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Model A chassis, introduction: In this series we will follow the construction of a Model A chassis. This isn't intended to be the definitive how-to, as there are multiple correct ways to perform any of the individual steps in this process, and we will not assume that our way is the only way. In this introduction, we will prepare the chassis for boxing.
One advantage we do have is a jig to which we can clamp the rails, which minimises the distortion that is inevitably created when so much welding is performed. This isn't a production jig, but it is square and dimensionally accurate to less than half a millimetre. The rails are RodTech reproductions, pressed out of 4 mm plate; the attendant boxing plates are also 4 mm, 25% heavier and stronger than original Model A rails.
Given that a stock, 76 year old Model A chassis, with stress cracks and serious rust, fetches about the same price as a the RodTech components, and that the guidelines allow the use of repro rails, that decision is a no-brainer. Some states have different rules relating to repro chassis - if you are planning a new chassis, check first.

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1. Zoom this illustration to see what we mean about the area at both ends of the rails that may need some attention with a 4 lb hammer.

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3. The Root nut requires a hole slightly larger than the clearance diameter of the fixing bolt.

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5. Here we use a high tech method to protect the thread of the body mount bolt while we were welding - a piece of scrap steel held in place by a clothes peg spacer.

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7. There are many ways to box a chassis; this method allows excellent weld penetration. It is a good idea to trim one edge of the boxing plate with a guillotine, if possible, otherwise the edges could be trimmed with a 1 mm cut-off disc in a hand held 125 mm grinder, then trimmed up. The person who welds the boxing plate will appreciate the straight edge.

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9. We tacked the top edge of the boxing plate to the top edge of the chassis rail, then worked on the bottom edge. Before we welded the bottom edge, we clamped the rail from the welded top edge to the un-welded bottom edge, to ensure the boxing overlapped the bottom edge.

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11. To have access to both sides of the boxing plate, we turned the jig on its side. You can see the plan view of the chassis here, with the rails running parallel at the rear, rather than continuing to splay outwards in the original fashion.

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2. A Root nut (or weld nut) is specifically designed for these situations. The over-sized hex, the turned collar that goes through the job, and the scallops under the hex that allow weld to flow under the nut, confirm that.

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4. We used M8 Root nuts for the running board brackets, and bonnet mount brackets, M10 for the body mounts. We would have preferred to use UNF, but Imperial sizes are not always available. If you use Root nuts, try to get them on a trade order - the retail price is almost 5 times the trade price.

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6. This shows the weld on one side of the Root nut. We used a cheap Chinese TIG welder to weld on two sides. The cheap & nasty welder had to have a break after welding just 6 nuts in place - a very poor duty cycle, and not what you would expect from industrial quality equipment. We then cleaned the surface rust off the interior of the chassis rails and boxing plates, then painted the interior surfaces, to provide some protection until the chassis is powder coated.

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8. 8mm plate is firmly clamped to the side of the jig, then the rail is clamped to the plate. We should have used better quality clamps; the hardware store cheapies worked well enough, but we may have to chuck them out before we do the next chassis.

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10. RodTech also supplied the pressed steel body mount brackets and bonnet (hood) catch brackets. Here they are bolted onto the rail before we welded the Root Nuts in place. This ensures that we won't need to drill out the bracket holes later, to get them to fit.

These rails were purchased 5 years ago, and as a result of sitting in a damp shed, are covered in a light film of surface rust, which requires mechanical removal, with a wire brush and sanding discs. New rails are shiny steel, so that step is normally not necessary.

Model A Fords have a simple chassis, consisting of steel channel, or C-section longitudinal rails, and formed channel cross members that are riveted to the rails. The chassis were designed to flex a lot, as the suspension was quite rigid. Our setup is going to have a RodTech independent front suspension (IFS) and a Jag rear, both supple examples of modern suspension, so a more rigid chassis is required.

The V8 engine has heaps more torque than the original 4 banger; another reason to box the rails. Boxing refers to converting the rails from C-section to a box section, by welding on a 4th side, the full length of the rails.

The jig was a fortunate find; not everyone will have access to such a handy item. This one will have at least 3 chassis on it in the next 6 months, so if you and some friends are considering new chassis, it might be worth your while to build your own. You might also put rotisserie style pivots at the ends too - the possibilities are endless.

Before we began boxing, we examined the rails for over-stamping, a situation that happens in production, where the extremities of a fold are bent at more than 90. We pounded the rails with a 4 lb hammer until the folds were 90 - a straight edge confirmed how straight the top and bottoms were. See illustration 1.

Root nuts have been around for decades, but some rodders prefer to weld in chunks of steel and drill and tap the holes after the boxing is in place. Either will work, but we find Root nuts a cleaner and simpler solution. If the rails had been made from RHS we would not have been able to use Root nuts, but then we wouldn't have such a large weld along each join either.

After the boxing plate had been tack welded to the rails, the rails fitted the jig without any force required to bend the rails to conform to the shape of the jig. That is important, as our next step is to remove the rails and weld them full length, outside the jig.


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