Rebuilding Triumph T20B Bantam Cubs

Rebuilding & restoring the Bantam Cub variant of the Triumph Tiger Cub

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.

The BSA Influence
Before You Start
How Bantam Cubs Differ

 
Restored 1967 T20B Bantam Cub

Restored 1968 T20B Super Cub

The BSA Influence

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.

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 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!

BSA logo

Triumph logo

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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.

The Tiger Cub Bible

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

Zeus Data Charts

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.

Triumph Parts Book for Bantam Cub & Super Cub

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.

Haynes Manual for Tiger Cub

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.

Consolidated Parts List

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.

Haynes Manual for BSA Bantam

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.

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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.

Terminology: 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.

Photo of Triumph T20B Bantam Cub

Photo of Triumph T20B Super Cub

Triumph T20B Bantam Cub, 1967-69

Triumph T20B Super Cub, 1966-68

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Paintwork

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

 
Restored 1967 T20B Bantam Cub
Restored 1968 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 Colours
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 of the same name used in the 60's and 70's.

Super Cub Colours
'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, and failing, to distinguish their own colour range?

 

1966 advert for T20B Bantam Cub stating 'Finished in Pacific Blue and white'.
1966 advert for T20B Bantam Cub
Note the comment that they are,
'Finished in Pacific Blue and white'.

 

Extract from the Parts Book referring  to Bantam Cub as 'Perrivale Blue'.
Extract from the Parts Book referring
to Bantam Cubs as 'Perrivale Blue'.

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Petrol Tank

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.

The 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. Importantly the T20B tanks 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.

T20B Bantam Cub Petrol Tank
Bantam Cub Petrol Tank
Similar to Bantam D7 tank,
but with fixing holes
for Triumph badges.

Triumph Bantam Cub mouth organ style tank badge.
Bantam Cub Tank Badge
'Mouth Organ' style

T20B Super Cub Petrol Tank
Super Cub Petrol Tank
Similar to Bantam D10 tank,
but with fixing holes
for Triumph badges.

Triumph Super Cub two-piece tank badge.
Super Cub Tank Badge
2-piece style

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Oil Tank & Oil Filter

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. This tank was also used on BSA's 350cc model B40. (Part number 40-8302.)

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.)

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.

The T20B oil tank is actually a BSA C15/B40 part.
T20B Oil Tank
Note the 'froth tower'
on the top of the tank,
under the seat.

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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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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.

Identifying Front Wheels and Mudguards

Bantam Cub front wheel and mudguard.

Super Cub front wheel and mudguard.

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.

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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!

Identifying Headlamp Shells - Top View.

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.

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.

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.

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.

 
Part
T20B Part No.
Ariel Part No.
Twistgrip & Brake Lever Assembly
368-23 (4-3/4")
T1152 (4")
Clutch Lever Assembly
18-1091
T1139
Throttle Rotor
366-004 (4-3/4")
T1377 (4")
Brake Lever
368-019 (ball)
29-8812 (plain)
Clutch Lever
18-1092 (ball)
29-8832 (plain)
Handlebar Grip - Right
16-070 (4-3/4")
6425-1 (4")
Handlebar Grip - Left
16-069 (4-3/4")
6425-7/8 (4")

T20B controls & similar Ariel controls

Identifying Amal combined 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.

 

Identifying Amal combined throttle assemblies.

Left: Ariel controls.

Right: Correct T20B controls.

Identifying Amal levers.

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.

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Seat

The steel base for the seat on the T20B was common for both models and is standard Bantam D7/D10/D14/B175 base. However, there were variations in the colour of the seat covers and in the thickness of the seat cushions.

Photos and drawings from the period suggest that the original colours and sizes were as shown in the table below, but I have not yet been able to verify these and several other styles exist.

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 are quite expensive and I have only ever been able to find plain black ones with thicker cushions as fitted to later models. 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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Frame & Engine Mounts

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 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.

Additional engine mount required for Tiger Cub engine in Bantam frame.
Third Engine Mount

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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".

The bracket for the left-side pillion footrest differs from the standard Bantam D7/D10 bracket. As discussed below in the 'Side Stand' section, this bracket is the same as the Bantam 'Bushman' models to accommodate the side stand.

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Side Stand

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.

Side stand bolted to rear footrest bracket.
Side Stand

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Engine

The engine is almost entirely a standard, late-type, T20 engine with points on the side, ball bearings fitted at both main bearings and the distinctive square barrel and head.

The Triumph Parts Books shows a roller big-end bearing fitted to all T20B engines, but this is not accurate! Most T20B engines did have the roller big-end, but early models came off the production line with the earlier plain bush big-ends. I'd be surprised if any of these plain bushes survive as this was a well-documented weak point. Most will have been replaced with Alpha roller bearing kits at least once in their lifetime.

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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Carburettor & Air Filter

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

 

Amal 375/61 mixing chmaber top screws

Amal Part No.

Description

4/035

Cable Adjuster Screw

4/137A

Plug Screw

5/077
(not shown)

Nut - Optional, used to lock the cable adjuster screw.

Parts for top of Amal 375/61 carb.
Correction to T20B Parts Book.

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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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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.

T20B silencer bulges inwards towards the back wheel.

T20 silencer bulges outwards away from the wheel.

T20B
Silencer bulges inwards, towards the back wheel.

Standard T20
Silencer bulges outwards, away from the wheel.

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Wiring Loom & Horn

The same wiring loom is used for both T20B models. This is very similar to the Bantam D10 loom and also 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. The Bantam D7 loom will not fit either model!

See additional infromation on the 'Electrical' page.

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