One of the known ailments on a Mazda MX-5 NB and NBFL (2nd generation) are the box girders (chassis beams) in the wheel wells. These suffer tremendously from rust.

When you get to the garage after doing a service or MOT and are told that the chassis is rusty you are naturally shocked. Your bolide that you take such care of can’t be completely rusted through, can it?

How does the rust form and why doesn’t an Mx-5 NA (1st generation) rust here?

The rust results from the design of the box girders Mazda chose. The Mazda MX-5 has 2-layer box girders, this is good for strength in case of an accident and chassis rigidity. Unfortunately, it also has a downside and that is that water can get between the 2 layers and then there is no way to go…. Stagnant water that has no way to go, you guessed it, that will rust!

The MX-5 NA (1st generation with folding headlights) does not suffer from this at all because it has only 1 layer of steel in the tubular beams. Water is therefore not “trapped” by this so they will rust little to not at all.

Lapping agent and bad repairs…

Let me start with how, in our view, it should absolutely NOT be done.
1. A so-called cheap MOT solution: Welding a plate over the rust so that it is “hard” again and passes the MOT.
With this method, you’ll postpone the problem and most likely have another trammel at the MOT the following year. This is a (bad) stopgap measure and not a solution.

2. Partial repair: Grind away the rusted part and weld a new plate in there.
The rust is probably mostly replaced but again it is not the solution. There are still parts with 2-ply steel that will rust through sooner or later.

How LucaCarMods performs this repair

As indicated, it is important that all rust is gone and it cannot come back either. The solution is to remove all the 2-layer steel at the front chassis beams and replace it with a repair panel which, like the Mazda MX-5 NA, consists of only 1 layer of steel.

In this chassis beam rust repair, we start by disassembling the front bumper, crash bar, headlights, wheels, radiator, power steering and many other items that get in the way. During the repair, the engine may remain in place but many attachments will be removed. Next, the engine and fuel lines will also be wrapped with a flame-retardant welding blanket. Safety is paramount in this repair.

After most of it is domed and the important parts are packed then the grinding work can begin. This can look quite severe because there will be a fairly large piece ground out of the chassis beams. What is often even more intense than the grinding is the rust that seems not too bad from the outside but is quite advanced inside.

Next, everything should be cleaned thoroughly so that it is free of old coatings or wax layers. This is because these catch fire easily and also make for a bad weld. If everything is clean then the repair panel can be made fit to the Mazda MX-5 NB. Again, you have 2 ways to do it. A weld where you overlay the panels or put them against each other with a gap of about 1mm between them. We always choose the latter option because then you have good weld-through, nicer looks and the steel is only 1 layer thick everywhere.

To spread the heat during welding, short welds are made each time and then you jump to another point. This prevents warping and there is less hit to the engine compartment paint job.

Finally, the most important step; apply a good moisture-resistant coating both inside and outside. Inside the chassis beam we use Dinitrol 1000, this is a cavity coating and on the outside a robust Dinitrol 447 anti-scratch coating. The welds and other seams where possible moisture or dirt can get between them are sealed with a body seam sealant.

After this, the MX-5 NB or NBFL is good for many years of driving fun again!

If you would like us to perform this repair please feel free to contact us! In terms of cost, you should consider approximately €1050.00 Incl. VAT

Of course, you can also perform this repair yourself with our repair panels which you can find below.