How do I know which QH bulb for my old bike?

British bikes used BA15d (dual filament dipping) bayonet cap bulbs from the 1920s until early post war. Some marques, inc. Velocette & Vincent carried on until the mid 1950s. Diag. 10

The bulb shown is a Quartz Halogen, the original Tungsten Filament type had a large round glass

 

These were then replaced by the British Pre-Focus bulb Diag. 4 on all bikes until the 1970s

The bulb shown is a Quartz Halogen, the original Tungsten Filament type

had a much larger glass

 

6V Dynamo systems On pre-War bikes it's usually best to stick with 25/25W
All post war bikes fitted with the Lucas or Miller dynamos can cope with the 25/25W headlamp. Bikes fitted with the Miller 50W & Lucas E3L 60W dynamo can run the 35/35W
6V dynamo systems converted to 12V If you've got the shorter Lucas E3H or Miller dynamos 12V 35/35W is the safe maximum.
If the later Lucas E3L as fitted to most twins then the 45/40W can be fitted. With LED stop/tail bulbs then the 60/55W will be OK, but note that you will be running the dynamo more or less at full output all the time.
If you're using coil ign. you're limited to 35/35W
6V Alternator systems All 6V alternator systems can cope with a 35/35W headlamp bulb.
6V Alternator systems converted to 12V With points ignition 60/55W but with electronic ignition 45/40W or even 35/35W depending on the effeciency & type of the alternator, it's usually best fitting a modern one.
250/350CC & everything made before 1959, 45/40W or less.
12V alternator systems T120, A65 etc.. 60/55W. High output or 3 phase systems may cope with 100W or even 130W. 60/55W is the legal maximum and 60% & even 80% brighter Xenon bulbs are available to fit the modern H4 type headlamp so there's no reason to exceed this.

 

 

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