Today in Motorcycle History

Triumph 3HW

14th November 1940 was the night that the Luftwaffe launched their infamous raid on Coventry, which military intelligence had forewarning of but were powerless to prevent without giving away the fact that Alan Turing had cracked the German Enigma codes at Bletchley. Or that's what I believed, but having read up on it just now it appears that is a misinterpretation of facts first made available in the late 60s or early 70s.

Along with many others, the Triumph factory was destroyed. With it went the 350cc 3TW, a primary product in Triumph's war effort.

An old foundry in Warwick was converted into a machine shop and production of Army service parts began. It was an uphill task in the very basic workshops but work progressed and by June 1941 complete machines were again rolling off the factory floor.

By then, Edward Turner had moved to BSA. The new machines in production had OHV 350cc power plants and were based on the pre-war 3H, and renamed 3HW. The only major difference was that whereas the 3H and Tiger 80 had separate alloy rocker boxes, the 3HW rocker boxes were cast integrally with the cast iron head.

The 3HW was employed with forces in North Africa, India and Greece. The Greek Army used these machines from 1942-1952.

A major problem with 3HW machines in N.Africa and India was the lack of an air cleaner, and the dust played havoc. A couple of thousand miles was about the limit before major servicing was required. This same problem was encountered by the G3/L Matchless.

It was an extraordinary oversight on the part of both the builders and the War Department.


Model 3HW


    O.H.V. Single Cylinder 343cc

    Bore/Stroke 70 mm x 89 mm.

    Tappet Clearance (Cold) Inlet ? Thous.

    Exhaust ? Thous.

    Lubrication Dry sump.

Petrol Tank Capacity 3.1/8 Gallons

Oil Tank Capacity 3/4 Gallons

Oil in Chaincase 3/4 Pint


    Con. Rod--Side play .003"-.015"

    Big-end diameter 1.4620"-1.4623"

    Crankpin-Bearing diameter .9620"-.9617"

Shafts And Tappets

    Timing Shaft diameter (bearing position) .9995"-1.000"

    Drive Shaft diameter (bearing positions) .9995"-1.000"

    Camshaft Bush diameter (internal) .5610"-.5620"

    Idler Gear Bush diameter (internal) .4995"-.5005"

    Tappet Guide diameter (internal) .3740"-.3750"

    Tappet diameter (external) .7495"-.7500"

Piston And Barrel

    Bore diameter 2.7555"-2.7560"

    Max. permissible wear before reboring .008"

    Piston Clearance-Top of Skirt .008"-.0085"

    Bottom of Skirt .004"-.0045"

    Piston Rings-Gap .010"-.014"

    Groove Clearance, Compression .001"-.003"

    Groove Clearance, Scraper .0005"-.0025"


    Valve Guide diameter (internal) .3120"-.3130"

    Valve Stem diameter-Inlet .3095"-.3100"

    Exhaust .3083"-.3088"

    Valve Seat Angle 45

    Tappet Clearance Nil

    Rocker Bush diameter (internal) .5620"-5630"

    Rocker Spindle diameter .5605"-.5610"

    Timing-Inlet Opens,B.T.C. 47.1/2"

    Inlet Closes, A.B.C. 68.1/2"

    Exhaust Opens, B.B.C. 69.1/2"

    Exhaust Closes, A.T.C. 46.1/2"


    Timing, fully advanced 3/8"

    Contact Breaker Point Gap .012"Sparking Plug Point Gap .015"-.018"

Oil Pump

    Body, Scavenge diameter (internal) .37475"-.37525"

    Body, Feed diameter (internal) .31225"-.31275"


    Amal Carburettor Nr: MT12/AM/276AD/1J

    Main Jet 150

    Slide 6/4

    Needle Jet 107

    Needle 6

Clutch & Gearbox Gear Ratios

    Top 5.80

    Third 8.40

    Second 13.30

    Bottom 17.80

Gearbox data

    Clutch Rod, Lost Motion 1/32"

    Clutch Cable, Lost Motion 1/16"

    High Gear Bush-Layshaft diameter (internal) .7498"-.7505"

    Mainshaft diameter (internal) .8115"-.8125"

    Clutch Spring, Free Length 1.5"

Front Fork

    Girder fork

    Spindle diameter .435"-.436"

    Spindle Bushes diameter (internal) .437"-.438"

Tyre Sizes Front & Rear 3.25-19

Wheel Base 52.1/2"

Overall Length 82"

Overall Width 28.1/2"NOTE. - All bush measurements are finished sizes in position where the bush is not floating.

Frame and Engine Numbers.
Frame no. H255 Engine no. 3HW 50853 (Bonhams Lot 294)
Engine Number is 3HW 10095 and Frame Number is TL 56999 (
Engine Number 3HW 38877S (Photo clearly shows full engine number and crankcase casting -
Engine Number 3HW 45027 (
Frame no. TEC 844 Engine no. 3HW 58794 (1948 model,

Some 3HW's have matching engine and frame numbers, many do not as engines were often changed.

Years of Manufacture.
Production began in 1941, and the article above suggests that the Greek army had them in service until 1952. A 1947 model is listed for sale at (no serial numbers provided).
1948 model listed at (numbers included)

Several sites list 1939 3HW Triumphs, but these dates would appear to be in error.

Source: Makis MC Classic