Ian:

I'm not sure how much experience you have using magnetic compasses on moving vehicles (ships, aircraft, motos), so, here are some things you will need to consider:

1) The compass will not be accurate during acceleration or deceleration. It generally takes about 10 seconds to settle down and become accurate again after acceleration or deceleration has stopped.

2) The same problem exists during and after turns (heading changes). That problem is exacerbated if the vehicle the compass is mounted to vehicles that bank during turns, as motorcycles and aircraft do.

3) You will need to 'swing' (calibrate) the compass once you have installed it on the moto. That is a time-consuming process. Make sure you have the engine running when you do this, because presumably the engine will be running when you intend to actually refer to the compass. It will either be accurate with the engine off, or accurate with the engine on, but not both.

4) You will need to know the magnetic variation of the area you are in, and correct for that mag var when reading maps... the maps are always oriented to true north, a whisky compass indicates magnetic north.

5) If you have heated handgrips on your motorcycle, the compass will be completely untrustworthy when the heated grips are operating (they set up a localized magnetic field when they are turned on).

Based on my experience as an aircraft pilot and aircraft mechanic, I suggest you find a cheap, used, beat-up GPS that has a compass display in it and attach it to your bike. You will then have a far more accurate & far more reliable indication of heading than you will ever get from a wet (spirit) compass. Chances are you can find an old, out of date Garmin device on eBay for $25. That would do the job just fine, without any of the problems mentioned above, because it uses vectors of movement rather than the Earth's magnetic field to determine your heading.

Michael