Wheelbase predicament

[Elliott000]

Hey guys, the car im building seems to have suffered a bump or two. Iv'e rebuilt the passenger side frame rail (engine bay) and i'm working on the driver, after comparing numbers vs the manual and squaring things up i have a 1/4" difference when it comes to driver vs passenger location of cross member holes (from front to back). Ive got the car level, and multiple string lines for reference on straightness etc. The rails are parallel and square to the car as a whole. My thoughts are since iv'e already located everything on the passenger side should I just match to it? At the end of the day I will be doing a bit of cutting on the fenders and using flares so if there is a slight off center in relation to the shape of the fender to the centerline of the wheel.... I can hide it. My only real concern will be panel fitment. I might actually install the doors and fenders to see how things line up.

 

Any opinions on this sort of situation.

[Patcon]

If you use the techno suspension parts you will probably be ok. I would be worried there is not enough adjustment to get the alignment right with the factory parts if the cross member is 1/4" out of square with the car...

That is provided I understood your description properly...

[Elliott000]

Yea u have it right, if i run a framing square off the passneger frame rail aligned with a bolt hole. The drivers side holes are 1/4 ahead of it. I had looked at all the techno front suspension. I've also thought of fabbing my own adjustable lower stuff. Tc rod and lower arm. If i can build stuff to handle jumping my bronco the datsun should hold haha

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[Patcon]

I would like to know where the 1/4" bust is. That would bug me something terrible...

[Elliott000]

It is bugging me aswell, looking at the upper structure that air horn part, everything seems pretty straight. I can see a bit of separation where the radiator mounts on some mating surfaces, the firewall is straight and there was some denting in the main frame member that had been hammered flat again. Im thinking I may have screwed myself by making the passenger side first. When I heck measurements in relation to the manual the driver side checks out pretty good. I may end up opening the frame i made and adjsuting the passenger side bolts holes to match the driver.... If i plumb bob down from the upper strut mount and compare those two points there is a variance aswell but it is only an 1/8" so theres less movement there up top.... AHHH dunno what to do haha, at least I have time on my side, i can just go down there and stare at it some.

[Patcon]

1 hour ago, Elliott000 said:

It is bugging me aswell, looking at the upper structure that air horn part, everything seems pretty straight. I can see a bit of separation where the radiator mounts on some mating surfaces, the firewall is straight and there was some denting in the main frame member that had been hammered flat again. Im thinking I may have screwed myself by making the passenger side first. When I heck measurements in relation to the manual the driver side checks out pretty good. I may end up opening the frame i made and adjsuting the passenger side bolts holes to match the driver.... If i plumb bob down from the upper strut mount and compare those two points there is a variance aswell but it is only an 1/8" so theres less movement there up top.... AHHH dunno what to do haha, at least I have time on my side, i can just go down there and stare at it some.

Yeah, I hate that. I don't have a good answer for you. A little far for me to stare at it with you...

[Namerow]

Would that denting in the main frame member have looked like this? (taken from Grannyknot's Z-M3 build thread)...

Does anyone know how this happens?  Hard to believe that this is collision damage.  It looks, instead, like damage cause by a hoist... but one would have thought that the rail would be strong enough here to withstand the load.  Also puzzling is how the damage appears to have been inflicted in two different locations.  Does that mean that a second mechanic (or owner) chose to ignore the damage created on an earlier occasion and then went ahead to prove that the same mistake really can be made twice? 

I'm thinking that maybe this type of caved-in rail results from the use of a bottle jack.  Comments?

[Elliott000]

My rails were beat on but not like that, my issue is rust. On yours I agree that's either a poor jacking job or someone drove it over a boulder! I can take a picture of the dent when I'm home tonight. Me and my boy who's 9 but has a good eye , were looking at the front end some last night. We found a few ripples that had been hammered out and also a little separation on some folded steel wherr the hood pivot mounts.

On that note, would one of u fine people be willing to measure the distance between the two panels where the hood pivot bolts on? I'll post up a pic tonight, it'll be easy to do when the hoods up.

 

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[Elliott000]

I'm back in the trenches on the 240z... Looking to get some intel from u guys again. In relation to the drawing found in the FSM it would "appear" that the pinch welds where the inner rocker ties to the outer are parrallel. I plumbed these fixtures down and snapped some lines and in reality on my car at least they are not they are wider at the front. These are measurements taken from the top weld, ill compare with the bottom tomorrow.

 

That's one thing, next is a true centerline, what do u think my best point is on the rear to drop a plumb bob and get a center mark. From center of rear suspension/spring bolts?

 

Then up front from the firewall are there any sure fire center marks or spots. With the differences im finding in the nose from this cars possible accident at some time. The only real way I can sleep at night and trust its straight when I start welding stuff is to have a dead to bust center line to work off....

 

Any and all input is appreciated.

 

I'll post a pic tommorow hilighting the areas in question

 

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[Namerow]

That's an interesting measurement.  My initial thoughts are:

• It's hard to believe that the LHS and RHS pinch weld lines weren't designed to be anything other than parallel.  Proving that with 100% accuracy is another matter, of course, because we have only the FSM 'Standard Body Dimensions' diagram as a reference and it doesn't even specify the left-to-right separation distance of the pinch weld lines.  I've just tried doing a blow-up of the diagram on my screen, so that I could scale off the separation distance and compare it to a marked measurement.  I get 1326mm.  The accuracy of a derived measurement like this is probably +/- 2mm.  Still doesn't solve your predicament.
   
• How do you find a centreline for a unibody?  That's a great question.  It would be nice if the design included an alignment hole at the front and the rear -- but it doesn't.  At the back, you have the two 'C' holes drilled into in the rear sub-frame piece.  Plumb those two, then measure and mark a centrepoint.  At the front, however, you don't really have anything obvious to work from.  Whatever you choose, you need to make sure it's not compromised by any of the collision damage -- everything ahead of the firewall probably needs to be treated as suspect.  Finding the mid-point between the pinch weld strips under the front edge of the doors might be as good a reference as any.
   
• Collision repair sources say that "3mm" is the accuracy that they shoot for.  I believe that, in practice, this means they work to +/- 3mm, rather than +/- 1.5mm).  So that means they're happy with a left-to-right discrepancy of up to 6mm ( about 1/4") on wheelbase, or any other pair of long-ish measurements.
   
• The rotisserie photos below (unknown source, so apologies for no photo credit) may give you or somebody else has some better ideas about how to find and mark a centreline.
   
• The racers in the crowd are probably the best source of information on how to do this and on, 'how close is close enough?'.  Track damage is pretty common occurrence and race cars usually get repaired rather than replaced.

 

[Elliott000]

Awesome info. I appreciate that. Can i co firm, the "c" holes, they are about 1" in diameter just by the seat belt buckle buckets

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[Namerow]

Nope.  I think the 'C' point is something that you have to mark for yourself, based on the visual info inferred (implied?) by the FSM diagram.  It seems to be in line with the outboard wall of the longitudinal rear sub-rail where that rail ties into the front transverse sub-rail, and then positioned dead-centre on the front transverse sub-rail.  Note that there is a hole (factory-punched) in the transverse rail in that vicinity, but it sits about 1/2" outboard of the location that the FSM diagram suggests.  Who knows?  Maybe the punched hole is the C-point.  It would certainly serve quite nicely for purposes of fore-aft measurements, but it would be off a bit for width-wise and diagonal measurements.  See pix below (and, again, apologies to the unknown owner of the newest photo I'm about to post):