Rebuilding Triumph T20B Bantam Cubs
Rebuilding & restoring the Bantam Cub variant of the Triumph Tiger Cub
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Bantam Cubs are very simple machines by today's standards and restoring them is pretty straightforward. However, because they're relatively rare models of Tiger Cub it can be difficult to identify and source some of the components, especially if you're aiming for an accurate restoration. The notes below are based on my personal experiences and the research I carried out while rebuilding my own bike. I hope you will find them useful. If you spot any errors or have anything to add then please post a message on the Tiger Cub Forum and I'll get in contact with you. Thanks. |
 
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The BSA Influence
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When I started to rebuild my 1967 Triumph Bantam Cub I didn't realise at first just how significant BSA was with these models. Although branded a Triumph, virtually everything on the bike apart from the engine and tank badges is BSA! The T20B Bantam Cubs and Super Cubs were based on the BSA Bantam D7 and D10 respectively. With the exception of the petrol tank, the front mudguard and the wheels, the two T20B models are almost identical to each other. Both models are also very similar to the Bantam D14 and the B175. A few parts were either taken from other BSA models or are specific to the T20B. For example, the oil tank was from the BSA C15 and the petrol tanks were unique. So, other than the engine, when you're searching for original parts, think BSA! |
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Before You Start
Before starting a rebuild I would recommend these publications, especially the three on the left - they really are essential.
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The Tiger Cub Bible, by Mike Estall - a really useful reference book covering all Cub models. |
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Roebuck Zeus Data Charts - a booklet with engineering data that includes useful thread details. It does not cover all types of thread used on Cubs, but is still very helpful when trying to identify nuts, bolts and screws. |
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A copy of the original Triumph Parts Book for the Bantam Cub & Super Cub - these are readily available via websites like eBay and from suppliers of Tiger Cub parts. |
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The Haynes Manual for the Tiger Cub - this does not give much detail that is specific to the T20B, but it does have useful information and pictures of the Cub engine assembly. |
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Mike Estall's Consolidated Tiger Cub Parts List - this gives useful sizes and references not covered in the Parts Book. For further information and dating on all Triumph Tiger Cub and Terrier models contact Mike Estall on mikeestall@hotmail.com. |
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The Haynes Manual for the BSA Bantam - a thin book that covers more than a dozen models so it's a bit light on detail. Also, the only sections relating to T20B are the frame, forks, wheels, brakes and electrics. Personally I didn't find it a lot of help, but there are a few useful specifications. |
How Bantam Cubs Differ
Both the Triumph Tiger Cub and the BSA Bantam are well documented elsewhere so I've just summarised below the main areas where the Bantam Cub and Super Cub differ from the more common models.
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Triumph T20B Bantam Cub, 1967-69 |
Triumph T20B Super Cub, 1966-68 |
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Headlight , Speedo & Switches
As far as I can establish, this particular headlight was only ever fitted to the T20B. It is based on the Lucas MCH56 shell and is chromed. Other versions of the MCH56 shell do exist, but the sizes and positions of the holes for the speedo, switches, etc, seem to be unqiue to this model.
Some late Bantams and a few C15 Sportsman models did have a similar headlight, but these were made by Wipac and have slightly different proportions. The Wipac shell is shorter and looks fatter than the Lucas version and the rim is deeper. Most importantly the Wipac model is made for a larger diameter speedo so they are not interchangeable unless you are willing to swap the speedo and accept that the appearance will not be original.
The T20B had the same Smiths SSM2001 speedo (2-3/8" diameter, 0-80mph) and separate Lucas SA88 ignition and lighting switches as fitted to other late Tiger Cubs. This differs from the Bantams which used Wipac switches and 2-1/2" diameter speedo.
As a consequence you can expect to pay a high price for a good, original T20B headlight!
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Identifying Headlamp Shells - Top View. Left: Correct T20B Lucas headlamp. Right: BSA Wipac headlamp. |
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Identifying Headlamp Shells - Bottom View. Left: Correct T20B headlamp. Right: Similar BSA headlamp. |
For more details of the electrical equipment fitted to Bantam Cubs and Super cubs see the 'Electrical' page.
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Handlebar Controls
The T20B handlebars are standard Bantam D7/D10/D14/B175. However, the levers and twist-grip are different. If you're not concerned about historical detail then any 7/8" levers and twist-grip will do. If you're going for accuracy then these will be difficult to get right. The T20B used an unusual Amal set-up with the brake-lever and twist-grip combined in a single assembly. The clutch lever used a cast bracket that matches.
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A similar set-up was also used on some Ariels - I believe certain Arrows and Leaders were fitted with them. However, most of the Ariels used plain levers while the T20B had ball-end levers. Also, the T20B used the standard 4-3/4" long Amal rubber grips while some Ariels used a shorter, 4" version meaning that, although the bulk of the assembly is identical, the length of the steel throttle tube differs in length. Finally the Ariel parts were usually painted grey while the T20B versions were chromed. Therefore, if you want an authentic restoration it can take a while to track down the right parts. Ariel parts may be used as short-term substitutes while you search for the correct originals. |
T20B controls & similar Ariel controls |
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Identifying Amal combined controls. Top: Correct T20B controls. Bottom: Similar Ariel controls. |
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Identifying Amal combined throttle assemblies. Left: Ariel controls. Right: Correct T20B controls. |
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Identifying Amal levers. Top: Standard Amal Levers - not suitable for T20B These levers are readily available from many suppliers,
but do NOT fit the original T20B set-up. They won't fit the brackets
properly and the cable will be too tight with no slack for adjustment. Middle: T20B Levers Bottom: Ariel Levers - alternative for T20B |
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Petrol Tank
The Bantam D7 and D10 used completely different petrol tanks. The Bantam Cub tank is based on the D7 tank and the Super Cub on the D10, but in both cases the Cub versions of the tanks are different from the original BSA versions to accommodate the different badges. This means that although the tanks are physically interchangeable, if you want the right shape with the right Triumph badge then you'll need the correct tank for your bike - there are no alternatives from other models. These tanks are still available, but are becoming quite rare now so expect to pay a high price for a good one.
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Oil Tank
The Bantam used a two-stroke engine so there was no need for an oil tank. Consequently the Bantam's right-hand side panel was almost a mirror-image of the left-hand battery box cover. The Tiger Cub engine is, of course, four-stroke so an oil tank was needed. BSA used the oil tank from their C15 model which was a 250cc, four-stroke. If you need a replacement you'll find that oil tanks described as 'C15' will be a lot more plentiful (and therefore cheaper) than those with either a 'Bantam Cub' or 'Super Cub' label. They are the same BSA part and are identical in every way.
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Front Mudguards
The Bantam Cub uses a common mudguard fitted to both the standard Tiger Cubs and most Bantams from D7 onwards. It has integral flat brackets that bolt to the fork legs and four identical, straight stays to secure it front and back.
The Super Cub appears to use a Triumph mudguard that was fitted to the Sports Cubs, Scramblers and Mountain Cubs. It also looks very similar to the mudguards used by many of the bigger Triumphs such as the Bonneville and Daytona. It is attached to the forks by three steel hoops.
See pictures in 'Wheels' section below.
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Paintwork
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Bantam Cubs were supplied in a two-tone, blue and white colour scheme - 'Nutley Blue' (*Note below) which is a BSA colour and 'Alaskan White', a widely-used Triumph colour. Super Cubs were finished in a BSA red known as 'Firecracker Red'. The petrol tank was chromed before the top half was painted, leaving the chrome exposed on the bottom half. Both models had gold lining on the front mudguards - see pictures in the 'Wheels' section below.
*Note: Some Triumph publications mention alternative names for the blue used on Bantam Cubs. The Parts Book, mentions 'Perrivale Blue', but I can't trace any evidence of that name being used in production. Also, shouldn't it have been spelt Perivale Blue? A 1966 advert describes the machines as being 'Finished in Pacific blue and white'. Pacific Blue was a colour that was used on some Triumph Twins of that era and it does look like it could have been the same as Nutley Blue. Was Triumph simply re-branding BSA's Nutley Blue to differentiate their models from BSA? Some modern, Hinckley Triumphs are also available in a colour known as Pacific Blue, but this is quite different from the colour used in the 60's and 70's. |
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Wheels
The Bantam Cub uses Bantam D7 wheels. On both the front and rear wheels, the side opposite the brake is smaller in diameter than the brake side meaning the spokes are different lengths on each side. Although the rear wheel looks very similar to standard Cubs, the hub is approxiamtely 10mm wider which means that neither the rear hubs nor the spindles are interchangeable with other Cubs. The front wheel is identical to standard T20 wheels.
The Super Cub uses the Bantam D10/D14/B175 wheels with 'full-width' hubs front and rear. These are the same diameter on both sides with a corrugated centre and they have spokes of the same length on both sides. If you are restoring these hubs then take care as the corrugated sleeve is quite thin and if corroded can be severely damaged by shot-blasting.
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Identifying Front Wheels and Mudguards |
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T20B Bantam Cub Wheel & Mudguard |
T20B Super Cub Wheel & Mudguard |
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Seat
The seat on the T20B is standard Bantam D7/D10/D14/B175. New seat covers and even complete seats are readily available, but are quite expensive. The original T20B seats did not have any logos on them, but new ones generally come with the BSA logo printed in gold at the back. However, this can usually be removed quite easily.
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Footrests
The front footrests on a BSA Bantam sweep up and out from their mounting points on the frame under the engine. However, the Cub engine is larger than the Bantam and so the footrests are different to avoid the outer casings. The T20B footrests come out horozontally and are then stepped up.
The pillion footrests on the T20B were fitted with the standard Bantam rubbers complete with the 'BSA' logo embossed on either side! Note that BSA produced two similar versions of these pillion rubbers for a wide range of different machines. The T20B uses the shorter, 3-1/2" version, not the longer ones that are 4-1/4".
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Engine Mounts
The BSA Bantam engine uses just two engine mounts - one at the front and one at the back. The Cub has a third engine mount underneath the engine. As a result, the T20B frames differ from the standard Bantam frames with the extra, middle engine mount. The two plates used to connect the rear engine mount to the frame are also quite different from the standard Bantam.
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Engine
The engine is almost entirely a standard, late-type, T20 engine with points on the side and the distinctive square barrel and head. The only difference with the main engine components fitted to the T20B appears to be that the engine number is stamped on the left-hand casing below the bottom of the barrel rather than on the front engine mount. (Is this a Small Heath trait perhaps?) Engine numbers all start T20B and would have originally matched the frame number exactly.
There are a few differences with some of the peripheral engine components as detailed below.
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Kickstart
The kickstart on the T20B is very slightly different from the standard Cubs. The difference is almost imperceptible, but it is important. Because the T20B exhaust pipe follows a different route to the standard Cubs (see below) there is a tendency for the kickstart to hit the exhaust. The problem is exacerbated by some manufacturers of replacement exhausts who appear to be using 35mm metric tubing which is marginally bigger than the original imperial size of 1-5/16".
There is therefore a different version of the kickstart for the T20B that curves slightly further out from the engine (part number T1679 for the complete assembly). The difference is small so unless you have a standard kickstart to compare it with, it can be difficult to determine that you have the T20B version.
New, pattern parts are available from some suppliers, but originals are hard to find mainly because they appear so similar to the standard version.
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Carburettor & Air Filter
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Most Cubs used an air filter mounted under the seat next to the oil tank and this was connected to the carb via a rubber tube. However, the Bantam tinware didn't permit this set-up. The T20B instead used a mushroom-shaped filter screwed onto the end of the Amal 375 Monobloc carburettor. Similar mushroom filters were used on several bikes of that era, but they all seem to be bigger and I can't find details of any other model that used this exact part. As you can see from the photo on the right, it's a pretty tight fit and it seems likely the filter was designed specifically for the T20B. The pressed steel bodies are still available, but the internal filter element is now very difficult to find. The version of the Monobloc carb fitted to the T20B had the rim of the intake threaded to take the screw-on filter. This contrasts with the carbs on standard Cubs that had a lip to hold the rubber connecting tube. Numerous versions of the Amal 375 Monobloc were produced, but the specific model to look for is stamped with the number '375/61' on the top of the flange. This has the correct 25/32" (19.8mm) bore and the right thread for the filter which is 1-7/16" (36.5mm) diameter x 24 tpi (threads per inch). Note that there is an error in the original Triumph T20B Parts Book. The carburetter is detailed on pages 52 and 53 and the illustration shows the 'Mixing chamber top' (reference number 6) with a single cable entry hole on a sloping top . This is not accurate as the correct Amal part number 375/064 actually has a square top with two cable entry holes. Also, the parts list does not show either the cable adjusting screw or the blanking screw that fits into the spare hole. These tops were also fitted to 375 carbs on other machines and the Amal drawing opposite almost certainly shows the correct parts that were fitted to the T20B as well. |
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Parts for top of Amal 375/61 carb. |
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Exhaust Pipe
At first glance the T20B exhaust pipe and silencer appear to be similar to the standard Tiger Cub, but there are some subtle differences that mean they are not interchangeable!
The standard Cub exhaust sweeps down under the engine and is tucked in close to the frame; the standard silencer bulges outward from the bike.
However, on the T20B the shape of the frame prevents the exhaust following the same route. The pipe has an outward kink to push it slightly further out and clear of the frame. The silencer looks similar to the standard Cub silencer, but it bulges inward towards the back wheel. Both components are unique to the T20B.
This means that the standard Cub exhaust system cannot be used on a T20B - if you have problems getting your exhaust to fit properly, check you have the correct one.
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T20B |
Standard T20 |
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Wiring Loom & Horn
The wiring loom for both T20B models is very similar to both the Bantam D10 loom and late, standard, T20 Tiger Cub looms. The only significant difference seems to be the location of the horn. The BSA Bantams and the majority of T20 Tiger Cubs had the horn mounted under the seat near the battery, but the T20B horn hangs down under the front of the petrol tank.
See additional infromation on the 'Electrical' page.
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