Oldbike: Customizing A 1970 CB350 Tail Light for the GS

Back in November I took the gigantic tail light off of theGS, and had originally planned on using a small tail light lens that I had literally found on the side of the road on a run.  After comparing it to the size of the stock turn signals (which I decided to keep and just shorten their stalks) it was pretty obvious that the proportions were not right.  The Turn signals looked huge in comparison, something had to be done.  I went out to the local bone yard to look around and found that they had probably a thousand huge tail lights just like mine hanging from the rafters.   Luckily after closer inspection they did have a few smaller ones too.  A lot of them had the mounting hardware to attach them to the fender as well.  Unfortunately the one that I really wanted, from a 1970 CB350 did not.  Undeterred I decided that it wouldn’t be too hard to modify the original tail light mount and mount the new tail light to it.

I brought the new tail light and the old tail light to my weekly (when my schedule allows) Wednesday night garage beer session at a friend’s house to start the work, which can sometimes be a good idea if the correct amount of booze and inspiration is applied, if there is too much booze best case scenario nothing gets done, worst case scenario something gets ruined!  After a lot of staring at the part and a good amount of bullshitting I came up with a plan of attack that would net me the desired results.  First, the license plate would have to go.  Mainly because with a tail light half the size as the original I decided that it would look ridiculous being that high up off the fender.  In the above picture you can see my plan of attack.  The scribbled out areas would go, where my thumb is, is the portion of the base of the mount that attaches to the fender.  Using a straight(ish) piece of metal that had a 90-degree(ish) looking corner on it I drew lines parallel to that portion of the base since the mount would be going back on the same part of the rear fender the angle needed to be the same.  Truthfully I eyeballed a lot of it and freehanded the curve connecting the two straight lines on either side of the mount.  I was a drafter for 9 years so I am pretty good at eyeballing things also the other drafter in the house concurred that my lines were good.  The little tab that you can see in the above picture is meant to be folded in and under, making the portion of the mount where I will drill the holes to attach it to the fender.

I used your basic run of the mill hacksaw to make most of the cuts, holding the mount in a bench vise and working my way around the piece.

After a little discussion we decided that the last cut on the curved portion of the mount should be done by a cut-off wheel.

After folding the tabs down (again in the vise) and drilling a couple of new holes in the back of the CB350 tail light to align with the mounts holes we were at a point where I was satisfied to focus on drinking and smart talk for the rest of the night.

The next morning I gave the CB350 lens a good cleaning in preparation for the next step I had in mind for it.  In looking at the lens from a side view I could see that there was about ¾” of clearance between the front of the tail light lens and the reflector inside the lens.  The bulb sticks out further than the reflector but conveniently it is centered on the round raised portion of the lens and is still a safe distance from the plastic!  My plan was to cut away that excess plastic slimming down the entire tail light assembly.

You can see how much of the lens material I plan on removing in this picture.

The Dremel workstation that I got for Christmas came in handy for this portion, it basically changes the Dremel into a drill press with adjustable height for the entire press assembly which can also be rotated in 15-degree increments up to 90-degrees off vertical for grinding and cutting sort of serving as a third hand, also it has an extension and hooks to hang the Dremel from when using the flex shaft.  What I did basically was set the desired height and then pivoted the piece around the cutter following the line.  Now if I would have had a larger diameter cutting wheel I would have been able to flip the lens over and cut the lens that way rather than having it teeter on the round raised portion of the lens.  This would have gotten me a better initial cut and resulted in less filing to smooth out the edges but you make do with what you have.

I was slightly concerned about cutting the transition between the red plastic and the clear plastic on the lens and took it easy on the transitions and things went smoothly.

This first couple of pictures look worse than it really is the plastic just got so hot when I was cutting that it melted.  The two pieces weren’t really stuck together and a little cleanup with a pocket knife took off the melted excess.

I did end up filing the edge of the lens I was keeping so that it wouldn’t let water leak into it and also so that it would look less cobbly.

The rubber gasket had shortened over the years and would not fit properly with the lens and the back cover of the tail light anymore, the way I got around this was to break out the arts and crafts hot-glue gun and glued one corner of the gasket down worked my way around the lens gluing and stretching the gasket as needed, the glue was strong enough to hold the gasket in place so that it properly sealed when reassembled.

For reference, on the 1979 Suzuki GS 425L the brown wire going to the tail light is the running light, the white wire is the brake light and the black wire with the white stripe is the ground.  On the 1970 Honda CB350 light the brown wire is also the running light and the green wire is the brake light the grounding point on the tail light is the little tab pointing down with the small hole in it.

Connections are soldered together and I soldered the ground wire directly to the grounding tab.

You can see the tail light and mount attached to the rear fender, the original mount used three holes in the fender to attach, but due to the having much less space I could only use two for them this time around which I lined up by hand and then marked the holes to drill on the mounts tabs to get the right position before drilling the holes.

I couldn’t be happier with the results, the tail light is basically the same width of the fender…

…and it is slightly taller than the turn signal and about twice as wide which I think are good proportions.

I also think that the ¾” I took off the depth makes a big difference too it just looks a lot better!  At a glance you really wouldn’t notice that anything is not stock and that is kind of what I am going for, subtle improvements without sacrificing functionality.  I did take a short video of the initial test of the tail light and a little walk around which you can see below.

I still have to make some spacers for the rear turn signals to properly mount them to the fender so that will be what the next update will be about in the next week or so.  Thanks to all who stop by and check out the blog!

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