Hey All,

I hope someone can help me out, this is a rehash from another Z forum, but I'm spreading it in the hope of finding a solution.

I'm in the early stages of restoring a 71 240z and thought a chassis jog would make life easier. As of now, step one is creating headaches, as described below.

As of now i have built a chassis jig/frame and am in the process of bolting up my bent 240z to it. And here is the rub.

I spent countless hours pouring over the only chassis dimensions document out there. That being the BF-3 204z dimensions PDF.

As described previously I was thinking that the dimensions are wrong, or my car is very, very bent. With what I describe below, I don't think the car can possibly be 'that' bent.

Some framing of the problem, with a crap pun:

I have used four mounting points (described from rear to front):

(1) Rear diff mount (not mustachio bar), (4 lateral bolts)

(2) Front mount to the rear lower control arm, (4 lateral bolts)

(3) Transmission mount, and then (two lateral bolts)

(4) Front Strut Tower Top ("A" point).

The dimensions (that don't work):

(1) Reference point.

(2) 465mm from ref. (1). this one is 15mm to short. (closer to 480mm)

(3) 1427.5mm from ref. (1). This one lines up.

(4) 2381.3mm from ref. (1). This one is 20mm to short. (closer to 2401.3mm)

To my point......what the heck is going on here??!!

Between point ref 1 and 2 my car is 480mm. In the BF-3 document, there is no reference to a 480mm dimension.... The car cannot be warped by 15mm, it's all structural mounting points. So, what am I missing?

Moving on to the dimension between 1 and 4. Again, too short by 15 to 20mm... So, what is the actual dimension?

All of the lateral dimensions are correct.

I scratch my head. It's great though as it gives me something to talk with my wife about over dinner.

Thank you in advance!

Ok. Replying to myself in order to hopefully help someone else out in the future.

This was SUPER painful to work out but the correct measurements were.

From back to front:

Rear of rear lower control arm mount to front of rear lower control arm mount: 456.5mm. This is 465-15+6.5mm.

Next is rear of rear lower control arm mount to gearbox mount:

1434mm. This is 456.5+15+962.5mm

The last one is from the rear of rear lower control arm to the strut towers. Mathematically its 2387.8mm.....

This is the number that doesn't work. It comes from 456.5+15+1585+331.3.

With my car however this number is closer to 2401mm.

For the above 4 points the vertical reference is from rear to front.

285mm,

69mm

86mm, and

130mm. (To underside of front frame rails).

I've mounted the car, its very stiff and ready to be deconstructed ....

Edited August 8Aug 8 by Timwell

Is it possible that you've misinterpreted the reference point for the 465mm measurement that appears in FSM drawing? The way I read the drawing, that 465mm measurement defines the distance from the so-called 'C-point' to the centreline of the moustache bar's chassis mount bushings (if you look really closely, you'll see the moustache bar's circular end loops defined just to the rear and inboard of the rear shock tower chassis mounts). Let's call this location on the vehicle the 'E-point'.

The 'D-point' refers to the centreline of the top of the rear struts . But, according to the FSM drawing the centreline of the top of the struts lies directly above the centreline of the 'gearcase mounting rear crossmember' (which is a more useful reference point when taking measurements off the vehicle).

The D-point sits 450mm aft of the C-point

The E-point sits 15mm aft of the D-point (so 465mm aft of the C-point).

The photo below (source?) provides a good visual reference for what all this looks like on the underside of a bare Z structure.

As far as I can see (I could be wrong), the FSM drawing makes no attempt to provide dimensions for the location of the rear lower control arm and its mounting points. You'd need to construct your own layout drawing for the diff, moustache bar, and rear suspension in order to specify (in mm) the locations of the rear lower control arm and its mounting points relative to the C-point.

Edited August 8Aug 8 by Namerow

Yes.... it's a mess.

I did use the wrong numbers initially measuring directly from the C point.

When I got under the car and meausred it, the only way I could square the circle was to use 465-15+6.5 to get the number between the two rear lower control arm mounts. That worked nicely.

That means the front mount is 15mm from C and the rear mount is 456.5mm.

Next problem is wether the front strut top distance is correct. My car is about 20mm closer than the 913mm measurement. Is the car bent? Or is the diagram wrong.....? I'd suspect bent car.

As an aside, that photo you provided is not a 240z ....perhaps a 260 or 280.....IDK. but very representative of what im working with.

Edited Thursday at 01:15 PM3 days by Timwell

The reference points used to generate that 913mm measurement are tricky to replicate in practice. If you look closely at the small sectional drawing of the front shock tower, you'll see that the 'A' point actually lies on the plane defined by the underside of the shock tower cap. You can't measure at this plane unless you remove both front struts and then use either a trammel bar or a plumb laser. So, instead, you have to measure across the tops of the shock tower caps. But...

Not so apparent from the sectional drawing, but very clear to the naked eye, is that the shock tower cap has a lip on the top (this is a little more evident in the sectional drawing of the rear shock tower that appears on the right side of FSM Dwg. BF-3). The top of this lip sits (by my measurement) 13mm above the bottom (reference) surface of the cap. So when you take the 13.5-degree cant of the front strut into account, it means that if you measure the A-A distance at the top of the lip, the result it will be somewhat less than if you were measuring at the (intended) bottom surface of the cap. My calculator says that 'somewhat less' = 7mm, so a measurement of A-A taken across the top of the lips should be 906mm.

Now, if you left the press-in black plastic trim buttons in place (did you?) and took your measurement across the top of the buttons, you'll be measuring at a plane that's even higher above bottom (reference) surface of the shock tower cap. 906mm will shrink to ~ 902mm (est).

Also: It's difficult to make this measurement accurately with a steel rule or (worse still) a tape measure. The correct way is to use a trammel bar*. If you used a steel rule or tape measure, your measurement accuracy will be no better than plus/minus 1mm (and more likely only plus/minus 2mm).

* You can make your own single-purpose trammel bar. Start with a 4-ft (1219mm) length of 1" x 2" lumber and two 6-inch lengths of straightened 0.032" wire. Drill a slightly undersized hole into the flat side of each end of the wood strip (drill all the way through) and then press the wire lengths into the holes. The holes should be about 900mm apart. Now, using 1-qt paint cans sitting on each front fender, support the wood strip (your 'trammel bar') above and across the shock towers. Adjust the projecting length of the wires as required. Then line up the tip of the LHS wire with the centre of the LHS shock tower trim button. Then -- taking care not to disturb the trammel bar -- bend the RHS wire until its tip aligns with the centre of the RHS shock tower trim button (you may need to go back and forth a bit, until you're satisfied that both wire tips are correctly aligned with the centres of the trim buttons). To get your final measurement, transfer the paint cans and the trammel bar to your shop bench (don't disturb the wires!). Then position a steel rule on the bench top so that it sits under the wire pointers and take your measurement.

Namerow got me searching...

You can buy trammel point on Amazon pretty cheap. Add a 1x2 thats long enough and you should be good to go...

https://www.amazon.com/s?k=trammel+points&crid=K19KM88E6XBD&sprefix=tammel+heads%2Caps%2C343&ref=nb_sb_ss_p13n-pd-dpltr-ranker_1_13

My frame man uses them alot to make sure the car is square and even. You set it for one side or one diagonal and the mirror side should be identical

I have also used a plumb laser to do frame measurements on my Z. I began by taping together a bunch of poster board sheets to form a single big rectangular sheet that was long enough to go from bumper to bumper and wide enough to just fit between the inner sidewalls of the tires. Once I had it in position under the car, I taped it to the garage floor so that it wouldn't shift while I was lying on top of it taking sightings on the various targets. Once I got the laser aligned with a target, I used a Sharpee to make an 'X' mark on the sheet (along with a little note to record what target it was for). After I had marks for all of the targets, I pulled the sheet from under the car and laid it out on my workshop floor so that I could make the various target-to-target measurements with a tape measure.

This system works pretty well for most the undercar measurements. The car can be sitting on its tires, but it needs to be elevated a bit (I used wheel dollies) so that you'll have enough room to slide underneath.

However, it won't help you with the fore-aft locations of the shock towers unless you remove the struts and sight the laser down through the hole in the centre of each shock tower cap (the procedure for this is to put a piece of wide masking tape across the hole, trace the hole outline with a pencil, mark two intersecting centrelines, and then poke a small hole at the centre point for the laser to shine through). You'll also need to set up some kind of the platform above the shock towers (I used a length of 1 x 6 lumber) to give you a perch for the laser to sit on. Additional requirements:

• You'll need to get the car up on axle stands.

• Don't do any of your measurements until the car is up on the axle stands.

• You'll need to get the lower control arms out of the way b/c they'll block the laser beam from reaching the floor. This isn't too hard to do at the front (the LCA's can be folded up to a near-vertical position after the struts have been removed). I'm not sure whether this can be accomplished quite so conveniently at the rear (I didn't try). It might be necessary to actually remove the lower control arm from the car ☹️.

Edited 3 hours ago3 hr by Namerow