Bikes | Features | Events | Books | Tech | Magazine | About | Messages | Classified | Links

more bike profiles...

Bike Profile

1966 Triumph T120 Bonneville
Home -> Bikes -> Road Tests and Profiles ->

John Craig is a professional mechanical engineer and a classic bike enthusiast who likes a challenge. ĎA challengeí is just what his written-off Bonnie turned out to be...

The bikeís basically a 1966 T120 Bonnie which I bought as a write-off back in 1983, when I was at university doing my Mechanical Engineering degree. It was rebuilt on a shoestring; money was pretty tight. It wasnít too bad in the main, although the front end was totalled. This didnít worry me too much, as I didnít much rate the Triumph forks! There was a bit of other damage too. The frame was OK, as was the tank and most other major bits.

Less is more 'The bikeís basically a 1966 T120 Bonnie...' Basically.

Less is moreI had (most of) the parts for a set of Norton Roadholder forks, so after some measuring I decided to use them. Of course, they had the usual cracked pinch-bolt lug problem, so I converted them to an end-cap arrangement by some careful welding and machining.

I also adjusted the length of the fork damper rods, as I found that they would top-out at full extension (so the pistons would hit the top of the damper tubes before the forks hydraulically stop as the bushes approach each other at full extension). I suspect this might be an original design error, or it could just be the result of some mix and match of parts over the years. However the forks were built with new hard-chromed Ďshortí stanchions and the rods hadnít been cut, so original error seems most likely. Norton owners might want to take note!

A little judicious welding and machining allowed me to fit the Norton yokes to the Triumph headstock (using standard Triumph bearings), and just for fun I machined up some fancy stainless steel top nuts too.

The clip-ons came from an RD250 Yamaha which had met an untimely end, and the handlebar levers, switches and throttle are of indeterminate origin, although I think theyíre probably from an early Honda.

You can't beat a bit of assymetry Yamaha bars, Honda clip-ons, British soul...

The front wheel came to me as a trade, in payment for some design and machining work I did for a friend; it needed a new set of bearings and brake shoes and a quick true-up and spoke tension. Itís an early Commando TLS brake and is extremely powerful when properly set-up, no matter what anyone says. The front wheel locked in the dry at over 70 on one panic-stricken occasion!

the more holes the better Early Commando TLS brake

I completely stripped the engine and gearbox, as I didnít know the history of the bike. This was just as well, as the drive side big end had just started to pick up due to lack of oil; explained by the state of the sludge trap tube in the crank. It was packed so full and hard with rubbish which had been centrifuged out of the oil that I had to drill the muck out before I could get the tube out!

Obviously there had been very little oil getting through for a long time so at this point I decided that a proper filter system was needed. I designed the system which is currently fitted to the bike, and has since been applied to quite a few others. Itís a full-flow full-pressure system which uses a car-type filter, arranged so that the filter lies between the pump and the crankshaft, where it should be. The filter is mounted under the swinging arm, out of harmís way.

Filter tip visible. The filter is mounted under the swinging arm, out of harmís way

A bonus is that itís now safe to use conventional multigrade oil (I use Duckhams 20/50), because thereís a proper filter to catch the dirt that modern high detergent oils wash out of your engine. The engine internals stay much cleaner too. This is the reason why low detergent monogrades are recommended for old bikes; itís not that the oilís better (itís not), itís because the engines donít have decent filters!

The crank was re-ground and rebuilt without the sludge tube (no longer required); the conrods were crack-tested and new bronze small end bushes made and fitted; bores and pistons were measured and found ok, so were just fitted with new rings; compression ratio is quite high enough!

The crankcases and head had the usual quota of stripped threads, scored faces and so on. As the faces tend to be quite narrow on most of the casings, I repaired the stripped threads by making aluminium inserts and fitting them to bring the holes back to original size. Doing it this way takes a bit longer than helicoiling, but means that the insert can be taken back fully flush with the face to give the maximum sealing area. Scored faces were carefully welded and then taken back by hand. It seems to have worked: it doesnít leak.

The cylinder head needed quite a bit of work; apart from the stripped threads etc, (repaired the same way), the head was also slightly warped, so I gave it a light skim to true it up (which also increased the compression ratio slightly). The exhaust stubs had worked loose and destroyed the threads in the head, and worn the ports oval (a common Triumph problem of this era). There wasnít enough material for an effective weld repair, so the ports were bored out to make them circular, stubs were machined and fitted to the pipes, and T-bolts were made and fitted to the head. These engage with collars (cooling rings) on the pipes and have completely solved the problem.

T-bolts engage with cooling fin collars.

The valve gear was stripped and checked; valves were dimensionally good and only needed to be ground. I made new valve guides and fitted them, threw away the Thackeray washers, ground the ends of the rockers square to remove all the digs and scoring caused by the washers, then reassembled the rockers with their shafts, measured it all up and machined up sets of shims to control the end float; and put it all back together. Itís never needed any attention since then and Iíve subsequently done this to quite a few bikes, both British and Japanese.

After checking and repairing or replacing the rest of the parts as necessary (a little bit of time spent with a fine file on the clutch basket works wonders for the gearchange), the engine/gearbox unit was carefully reassembled with new bearings, seals etc, and a Morgo high capacity oil pump was fitted. Particular attention was given to fitting and assembly at this stage, as care taken here pays dividends in the future; demonstrated by the fact that the engineís never been out again since it was built.

The crankcase breathing arrangement was revised to include a breather taken from the Ďtiming portí at the back of the cylinder block; this links to the timed engine breather and oil tank breather, and also provides some oil mist lubrication to the chain. Some time was spent Ďbalancingí this system, which was well worthwhile.

The crankcase breathing arrangement was revised to include a breather taken from the Ďtiming portí at the back of the cylinder block.

Previous experience of Lucas alternators convinced me that the crankshaft mounting wasnít the best idea: along with everyone else, Iíve seen plenty of them come loose and make a bid for freedom! I decided that the problem was due to the loose parallel fit of the rotor on the crankshaft nose, with drive by a parallel key: the inertia of the rotor combined with sudden acceleration/deceleration is bound to make it slacken over time. I made and fitted drive pegs to the engine sprocket, and drilled a couple fairly shallow flat Ė bottomed holes in the back of the rotor, which the drive pegs engage in. Now the sprocket drives the rotor and the shaft only locates it, and nothing ever comes loose.

I revised the ignition system by doing away with the Lucas contact breakers and coils (one coil was dud), and fitted a Boyer Bransden kit, which drives a pair of Honda C90 6V coils which are wired in series. This system has been 100% reliable over the years, and never needs any attention.

The carburettors were completely worn out, and were replaced with a (less worn) set; I bored the bodies on the lathe and sleeved the slides in brass, which has worked well, although it was a time consuming job.

The exhaust is a stainless steel Siamese system, with a Ďstandardí Triumph silencer from Armours. This was chosen to boost mid-range power as well as to reduce weight, and seems to be effective when compared to bikes which use the normal full system. It was cheaper too, and sounds good!

The frame was checked for damage and found to be ok, although the swinging arm bushes were worn out (probably due to lack of greasing). I made new bushes and fitted them. I then dropped the engine unit in the frame, and made sure everything lined up exactly; reaming through the mounting bolt holes meant that all the bolts could be made a tight fit, which helps to prevent slackening Ė off caused by vibration. I also used ĎAerotightí nuts wherever possible.

The frame was cleaned up by hand (a long job), and brushĖpainted using Woolworths polyurethane paint, which has proved to be highly durable! The tank was rubbed back by hand and painted with Halfords aerosols, and it has lasted well too. Nowadays there are better paint options, but I doubt if thereís anything cheaper.

Like everything else, the back wheel got new (sealed) bearings; it also needed a couple of new spokes and to be trued up. The QD sprocket/brake drum had broken teeth (probably due to a slack chain), so was replaced with a new one. The rear shocks were shot, and were replaced with a set of Marzocchis from a T140; these have since been replaced (they were worn out) by a set of Hagons.

Mudguards are period accessory alloys with self-made brackets, stays, etc; some time was spent making rubber mounts for vibration isolation, as otherwise these crack double-quick. The same attention was given to fuel and oil tank mounts, battery carrier, and so on. Strangely enough, the headlamp unit seems to be immune, although itís not rubber mounted (itís the original shell fitted with the halogen light unit from a Honda CX500 Ė so you can see in the darkÖ)

The original speedo and rev counter were destroyed in the crash that wrote the bike off, so I fitted a Smiths Chronometric speedo of unknown origin, recalibrated to read correctly, and an oil pressure gauge. Thereís no rev counter fitted, the approved technique (so far) is to rev it till the valves bounce then change up Ė itís taken it ok up to now!

The bike runs on Super Unleaded with a lead replacement additive, engine oil is Duckhams Q20-50, gearbox oil is Castrol GP50 (never use EP or Ďhypoidí oil in an old British gearbox, it will eat the bushes), and the primary drive uses Automatic Transmission Fluid (ATF), which improves the clutch action and hence the gearchange.

Bonneville stuff on

Of course, there are a thousand other little jobs which went into the bike, like making a new wiring harness, making up cables and so on; but it was great fun, and Iíve ended up with a bike thatís given me lots of pleasure.

Itís taken me to lots of places and I can thrash it to within an inch of both our lives. What more could you ask from a £250 wreck? Iíll certainly never sell it!


Need Help?

If your classic motorcycle needs this kind of attention then John Craig is happy to help. He runs Johns Engineering Ltd and will take on any motorcycle-related job for the restoration of classic, vintage and veteran motorcycles including engine, gearbox, frame and suspension repair; one-off designs, performance / racing-quality components, manufacture of special purpose parts, plus universal upgrades such as full-flow full-pressure oil filter systems and fully shimmed valve gear:


Interesting BSAs on Right Now...


Like what you see here? Then help to make even better

Bikes | Features | Events | Books | Tech | Magazine | About | Messages | Classified | Links

More Bike Profiles...

RedLeg Interactive Media

© 2002 The Cosmic Motorcycle Co. Ltd / Redleg Interactive Media

You may download pages from this site for your private use. No other reproduction, re-publication, re-transmission or other re-distribution of any part of this site in any medium is permitted except with the written consent of the copyright owner or in accordance with the provisions of the Copyright, Designs and Patents Act 1988.