I'm looking at an AIM for a 67 model year and I'd bet the same thing occurs in all. Many times there are measurements to specific items (or centerlines), like front fender flags for example, that show an apparent reference line such as the numbers 20 and -35, in a circle with the specific item location referenced from that. Anyone know what these major reference lines represent? From other drawings, the horizontal number, (-35) in this case, is some measurement from a "0" point with negative measurements going forward and positive numbers going aft; vertical reference numbers, using 15 as a starting point, is 1.76" above the lower belt line crease.

Take this example...
http://www.chevelles.com/showroom/396guy/327_engine.jpg
...it shows the centerline of the two mounting holes to be 1.86" above the referenced "20" and the right-most mounting hole is 1.43" left of the referenced "-35". Question is what are those referenced, circled numbers referring to?

Dale, the reference points/lines they refer to can be found on the 'Sheet Metal Alignment' pages. I don't have my book with me here at work so I can't give you a section/page number.

Originally posted by NITRO:
Dale, the reference points/lines they refer to can be found on the 'Sheet Metal Alignment' pages. I don't have my book with me here at work so I can't give you a section/page number. Ah HA! Found a "SHEET METAL CHECKING" page but it only shows the 0 reference mark with no other circled reference marks but comparing some measurements (like 0 to front of fender of 38.92") seems to correlate to another drawing showing -40 to be 1.00" inch in front of the same (apprarent) location. Can I safely assume each circled number is in inches from the 0 reference line?

Yes, the circles numbers are dims from the zero reference (Datum) lines.
Also, I have'nt snapped a picture of the Fremont POP we discussed, but I will shortly. Been to busy disassembling and cleaning the 69.

The datums with a circle around them are "inch" or body lines. They were numbered relative to specific features of the vehicle. For width or cross-car dimensions the inch lines are numbered relative to the centerline of vehicle. The body zero for width is the centerline. Negative values are lh side, positive values are rh side.
The body zero for length or fore-aft dimensions were based on the front of dash, or firewall. Negative values for length are forward of the firewall, positive values are rearward of the firewall. I'm not sure about the body zero for height. I think it was based on the horizontal centerline of axle.

Maybe there are different reference lines for each portion of the car. My 67 AIM only shows SHEET METAL CHECKING for UPC 11-13 for the front end. The 0 reference for front-to-rear is shown 38.92" to the rear of the centerline of the radiator support and the 0 reference is 30.76" down from the fender peak line.

The body zero should be common for all vehicles. The body "grids", or "inch lines", are numbered sequentially every 5 inches from zero. For height, from zero up would be numbered 0,5,10,15,20, and so on. Dimensions should be based on the nearest body grid.
In the pic you show of the fender emblem location, the dimensions are relative to the nearest "inch" line. For height, the holes are located 1.86 inches above the 20 inch grid line, or 21.86 inches above the horizontal body zero.
The last example you mentioned for the rad support fore-aft dimension should have been referenced to the -35 body grid, at least according to body drafting standards that I'm aquainted with. I've been a body draftsman/ designer at GM for 25+ years.
Hope this helps
Dennis

Here's the diagram from the AIM. I assume the first diagram posted is referenced from these "0" points...?

http://www.chevelles.com/showroom/396guy/sheet_metal_alignment_small.jpg

yes. Normally a drawing of that size would have included all the body grids encompassing the area. For some reason the last drawing you show only references the body "0" for width (centerline), height, and length. The body "0" is common for all the drawings.

Thank you for all the info. Just trying to determine measurements when someone asks where things like front flags go.