bianchi, the breather hole was plugged with a cork, probably inserted after the water had entered. Or maybe it was wine in the sump, not water!
The big end had obviously seen better days as there was about 1mm of play. It uses a bronze bush, not rollers, so must have rattled a bit up until the bike finally died. I sourced an NOS big end from Italy, complete with pin, conrod and little end. Then I found out
that they are a stock Item at Alpha Bearings in the UK! It pays to check and not make assumptions about availability.
The pin in the big end is parallel, so there is no scope for grinding and fitting an oversize bush. The crank was disassembled, reassembled and aligned by a local engineering firm. I don’t have an hydraulic press otherwise I would have had a go myself. I got an NOS oversize piston and rings from King of Piston in Italy, and the local engineers rebored the cylinder for a very reasonable £25 per cylinder. The piston was to the original design with four piston rings; two compression and two oil control rings.