@Andrew Shadow do you still snap off 2 of the lower locator tabs on the bulbs when using the rings for the st1100?
Yes, as Larry wrote, a standard H4/9003 automotive bulb has lower tabs that are in different locations than the OEM bulb. Their removal leaves the bulb with improper lower support against the headlamp housing. The adapter rings are to re-establish this support.
Note 1:
Since there are many people on this forum who are not riding North American standard ST1100's, it is important to point out that this only applies to ST1100's equipped with the North American headlamp housing. I do not believe that any other market used this housing, and that it is a result of the lighting regulations that exist in North America.
The adapter ring is not designed to position the bulb at the correct depth however. The addition of the adapter ring actually causes the bulb to be at an incorrect depth by the the thickness of the adapter ring. The correct bulb depth is having the bulb base sitting directly against the headlamp housing where the adapter ring has to go, and where the OEM bulb sits. I have seen some of these adapter rings that are thicker than others. The thinner that adapter ring is, the closer to the correct focal point will be the H4/9003 bulb.
Note 2:
If using the adapter rings, and if the new bulb has curved tabs as most Halogen bulbs do, the remaining tab of the new H4/9003 bulb that was not cut off, as well as the area adjoining where the two lower tabs were cut off, should be flattened so that they do not protrude beyond the surface of the rest of the bulb base. If this is not done, the bulb will be positioned even further away from the correct focal point in the housing.
If the standard automotive H4/9003 bulbs are going to be used, there are only four ways to achieve this as far as I know.
1- Remove, or bend out of the way, the two lower tabs of the H4/9003 bulb so that it will fit in to the North American ST1100 headlamp housing with the use of adapter rings. (See post # 14)
Doing so means that the lower portion of the bulb will now go to far in to the housing and will not sit correctly at the bottom. This is because its lower tabs are no longer supporting it by resting against the headlamp housing. These adapter rings re-establish a place for the lower portion of the bulb to rest against.
See note: 2 above.
2- Replace the base of the new H4/9003 bulb with a base that properly fits the ST1100 headlamp housing.
This is possible with many of the regular Halogen bulbs. This is probably not possible with most of the LED bulbs however, so whether or not this can be done is dependent on what replacement bulb is being installed. If it is possible, remove the base from the new bulb. Then remove the base from a Honda OEM bulb and assemble it to the new H4/9003 bulb being installed. This allows installation of the new bulb without having to use an adapter ring because an OEM bulb base with the correctly orientated lower tabs is being used.
3- Modify the headlamp housing to accept the standard H4/9003 bulb base.
This involves removing the housing and machining, by whatever means one chooses, notches in the headlamp housing at the correct location and the correct depth to accept the standard H4/9003 bulb lower tabs. Once done, this allows the use of any H4/9003 automotive bulb without any modification to the bulb and without the need for the adapter ring.
4- Purchase a European standard ST1100 headlamp housing.
This odd ball lower tab configuration is to comply with North American lighting regulations. They do not have this requirement as part of the lighting regulations in Europe. The standard H4/9003 automotive headlamp bulb fits directly in to a European standard headlamp housing with no muss, no fuss. It is a direct fit. Despite the expense, several North American ST1100 owners on this forum have gone this route.
Note: 3
Anyone considering this European headlamp housing option would be well advised to do a little research before purchasing one. I do not know if the headlamp housing from all European countries are designed for the standard H4/9003 automotive bulb, or if it is only certain countries. Maybe
@John OoSTerhuis might know, as he has installed a European headlamp housing on his ST1100.
