This is my VDO boost gauge mounted in the stock location. My
ancient camera doesn't really do it too much justice. It took
a lot of time and the dash really had to be hacked up quite a bit to get
it in there. Here is how to do it :
My VDO boost gauge is 52mm(2 1/16"). It is much deeper than the stock gauge and any aftermarket gauge will be, too. What I did to get it to fit was cut out the back of the gauge cluster("hack" would be a better word for it) w/ a dremel tool. When I got the hole wide enough for the gauge to fit, I hot glued it in place because there was no way any conventional mounting methods would work. I literally used gobs and gobs of hot glue. It really isn't pretty behind there :-) **Before hot gluing it in place** To fill in around the edges, I cut up(hacked) a 5 1/4" floppy disk casing(remember those?) in little 1/4 moon shapes. I glued it around the outside of the gauge and fitted it so that it would look like an extension of the black part in the inside of the gauge cluster. The color and texture matched pretty well and made it look pretty good.
Also, behind the gauge cluster, there is a piece of green flexible PCB-like material(you'll see what I mean) that has all the traces for electrical connections of the gauges. I had to cut it and resplice some of the traces w/ wires so that the gauge would fit in there. I just scraped the green material from the trace and soldered the wire to it, being careful not to keep the heat from the soldering iron on it too long because the green material melts easily. I used the illumination connections from the original gauge. Since the boost gauge is at the end of the gauge cluster, there aren't too many wires running behind it. Just the security LED wires, and the boost gauge illumination wires, I think.
You'll have to remove the glass from the face of the VDO gauge when you put it in there, or else you'll have a bad glare from the light having to pass through both the plexiglass of the gauge cluster and the glass of the VDO gauge. Getting it out is no easy task. I had to carefully smash it and pluck out the small pieces of glass with needlenose pilers. I think I used a dremel tool to cut a small hole in it first, then put a flat head screw driver in the hole and twisted. On some gauges, I would guess that it may be easier to remove the glass than this.