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I'm starting a Mark 3 - mid 60s, 250 - semi-reproduction. I've read numerous references here to a "modification" or "dent" to the underside of the tank to accomodate the larger carb. Does anyone have a photo of this "dent"? My Diana/MKIII tank has been "modified" at the rear of the tunnel but I think it's gone a bit too far. I'd like to see what "original" looks like before I start re-doing it. I'll be using the big Dellorto 30mm carb. I'll try getting a pic or two of mine and post it.

Hi,

the Mach 1 tank had a bump not to clear out for the carb (ssi 27, 29 and 30mm are the same dimensions), but to help with the removal of it as quick as possible without removing the tank.
the bump is similar to the one on the left (picture courtesy of Tom Bailey):

I have a Mach 1 tank at Ross Thompon's in Canada right now. It is getting numerous small dents taken out and the bottom is being checked for thickness and reinforced where needed

I have a non-Mach 1 tank here at home that has a stress fracture high up on the tank.

Ross is going to record the indent on the M1 tank (I am not sure how he does this) in the event I want to convert the non-M1 tank to have the cut-out.

I use Ross as he uses no body fillers (a.k.a. bondo) in repairing tanks. All repairs are metal. Ross in not cheap.

But the point of this is you could inquire of Ross while he has my M1 tank what he would charge you to add the cut-out if it's beyond your capabilities as it is mine.

Ross' website:

http://www.execulink.com/~rosst/

Jim and Greg -- Thanks much both of you for the greatly helpful info. Greg's comparison photo is just great - and that is a huge indent! I'll get a photo of my tank underside and post it -- it's indented on BOTH sides of the tunnel and I'm trying to figure out why. I'm in Denver, Co area and actually have two very good metal masters that can handle it if I decide it's too complex. One can remove dents from hollow items - expansion chambers, fuel tanks, etc by pressurizing with air to around 30psi, then carefully heating the dented area with a torch. Now a motorcycle tank with it's folded tunnel shape - if pressurized - will"unfold" or spread open and try to flatten out - and ruin the tank. One must install quite a number of long hose clamps around the tank to prevent it from "spreading" when it's pressurized. Must also make sure that there is no residual fuel or varnish inside.