Rebuilding Triumph T20B Bantam Cubs
Rebuilding & restoring the Bantam Cub variant of the Triumph Tiger Cub
The BSA Influence
When I started the rebuild of my 1967 Bantam Cub I did not at first appreciate the significance of the Bantam aspect. Although branded a Triumph, virtually everything on the bike apart from the engine and tank badges is BSA Bantam! It took me a while to realise that. When you're working on a Bantam Cub or a Super Cub you are primarily dealing with a BSA machine, not a Triumph.
In the case of a Bantam Cub it is a Bantam D7 while a Super Cub is a D10. However, with the exception of the tank, the front mudguard and the wheels, the two models are almost identical to each other. They are also very similar to both the Bantam D14 and the B175. So, other than the engine, when you're searching for parts, think BSA!
Before You Start
Before starting a rebuild I would strongly recommend these publications:
 |
The Tiger Cub Bible, by Mike Estall - a really useful reference book covering all Cub models. |
 |
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. |
 |
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. |
 |
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. |
 |
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. |
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.
To avoid confusion I have used the following terminology to identify
the different models when describing parts:
'T20B' - Applies to both Bantam Cub and Super Cub.
'Bantam Cub' - Applies to Bantam Cub only; not Super Cub.
'Super Cub' - Applies to Super Cub only; not Bantam Cub.
|
Triumph T20B Bantam Cub Triumph Tiger Cub engine in a BSA Bantam D7 frame. |
Triumph T20B Super Cub Triumph Tiger Cub engine in a BSA Bantam D10 frame. |
Headlight
Handlebar Controls
Petrol Tank
Seat
Oil Tank
Front Mudguards
Wheels
Footrests
Engine Mounts
Engine
Kickstart
Carburettor & Air Filter
Exhaust Pipe
Headlight
As far as I can establish, the T20B headlight is unique to these models. It is based on the Lucas MCH56 shell and is chromed. Other versions of the MCH56 do exist, but I have never found one with the correct holes for the T20B.
Some late Bantams did have a very similar headlight, but they are shorter and look fatter than the MCH56. Some Bantams also had a larger diameter speedo than the Cubs so they are not interchangeable unless you swap the speedo as well. As a consequence you can expect to pay a high price for a good, original headlight!
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.
A similar set-up was also used on some Ariels - I believe some 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" long Amal rubber grips while some Ariels used a shorter 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 often painted grey while the T20B versions were chromed.
It can therefore take some time to track down the right parts.
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.
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. They also generally come with the BSA logo printed in gold on the back of the seat. The T20B seat did not have any logo on it, but these can be quite easily removed.
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.
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 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 has three separate steel hoops to hold it to the forks.
Wheels
The Bantam Cub uses Bantam D7 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. 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.
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 T20B footrests come out horozontally and are then stepped up to avoid the outer casings.
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, 3.5" version, not the longer ones that are 4.25".
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 Cub's rear engine mount to the frame are also quite different from the Bantam.
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 (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.
Kickstart
The kickstart on the T20B is very slightly different to 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 which curves slightly further out from the engine (part number T1679 for the complete assembly). The difference is only a few millimetres so unless you have a standard kickstart to compare against, 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 are so difficult to identify.
Carburettor & Air Filter
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.
These mushroom filters were used in a variety of bikes of that era so are readily available. You should be able to source both the filter body and the internal mesh at a reasonable price.
The version of the Monobloc carb fitted to the T20B had the rim of the intake threaded to take the filter, unlike the carbs on the standard Cubs that had a lip to hold the rubber connecting tube. There were numerous versions of the Amal 375 Monobloc, but if you can trace a body that is stamped with the number '375/61' on the top of the flange then it should be the right size and have the correct thread for the filter.
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.
Questions about Tiger Cubs? - I suggest visiting the links page or ask via the independent Tiger Cub Forum
All information on this website is provided in good faith. The accuracy of the information cannot be guaranteed.
No responsibility can be accepted for any consequence arising from the use of
this website. I recommend that all information is independently verified.
The owner of this website has no connection with the Tiger Cub Forum other than
as a general subscriber and contributor.
Copyright © 2010