wiring harnesses 54955719 (for the 1969 tr6c) and 54955257 (for the 1969 t120r)? Can they be interchanged without modifications?
How long is a piece of string? Are the ones you're looking at original? If so, they're fifty years old.
If they aren't original, who were they made by? In theory at least, it should be possible to fit a T120R harness to a TR6C and simply join the Brown/Blue and Brown/White in the headlamp (that attach to the ammeter on a T120R or TR6R). In practice, some modern harness makers' interpretations of 'original' vary ... somewhat ...
Also bear in mind that certainly originally, a single harness didn't always make all the connections between everything; there were small harnesses to connect, say, the main harness and the ignition switch. Modern harness makers don't always supply these, or do so at extra cost ...
what gage wire is used in these harnesses.
Original wire gauges vary. Ime, original wires were based on 32SWG (No.32 (British) Standard Wire Gauge) strands; wires to individual components were 14-strand, rated for 7.5 Amps, the wire between ignition switch and the junction of individual components' White wires was 28-strand, rated for 15A, the Brown/Blue, Brown/White and main Red were 42- (-ish) strand, rated for 22.5A.Lucas
did supply original harnesses with all-14-strand 32SWG wire but ime only '71-on. Note that the rating of either 14-strand Imperial or metric wire (that Brett posted) used to link rectifier and Zener is exceeded by the alternator output above about 2,000 rpm. In a '69 harness, if you find 14-strand used for main Red (to battery), Brown/Blue, Brown/White and either White or White/Blue from the ignition switch, the harness is poor-quality.
The problem of modern wire gauges is vexing but worth taking the time to understand, if only because it'll inform your choice of harness and also you will need to make changes and/or repairs at some time in the future:-
. The vast majority of connections in Britbike harnesses, including '69, are 3/16" o.d. (4.7 mm. equivalent, be wary of any supplier that quotes "5 mm.") 'bullet' terminals joined by tubular black-insulated 'snap connectors'. The most important criterion certainly of original-looking bullets is they stay on the conductor by friction, having been squeezed by a hex. crimping tool - Lucas
stopped soldering years before '69.
. Brett mentioned the modern metric equivalent of one of the original Lucas
wire gauges - "14xSTRANDS, RATED 8.75 AMPS, OD 1MM SQ". Original-looking crimp-on bullet terminals are only available as standard for this wire gauge, plus the metric 28-strand and 44-strand equivalents. None
are available for any other wire gauges, including original Lucas
, AWG (American Wire Gauge), the one Brett says he was "recommended to use" or any of the weird ones some pattern harness makers seem to be able to find. Modern bullet terminals vary a little by how much they can be squeezed when crimping; in certain circumstances, certain bullets can likely be make to 'work' on conductor they aren't sized for but it's a minefield.
Finally, bear in mind that almost any off-the-shelf harness will be a good or bad copy of an original, so it'll have connections to separate rectifier 'n' Zener, points, condensers, etc.; if you're considering modern reg./rec. (modern Zeners are particularly poor-quality
) and/or e.i., as you're in the US, I'd strongly recommend at least talking to British Wiring
; I know they use the aforementioned modern metric equivalent wire, terminals and connectors; aiui they can supply a harness without rectifier/Zener/points/condensers connections you then otherwise have to tape up ...