BIANCHI 125 REBUILD
Stu Thomson has been rebuilding a 1960 Bianchi Bernina lightweight. Now he needs an unobtainable, obsolete bearing, and has to refurbish the forks. Lateral thinking pays off…
Stu Thomson has been rebuilding a 1960 Bianchi Bernina lightweight. Now he needs an unobtainable, obsolete bearing, and has to refurbish the forks. Lateral thinking pays off…
All the engine bearings were replaced. Most were standard sizes from Simply Bearings, but one bearing in the gearbox is very special. It is a very narrow bearing with a large outside diameter and a small inside diameter. The old one was rusty and could not be re-used. No one could supply this apparently obsolete size. So after some lateral thinking I bought a standard bearing of the correct outside diameter and modified it.
I ground it to the correct width, removing the same amount of material from both sides and making a sleeve for the inner bore from EN24 steel. The grinding was carried out using my trusty Dremel and the even trustier Myford lathe. Then I washed out the bearing several times with solvent before blowing it clean with the air gun and making sure it ran cleanly.
The rest of the engine went together very easily, with newly made phosphor-bronze valve guides to suit the existing valves which had no discernible wear on their stems. New pushrod assister springs were made by Alberta Springs from patterns I supplied. These fitted perfectly: a great company with old style workmanship and values.
Gaskets were a little bit of an issue. They are available but quite expensive so I made my own, including the cylinder head gasket from alloy sandwich gasket material. To cut this and get a good finish on the cut edges, the material was clamped between two pieces of plywood then the pattern cut out using drills and a fine tooth jigsaw.
Torque settings were used relative to the stud diameters from online data and info from various manuals.
There were some other mods and components fabricated along the way, including a stainless steel inlet tract, exhaust collar and a new engine breather filter assembly from aluminium alloy. That inlet tract
is very long with cooling fins; I’ve never seen another small bike with a tract so extended. Maybe this is to improve engine torque?
After rebuilding, the engine was filled with 20/50 mineral oil and turned over several times with the kickstart to ensure oil was getting to the valvegear. There was a very good flow, so the oil pressure to the big end should also be pretty good.
Next step: onto the cycle parts. A new silencer and exhaust pipe were sourced from Motostoriche Italiane in Italy: great service and quick delivery and the parts nearly fit perfectly (strange how things for old bikes ‘nearly’ fit so often!) I made a stainless support bracket to ensure clearance for the kickstart.
You might remember that I decided to change the size and type of the wheel rims and the bike’s colour scheme. I went for 18” alloy flanged wheel rims with stainless spokes. The rims came from BEB Meccanica in Italy who I’ve used before and have been impressed by the high quality and low price. I got the details for spoke lengths from an English-language Bernina manual which came from the National Motorcycle Museum. These were almost correct and just needed a couple of millimetres adjustment – probably down to the difference between steel and alloy rim profiles, and the UK and Italian market machines had different size front drums. The rims were designed for full-width hubs and they went together very well, with the drilling angles spot on.
I checked the brake shoes and drums which were worn, and probably asbestos. To be on the safe side I sent the shoes off to Villiers Services and gave them the exact drum internal dimensions. The shoes were rapidly returned and fitted perfectly; no drag and instant contact on the lever. Great service.
I rebuilt the forks with new bushes of alloy and phosphor bronze made on the lathe. I considered having the fork legs ground and chromed but this is very expensive at £180 or more. So I came up with a cost effective solution. The chromed fork legs were true with no bends and maintain their diameter and wall thickness for their full length. The top has a thread cut into it, while the lower end has circlip grooves to hold the bearings / damping valves, and a couple of oil flow holes. I turned them upside down, cut a 1mm pitch thread into one end and circlip grooves in the other. This provided almost new chrome running surfaces at the lower end. I also made new top nuts from stainless with threads to match the tubes as the existing ones were well-chewed alloy. When I tried
them with oil in, the damping functioned OK even though the lower diameter was slightly larger and threaded.
Like other small Italian bikes, there was no easy way to drain the fork oil. I really can’t fathom why this small detail was missed as the stanchion sealing is not brilliant from new. Being able to drain and refill the forks easily would have extended fork life and seals dramatically. So I added small bosses and drain screws. I also made a stainless wheel spindle, spacers and nut. Where the front mudguard stays had been welded to the lower fork legs, a repair was required on the sliders. I also needed to make new mudguard stays to fit to the mudguard.
At the back of the bike I thought I might be able to rebuild the old shocks, but once they’d been removed I realised they were different makes and lengths! That can’t have improved the handling. I found some good quality lightweight shocks for £40 from Race and Rally Spares. These were the same as the ones I used on my Morini 125. They’ve done
2000km and give a firm and well damped ride. So I bought some more for the Bianchi and made stainless bushes to go in the rubbers.
The swinging arm bearings were good with virtually no wear. That’s probably because there are grease nipples at both ends of the hardened shaft and plenty of through-ways for the grease. The pressed steel plate that held the rear sprocket in place was very damaged, so I made a new one from alloy with counter bores for the bolt heads. These were uprated to 7mm diameter from the 6 that had failed to keep everything secure before. The thin-gauge mild-steel footrest hangers were very bent so I made some alloy / stainless replacements. The rear wheel adjusters seemed a very strange design so I made some new ones, again from alloy / stainless.
This bike would have minimal chromed parts: the front fork seal housings, carb filter grille and petrol tank cap. I used a local company but was not really impressed with the finished articles. I think they normally handle industrial chroming onto new, pristine parts, not bits from old bikes that need good preparation.
Most of the metal areas on the frame and chassis components were in reasonable condition with only light rust where the paint was thin. There were several dints, bumps and repairs needed, new front mudguard stays. The frame showed quite a lot of weld spatter so I ground this away before coating – the Bianchi’s welding wasn’t to the same standard as other Italian bikes I’ve rebuilt. My Mival has a much higher quality frame with cast lugs and brazing.
The Bianchi’s headlamp had several holes surplus to requirements, filled by making small discs and brazing them into place. It boasted four switches in the headlamp, all serving different purposes. Normally there is one multifunction switch…
After these repairs, the frame, mudguards and other chassis bits were powdercoated jet black. The finish was so good that little needed to be painted with 2K – just the front mudguard with its white flashes, the petrol tank with its complex ivory artwork and the sidepanels which need the Bianchi Bernina transfers applying and then lacquering.