Shrinkage ? Options

[cary]

We're getting ready to move the project off the rotissere to the lift.
We'd rehung the doors . Pretty scary hanging there on the rotissere.
How many 100's should we allow for shrinkage when we reinstall the long?

[Jeff Hail]

There is a lot of good feedback and direction here on this thread. As I have read everyone’s replies the seasoned guys here have wisdom to share. That wisdom is learned through experience of trial and error.


Andy is correct "I would not use the doors as a guide to building the frame" ...what you can use them for as an indicator of what is shrinking and shifting as you repair, replace and weld.


The main control points are the most critical parts. Suspension mounting and load bearing areas are primary. Remember when the car is assembled and all four tires sitting on the ground the center section of the tub is not bearing the entire weight of the car, its loaded from the suspension mounting points and then spread to the center section. The center section is critical to being square an level but the entire chassis is a concerted system as a whole.


If you are repairing a tub and only the center section is supported both front and rear overhangs are sagging. If you support the car from the ends such as a rotisserie the center section sags. 8 points are recommended at a minimum (center section 4 points and 2 points on each end of the tub). With a Celette you can add as many fixtures available that are made for the chassis which will provide for consistent support across the datum line through both ends of the tub, torque box’s and center section. Many cars have up to 20+ control points that can be supported. 4 point pinch weld clamps at the rockers is insufficient for major structural repairs on a 914. Think of a Celette as a surface plate with predetermined stand offs.


Many cannot afford or do not have access to a dedicated bench. What do you do? Take your time! Watch panel gaps to see what’s happening during welding, use them as a visual aid. Improvised support jigs, brackets, braces can be made and are effective. Some kind of improvised support like an H beam frame, box tubing, truss under the chassis to keep things flat and level are a start. The datum line under the car will give you a level line to reference. If your support is uniform on both sides of the tub it can indicate twist in the chassis also and height differences. Tape measures are indispensible and so are plumb lines. All of these are old school and can be done on the cheap with minimal fabrication. Might take a little longer than a bench system but if done properly will result in a straight project. X, Y and Z axis did not originate on CNC machines!


Door bars are great for roofless cars. If you use door bars I advise bracing at the windshield frame to targa bar also as door bars alone can actually spread the door openings unevenly when the pressure is cranked up. Cross bracing across the center section long to long is also recommended. It’s a lot harder for the chassis to move around when something is helping it resist or oppose forces.


When you introduce heat you will have shrinkage of some kind in the area its applied. It’s one of those laws of the galaxy called thermodynamics so get used to it. Heat goes to cold with metal. Just visualize your testicles when it’s really cold out!!! The cold sucks the body heat away resulting in turtle syndrome! Same principal applies to metal, how its controlled is key. Do not spray water or use cold wet rags on structural areas –just don’t! I have stuck and air hose with a slow blead nozzle inside of longs to “move air” and it seems to work for minimizing shrinkage.


I have also learned that human nature says add more heat when things start sputtering and popping –stop now, go back and clean your metal better at the weld site and slow down. Speed welding always causes more work in the end. Another thing- if your panel’s gaps are perfect before you start welding they are not going to be perfect when you finish. Sometimes you need to fit and spread a gap (say door opening) wider and account for shrinkage where the end result will be where you want it. Learn to control it.


One thing about metal that’s a fact; material can be added or subtracted. Spot welds can be drilled out a second time if need be. Metal is forgiving, stretchable, shrinkable and submissive. If you learn to read it you can make it do whatever you want. What’s the worst that can happen a do over?