Rebuilding Triumph T20B Bantam Cubs

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.

The BSA Influence
Before You Start
How Bantam Cubs Differ

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 also have a similar appearance to the Bantam D14 and the B175.

When originally manufactured, many parts were used from other BSA models, plus a few are specific to the T20B. For example, the oil tank was from the BSA C15 and the side stand from the Bushman version of the Bantam, while the petrol tanks and headlight are unique.

So, other than the engine, when you're searching for original parts, think BSA!

The Tiger Cub Bible, by Mike Estall - a really useful reference book covering all Cub models.

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.

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.

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.

Mike Estall's Consolidated Tiger Cub Parts List - this gives useful sizes and references not covered in the Parts Book. These used to be available directly from Mike Estall himself, but sadly he passed away in 2017. Updated availablity to follow.

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.

Triumph T20B Bantam Cub, 1966-68

Triumph T20B Super Cub, 1967-69

Bantam Cubs were supplied in a two-tone, blue and white colour scheme - 'Nutley Blue' (see notes 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' (see notes below). 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.

	T20B Bantam Cub	T20B Super Cub
Petrol Tank:	Top: Nutley Blue Bottom: Alaskan White	Top: Firecracker Red Bottom: Chrome
Oil Tank & Battery Box:	Nutley Blue	Firecracker Red
Front Mudguard:	Alaskan White with Gold lining	Firecracker Red with Gold lining
Rear Mudguard:	Alaskan White	Firecracker Red

Bantam Cub Colour Notes
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 colours from BSA?

Some modern, Hinckley Triumphs are also available in a colour known as Pacific Blue, but this is quite different from the colour of the same name used in the 1960's and 70's.

Super Cub Colour Notes
'Firecracker Red' was a standard BSA colour. However, when the Super Cubs were first introduced Triumph re-named it 'Bushfire Red'. This name was short-lived and quickly reverted back to 'Firecracker Red'. Is this another example of Triumph trying to distinguish their own colour range, but failing?

The Bantam Cub petrol tank is based on the Bantam D7, and the Super Cub on the Bantam D10. The two tanks are completely differerent shapes and in both cases the T20B versions are not exactly the same as the original BSA models because of the difference between the Triumph and BSA badges.

BSA tanks tend to have recesses on either side of the tanks to take the BSA badges. However, the Triumph badges sit on the surface and follow the contours of the tanks. Consequently the T20B tanks have no recesses for badges and instead have threaded holes in the correct positions for the Triumph badges.

This means that although all 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.

The Triumph tank badges (referred to as 'styling units' in the parts book) are also different for each model. The Bantam Cub used the popular 'mouth organ' style found on many Tiger Cubs while the Super Cub had the later two-piece badge.

Only the Bantam Cub had rubber knee pads fitted to the tank. The Bantam D7, D10 and the Super Cubs had no knee pads.

Bantam Cub Petrol Tank
Similar to Bantam D7 tank,
but with fixing holes
for Triumph badges.

Bantam Cub Tank Badge
'Mouth Organ' style

Super Cub Petrol Tank
Similar to Bantam D10 tank,
but with fixing holes
for Triumph badges.

Super Cub Tank Badge
2-piece style

The BSA 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. However, the Tiger Cub engine is, of course, a dry-sump four-stroke so an oil tank was needed. On both models of T20B, BSA used the oil tank from their latest C15 model which was a 250cc, four-stroke.

Some earlier C15 oil tanks are the correct size and shape, but they don't have the 'froth tower' projecting from the top of the tank where the breather pipe connects.

The version fitted to the T20B seems to have appeared on the C15 in the mid-1960's, around the same time as the T20B was introduced. Did the T20B simply use the latest design that had evolved for the C15, or did the C15 benefit from improvements developed for the introduction of the T20B?

Whatever the reason, if you need a replacement you may find that oil tanks described as 'C15' are cheaper than those with a 'Bantam Cub' or 'Super Cub' label. Provided you get the later version with the froth tower, they are all the same BSA part. (Part number 75-8345.) A very simlar tank was also used on BSA's 350cc model B40 and this may also be suitable. (Part number 40-8302.)

The oil filter in the bottom of the oil tank is a widely used BSA/Triumph part. The original oil filter part number F3179 is equivalent to 82-3179 and 40-8328. Replacements are readily available at reasonable prices so shop around.

Identifying Front Wheels and Mudguards

T20B Bantam Cub Wheel & Mudguard
Hub is smaller opposite brake.
Spokes are different lengths on each side.
Flat brackets attach centre of mudguard to forks.
Mudguard stays front and back are equal length.

T20B Super Cub Wheel & Mudguard
Full-width hub with corrugated centre.
Spokes are all the same length.
Three hoops support the mudguard.

Identifying Headlamp Shells - Top View.

Left: Correct T20B Lucas headlamp.
Longer, 6-1/4" body with thinner, 1" rim.
Hole for 2-3/8" diameter speedo.
'D' shaped switch holes, slightly raised.
Central hole set back from switch holes towards speedo hole.
Small hole for rim clamp screw.

Right: Similar BSA Wipac headlamp.
Shorter, 5-3/4" body with deeper, 1-1/2" rim.
Hole for 2-1/2" diameter speedo.
Round switch holes with small locating holes on pronounced, raised bosses.
Central hole in line with switch holes.
No hole for rim clamp screw.

Identifying Headlamp Shells - Bottom View.

Left: Correct T20B Lucas headlamp.
Longer, 6-1/4" body with thinner, 1" rim.
Two separate holes for speedo cable and wiring.
Stamped with markings, "MCH 56 LUCAS".

Right: Similar BSA Wipac headlamp.
Shorter, 5-3/4" body with deeper, 1-1/2" rim.
Single hole for speedo cable and wiring together.
External rim clamp.

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

Identifying Amal combined controls.

Top: Correct T20B controls.
Longer, 4-3/4" throttle tube and rubber.
Chromed castings.
Ball-end levers.

Bottom: Similar Ariel controls.
Shorter, 4" throttle tube and rubber.
Painted castings.
Plain levers.

Identifying Amal combined throttle assemblies.

Left: Ariel controls.

Right: Correct T20B controls.

Identifying Amal levers.

Top: Standard Amal Levers - not suitable for T20B
Common version of Amal ball-end levers as fitted to many British bikes. Note the cable installation slot at the top.

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
Smaller ball-end levers as fitted to T20B. Note the cable installation slot at the side and the indentation opposite the slot.

Bottom: Ariel Levers - alternative for T20B
Small, plain levers as fitted to some Ariel models. These fit the brackets perfectly, but look a little small and don't have the original appearance.

T20B Model

Part No.

Seat Top

Seat Sides

Top Piping

Base Edging

Pillion Strap

Cushion

Bantam Cub

75-9164

Grey

Black

White

Grey

Not fitted

2 inch thick

Super Cub

71-9171

Black

Black

White

Black

Black

3 inch thick

New seat covers and even complete seats are readily available for Bantams, but can be expensive.

The original Bantam Cub covers with the grey top and no strap are incredibly rare! Standard Cub seats are available with the grey top but they are, of course, designed to fit the Cub seat base and not the Bantam base, which is a completely different shape.

The T20B machines use standard BSA Bantam frames. The only difference is the addition of an extra engine mounting point.

The Bantam engine has just two engine mounts - one at the front and one at the back. The more powerful Cub engine has a third mounting point underneath the front of the engine. As a result, the T20B frames differ from the standard Bantam frames with an extra engine mount.

The two plates used to connect the rear of the engine to the frame are also quite different from the standard Bantam plates. These are uinqie to the T20B.

Frequently BSA Bantams relied on just a centre stand. Where a side stand was fitted, it was usually attached to the two front engine mount bolts. Alternatively some owners fitted a common after-market stand that clamped to the frame underneath the engine. Neither of these options were possible on the T20B because the front mount on the Cub engine only has a single bolt and the larger size of the engine means there is not enough room between it and the frame for a clamp.

Around the same time as the Bantam Cub was introduced in 1966, BSA launched their 'Bushman' model of the Bantam and both machines shared a new side stand. This was bolted onto a re-designed bracket for the left-side pillion footrest.

It's not clear whether the introduction of the Bantam Cub prompted the re-design and this was adopted by BSA for their own new model, or if the stand was already in place for the Bushman and it offered a simple solution for the Bantam Cub too. Either way, the Bushman and both T20B models used the new design and they are therefore different from other Bantams and Tiger Cubs.

Both models of T20B were fitted with an Amal Monobloc 375 carburettor fitted with a 'mushroom' air filter.

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 Ref. No. 6, 'Mixing Chamber Top' (375/064) with a single cable-entry hole on a sloping top. This is not accurate because that part actually has a square top with two cable-entry holes.

Also, the parts book does not show either the Cable Adjuster Screw (4/035) or the Plug Screw (4/137A) used to blank off the unused hole. The same parts were fitted to 375 carbs on other machines and the Amal drawing opposite almost certainly shows the correct parts fitted to the T20B.

Although not shown in the illustrations, Amal also supply a Nut (5/077) which may be used to lock the cable adjusting screw. This is a sensible addition to prevent the screw turning by itself.

Many of the more common models of Tiger Cub used an air filter mounted under the seat next to the oil tank and this was connected to the carburettor via a rubber tube that fitted over a lip on the carb inlet. However, the Bantam tinware didn't permit this set-up and so instead the T20B used a mushroom-shaped filter screwed onto the inlet.

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, 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.

Amal 375/61 carburettor
and 'mushroom' air filter.

T20B
Silencer bulges inwards, towards the back wheel.

Standard T20
Silencer bulges outwards, away from the wheel.