Fuel Tank Mount Positioning

[DJinCA]

I am looking to make a little progress on my RGS project, especially if it doesn't cost much.  Some DPO has brazed a nut on the frame in place of the fuel tank mount.  I have a proper saddle mount (from ignorance or lack of a better term) shown sitting on the frame rail in the attached photo.  I am looking for a measurement from, say the outside surface of the steering head to the center of the threaded hole for the tank mount.

I have a frame drawing with dimensions, but would rather not try to figure out placement by establishing the position of everything from vertical and horizontal datum lines, especially without a frame jig.

The frame has been trued up by a reputable frame man and  why, you may ask did I not take care of this problem then.  You may ask, but don't expect an answer (see ignormance comment above)  This can't be a terribly critical dimension so I am thinking that my short cut is reasonable.

Thanks for your attention.

DJinCA

[Peter in Aus]

On my 58 frame from the centre of the tank bolt to the out side of the steering head tube is 244mm, if that helps.
Peter

[DJinCA]

Thank you Peter,

That's what I was looking for, unless others put forth some argument against this approach.

DJinCA

[Swarfcut]

     Got this from the Tech Pages of Old Thumpers.

     A BSA Frame Drawing shows a distance from the centre of the seat nose mount tube to the tank mounting saddle  centre. This is given as 9 13/32" This is in  the vertical  plane, the top tube slopes, so to the centre of the mounting thread measured along the top tube is a little less. I'd be inclined to assemble  the seat and position the tank for the best aesthetic, mark the position for the tank mount. Mine has already been played with so is a non factory best guess, done badly I might add.

 Swarfy.

 

[BSARGS650]

From three frames; B31, SR and GS as close as I can measure, vary from 243 to 244 mm...

[BSA_54A10]

Simple trick is to weld a slot onto the top rail from an old muffler
Wekd a stud inot a piece of thick plate about  1" to 2" long so you now have a very much captive bolt that will have 1 to 2 " of "fitting space"

[BSARGS650]

Yes, a slotted arrangement on the top tube with an "inverted" bolt is as per my BSA WD B40.

[DJinCA]

Gentlemen,

Thanks for you replies.

Swarfy, I looked at a drawing and saw the dimension horizontal from the seat mount tube, and thought that the longer hypotenuse leg for the actual measurement would require determining the angle of the spine from the datum lines.  It seemed easier and about equally accurate given my shop limitations to get a measurement from another fixed point on the spine.

BSARGS650 has given 3 more measurements consistent with Peter in Aus, more or less confirming (to me, anyway) that my approach is reasonably legitimate.

Trevor's slotted plate trick is a good approach, and used by BSA in other frames, but as this is a genuine Rocket Gold Star frame with its original engine (crankcases anyway) I will do my best to make it as original as possible.  I would add the seat and tank as a sanity check before attaching, but those parts, as well as most of the rest of the bike, wheels, etc. are in a friend's warehouse 100 miles away.

Thanks for your help, I can make a little progress on the RGS while waiting for paint or adhesives to dry and welds or brazes to cool in the Gold Flash project.

DJinCA