Chassis repairs

Penned by resident technical adviser “Assassin”, this lengthy yet comprehensive article concentrates on a host of chassis repairs from section renewal to crossmember replacement.

Many people contemplate chassis repairs on 4×4’s but are put off by them. They require nothing more than basic fabrication and welding skills, a welder, angle grinder, some accessories, and a little ingenuity along with some basic fabrication tools.

Chassis sections on most 4×4’s are 3mm or thicker section and can be welded with either MIG or arc welding. I would recommend a MIG welder of around 150 amps and 0.8mm wire or an arc welder of no less than 100 amps, plus a selection of 1.6 and 2mm electrodes.

Many people are aware that I support British engineering companies and this includes engineering supply companies, and I make no apologies for this. One such company is a company called R-Tech Welding Equipment Ltd. who manufacture and supply quality industrial welding equipment at the price of mid-range DIY equivalent and are worth a look. They may seem expensive next to the cheap Chinese welders but they incorporate the latest inverter technologies among other things. Having used a number of them I can vouch for their quality; and being much easier to use, they are ideal for beginners to welding. They also do a nice range of dual purpose machines which are MIG and stick welders in one machine.

Angle grinders should ideally be 115mm or 4.5” as these hold the industrial-sized accessories as they have the M14 thread instead of the smaller M10 thread of the 4” angle grinders.

Angle grinder accessories should include:

  • Backing pad for resin bonded discs as these flex and bend.
  • Resin bonded discs in a coarse and fine size, as these are more delicate than a traditional abrasive disc and clean up just as quickly without leaving the score marks, for they remove less metal and leave a finer finish and are better suited to thinner metal sections and sheet metal. Once you have used them you will rarely use abrasive grinding discs again.
  • Wire brush of around 3” / 75mm diameter, these simply screw onto your angle grinder and are great for initial removal of contamination and can be cleaned.
  • Thin abrasive cutting discs often called plasma or slitting discs. These are much thinner than the traditional 3mm thick cutting disc, generate much less heat, also going for 0.8 or 1mm thick discs is the better option.

Fabrication tools should include:

  • Scriber to mark score lines onto metal. Avoid pencil as it rubs off and the lines are much thicker than a scriber, you cannot rub a scribed line off as it scores the metal. (Editor’s note: Engineer’s French Chalk or Soapstone as it’s sometimes called, can also be used as it is designed specifically for welding as it doesn’t burn at high cutting temperatures, plus it won’t rub off as easily as pencil or blackboard chalk. However, as eluded to by Assassin – a scribe scores the metal, hence is totally foolproof).
  • Steel rule 12”/300mm long as this provides a straight edge for scribing against for accurate lines.
  • 5 metre measuring tape as this is long enough to measure most chassis.
  • Small centre pop or (also called a centre punch), this is used to put a pop mark into steel for future reference for datum points and acting as a centre for accurate drilling to stop the drill bit from slipping.
  • Permanent black marker for marking dimensions onto a cleaned chassis – leave a job overnight and it’s surprisingly easy to forget your measurements.
  • Hole-saws for metal. Kits are cheap enough and most cordless drills will power them at slow speeds.
  • Several magnets, these are very useful for holding repair sections in place, particularly for invisible repairs.
  • Combination square. Only buy the 12” /300mm version with a cast iron head, not the cheaper aluminium headed type, the better quality cast heads cost much more but will last a lifetime. These act as set squares for accurate angles, and as the rule slides you can have as little or as much protrusion as you like to get into awkward spaces, plus you can set the length for repetitive depth markings.
  • Various clamps and bits of metal such as stout angle iron for marking out or bending metal.
  •  Spreader bar and a large heavy duty ratchet strap. If you replace a cross-member the chassis may distort, so if it closes up you can open it up with your spreader bar, if it opens up you close it with your ratchet strap. Alternatively, you can use both together for a very accurate positioning. MAKING A SPREADER BAR: Making a spreader bar is easy – take a piece of 1” steam pipe 6”/150mm shorter than the distance between your chassis rails, get some 20m threaded bar and nut, weld the nut centrally and squarely onto one end of the pipe. Cut 12” / 300mm of threaded bar and carefully grind two flats on one end wider than the jaws of a 12” adjustable spanner. Make 2 square sections from 5mm or thicker plate at 50mm square. Weld one centrally onto the opposite end of the pipe to your nut, then weld the other one to the end of your threaded bar with the flats on; these spread the load on your chassis. Screw your 20mm bar on to the nut and down into your pipe, wind it out and use a 12” adjustable on the flats to wind the thread out to spread your chassis.

Basic chassis repairs:-

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How NOT to weld!

We have all seen the bodged chassis repairs where someone has thrown on a plate over corroded metal and attempted to weld it. The weld looks like what is known in the fabrication profession as ‘pigeon shit’. This is where lumps of metal are along the weld, but it hasn’t welded properly.

Making an invisible repair involves more work, but not much more! In addition, it removes the corroded metal and leaves only good metal … and as we all know, the only way to get rid of rust is to remove it.

Clean your chassis in and around the vicinity of the repair using your angle grinder fitted with the wire brush as this will remove both paint and underseal; ensure you clean a good area around the affected area of your repair. Swap your wire brush for the backing pad and a fine resin-bonded disc and go over the area as this will buff it and lightly grind it – do not use too much force on it.

Mark out the area with your combination square and steel rule in either a rectangle or square, ensuring you scribe the lines on accurately. Place a piece of angle iron on a line, clamp tightly and use this as a guide to rest your slitting disc on for an accurate cut. Change your resin bonded disc for a slitting disc. Start your angle grinder and lay the disc flat onto the angle iron and slit into your chassis. Keep your slitting disc flat to the angle iron and slit one way to the end of your cut, then go the other way. Repeat with the opposite line and you now have your top and bottom slit. Repeat at one end cut, thus leaving three sides cut, then pull the slit end of the metal out and put a piece of plate behind it holding it in place with a magnet. Then set up your angle iron and do your final end cut. Your cut piece won’t fall inside your chassis and you won’t be digging about inside a chassis rail to recover a hot and sharp piece of metal. You will however have either a perfect square or rectangular hole in your chassis.

Cut your repair section from your repair plate, use your angle iron as a guide as you did cutting out your chassis. Offer it into your hole by putting two magnets on it, one either end, and ensure it’s a snug fit. If the cutting of the repair piece isn’t accurate then sandwich it between two pieces of angle iron. Leave only a slight protrusion and using a fine resin bonded disc you flatten the edge of the repair section down to the angle iron for a perfectly flat surface. Repeat until the repair section is a good fit.

Insert the repair section into the hole in your chassis and use your magnets to position it squarely into the hole. Use very thin sheet steel to pack if necessary. Once aligned you then tack weld one top corner with a small tack and check your alignment. Ideally you want a gap of 0.5-1mm between the new metal and the chassis for full weld penetration. Tack the bottom corner on the same end and then the middle at that end. Work along the top and bottom seams alternately, placing tacks every inch or 25mm. This equalises the heat to minimise distortion. TIP: If you keep a wetted cotton cloth you can dab the welds to cool them.

With the repair section fully tacked in place, you can then seam-weld round it, allow to cool, grind the welds flush with a coarse resin bonded disc and finish with a fine resin bonded disc for a full strength. The result is invisible repair and all your corroded metal is removed.

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Basic chassis repairs can be relatively easy and straightforward for the amateur enthusiast – this photo being a classic example – but if at all unsure about your skills, let a professional tackle the job.

Repair Through The Chassis Mountings:-

Many items require repair through the chassis mountings, for instance side steps or tow bars. If you just drill a hole right through the chassis, push a bolt through and tighten it up, it will result in crushing the chassis. Where tow bars are concerned they are subject to a massive amount of load where failure could be fatal.

Repairs through the chassis mountings are simple to make and install properly, and all that is needed is to install a tube right? No! Importantly, you need to install a tube and spreader plate to spread the load; again this is due to the strain the area can experience. Measure the width of your chassis and add the thickness of both your spreader plates to this measurement. If we assume your chassis is 100mm wide and you intend using a spreader plate of 3mm thickness, your tube length is 100mm + 3mm + 3mm as this is the thickness of your chassis, plus the thickness of your inner spreader plate, plus the thickness of your outer spreader plate. Personally, I would also add another 3mm to this to allow for any chassis distortion.

Remember – you can always grind a bit off, but you cannot grind a bit on!

For basic or low loadings through the chassis mountings we need a steel tube with a bore large enough to take the bolt of your prescribed size and two spreader plates of 3mm thickness. Cut two plates 50mm x 50mm square or 60mm x 60mm square depending upon the outer diameter of your tube. Scribe across from corner to opposite corner to find the centre. Centre pop the point where the lines intersect and drill a hole large enough to clear your tube. Add up the thickness of your chassis and spreader plates plus another 3mm, then cut your tube to length. Chamfer one end of the tube to give weld penetration and set the tube in a vice, inserting one of the spreader plates over the end and weld on squarely and grind flush.

Mark out your chassis for your mounting point and ascertain your tubes outer diameter. Mark one side of the chassis and centre pop the hole centre. Transfer that down using your combination square to the bottom of your chassis, across the bottom, and up the other side of your chassis. This should give a reasonably accurate marking to centre pop the mark on the opposite side of your chassis.

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Now we need to find the centre of the hole for accuracy in installing your tube. To achieve this we make a couple of datum points by taking our combination square and scribing a line from the top to the bottom of the chassis, ensuring it passes directly through the middle of the centre popped mark. We do a second line at 90 degrees to this first datum line using our steel rule ensuring it passes through the middle of our centre pop mark, ensuring these cross hair lines are longer than the spreader plate.

With both sides of the chassis marked, centre popped and datum lines scribed, we drill the holes. However, never drill a hole just large enough to accommodate our tube diameter. We make it around 3 – 5mm larger, hence the need for our holesaw. By cutting the hole for the tube larger than necessary we have adjustment in the hole to allow for slight inaccuracies.

If you take your spreader plate welded to your tube and measure it along each flat side and mark it, then place a scribed line from side to side and top to bottom, they should pass through the centre of your tube. Repeat this on your loose spreader plate. Install your spreader plate with tube through your chassis. If you line up the datum marks on your spreader plate with those on your chassis, the centre of your tube will be exactly on your centre popped mark. Fit the loose plate on the other side and align this and clamp them both. Grind off any protruding tube flush with your spreader plate, remove both plates, grind a bevel or chamfer on the end of the tube and reinstall. Tack the spreader plate with the tube attached to your chassis, tack the tube on the opposite side to the loose spreader plate and weld. Now weld both spreader plates to the chassis and grind the tube welded to the loose spreader plate flat with a fine resin bonded disc. You now have a ‘through the chassis’ tube perfectly aligned and structurally sound.

For heavier loadings you simply follow the above procedure and make your tube from thicker wall material and use thicker/larger spreader plates.

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Opps … this Toyota has certainly seen better days!


Crossmembers are often the hardest things to replace as many people feel they lack the necessary skills to make new crossmembers, or feel it may be too difficult to fabricate new items and install them correctly.

Datum points are the essential component of any crossmember replacement as these are what provide the accurate alignment of the chassis if it moves, and datum points are easily dealt with if we know how to create and use them. Before we remove any crossmember we create our datum points using our combination square, accurate measuring and a centre pop. We do this by creating two centre pop marks on exactly the same position of both chassis rails and two pop marks at the exact centre of the crossmember on each chassis rail.

Measure the exact centre of your crossmember and scribe this on the underside of your chassis rail. Repeating this on the other side, on the underside measure at least 12” (300mm) front and rear of this line on one chassis rail, and using your combination square mark it right across the underside of your chassis rail. Repeat this on the other side on the chassis rail.

You should now have three scribe lines running across each chassis rail at equal distances. Measure the width of your chassis rail and set your combination square to exactly half that distance, working from the OUTSIDE of each chassis rail you mark across each scribe line at 90 degrees. This should leave you with three X markings on each chassis rail. At the intersection of these marks you carefully centre pop them, hence leaving you with three centre pop marks on each chassis rail. You now measure them.

Begin by measuring from the left hand chassis rail front, centre pop mark to the front centre pop mark on the right hand chassis rail and note the measurement. Repeat at the centre and rear centre pop marks.

Now we cross measure, we measure from the front centre pop mark on the left hand chassis rail to the rear mark on the right hand chassis rail, and then from the rear centre pop mark on the left chassis rail to the front mark on the right hand chassis rail and note these measurements. Always take these dimensions as they are crucial to alignment of the chassis rails upon installing the new crossmember.

If your crossmember is a straight section such as a box section or tubular section you simply obtain a new piece of steel box section or tubular section of the correct size and cut to length. Cut oversize by about 1-2mm as you can always grind a bit off, but again, you can’t grind a bit on. If your crossmember isn’t the same height as your chassis rails you will need to measure the height of the crossmember from the bottom of the chassis rail to the bottom of the crossmember and note the measurement.

You now cut out the old crossmember using a slitting disc by working round it. Cut carefully and ensure you wear goggles at all times while cutting or grinding. Fit a coarse grit resin bonded grinding disc and grind off anything the chassis, swap for your wire brush and clean the entire area.

Once the area is cleaned of all the old metal it’s time to examine the chassis. Often they contain a hole inside the crossmember for initial protection inside the crossmember from the factory applied corrosion inhibitors. If it has such a hole it needs filling flush to the chassis. Never weld a plate over the top as this can impede you when you install the new crossmember, cut to the exact length and try to slide it in, this will make you force the chassis apart with your spreader which is unnecessary work.

With your chassis cleaned and plugged you need to prepare your crossmember for installation. Most crossmembers have holes or slots cut in them, if you have services installed through your crossmember you have to replicate them. In addition many manufacturers fit holes or slots in their crossmembers and it is prudent to do the same. Where services pass through it’s advisable to install either tubes or other hollow sections and seam weld them in. If you have no services then you will need a couple of drain holes. I prefer to drill a couple of 12mm holes at each end of the crossmember near the end on the lower side, but not too near the end to impede the welding, and another 12mm hole in the top centre of the crossmember. It is now ready for installation.

To install you need to get the crossmember in position. If it is smaller than the chassis rail section I clamp packing to the chassis, the crossmember then sits on top of this packing.

You now do the crucial bit, this being a check all the datum point measurements. If the chassis rails have closed up then you need to spread them with your spreader you made earlier. On the other hand, if it has splayed you have to close it up with your ratchet strap.

It is important to take extreme care with this stage of the process, taking time to get it right.

Once your crossmember is installed and the datum point measurement points match its welding time, always seam weld these types of crossmember in. At this point I apply a good coat of industrial paint to the outside of the crossmember and let it dry, once dry I wrap masking tape around the crossmember to cover the two bottom drain holes, I then pour old engine oil in through the top hole until the new crossmember is full of oil. If this is left overnight it will coat the unexposed sections of chassis rail and the inside of the new crossmember. Leave overnight and pull off the masking tape at one end and drain the old engine oil into a bowl. Now plug all three holes with 12mm grommets.

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Prefabricated shaped crossmembers are available for most 4x4s , hence making repair a lot simpler.

If you have a standard section crossmember which is shaped and not straight it becomes useful to buy the section you require, you then take a piece of plywood and draw your crossmember onto it. I do this by placing a straight edge across the bottom and one at the top. Measuring between them you then have the maximum height. Place your section inside this and scribe along the edge from your top line and from your bottom line. Measure to the centre of any bend and translate this onto your plywood, this will give you your angles. Put your section inside it and you will have a full size drawing of your section and a template to set it out.

Where you have an angle you measure an equal distance, approximately 6” (150mm) along the outer lines and mark them. Take your combination square and set it flush on your line at the marked point and scribe a line at 90 degrees to the outer lines on the inner ‘edge’ of the angle, NOT the ‘outer’ of the angle. Where these lines intersect you take a straight edge and run it through this point and the outer line of the section lines you marked, this then gives you exactly half the angle – these are the datum lines.

Place your section onto the plywood and using your combination square you follow these lines and scribe them onto your steel section to give you the exact cutting angle for your joints. Scribe this all the way round your steel section, cut this angle using your slitting disc and a straight edge on your section. You now have your first angle cut, so lay this back onto your template and align the cut end on to your angle datum line. Scribe your next angle datum line exactly from your template, repeat as for your first cut, you will then have your first piece cut and your next angle cut on your second piece. Align this cut end to the first piece on your template and mark the uncut end. Repeat until you have all the pieces cut and line them up on your plywood template. Hopefully everything should match, if it doesn’t, then fine tune with your grinder.

With everything perfectly aligned on your template you tack everything together and check it hasn’t pulled out of alignment. Turn it over and tack the opposite side and place it back onto your template and check alignment again. Once perfect, you weld it all together. Finish by drilling any through holes as previously described, you will have a jig-built and perfectly aligned crossmember; fit as above.

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Nice job!

If you want a shaped crossmember fabricated from lighter material you follow the same procedure for making a template in plywood, but you cut it out and copy this template onto your sheet metal. Do this twice for both the front and rear shaped sections of your crossmember. On the top section you need internal flanges, so on any straight sections you add a 6mm wide piece to form this flange. If the bottom is flanged, then add the required amount on the bottom of your shaped section to form a flange.

Place both shaped sections between two stout pieces of angle iron in a vice. If your vice isn’t deep enough to accommodate your shaped sections you move your angle iron one way, so your vice only clamps one end, then clamp the other end of your angle iron with a good clamp. Ensure both pieces of shaped crossmember section and both pieces of angle iron are perfectly aligned. Drive a metalworking chisel between the two pieces of crossmember to open them slightly, move the chisel along these sections and form an opening all the way down their length. Repeat this moving from end to end until the two flanges are far enough apart to use just a hammer on them and work slowly from end to end of the flange. Never hit too hard or work too quickly, as you will stretch the metal and you want to avoid this. Once the long flanges are formed it is time for the shorter flanges on the angles. Set them up in your angle iron as you did for your long flanges and form in the same way. Repeat for the opposite angled section, you will now have two shaped sections of crossmember handed, with flanges.

Measure along the top edges and add a couple of inches, then measure the width of your crossmember and cut this piece of flat strap from your sheet metal. Mark the middle and scribe a line across, taking one of your formed and shaped ends and place this on the centre mark with the flange facing inwards. Align and put a tack on the inside edge of the flange. Get your second shaped section and repeat, you will now have two shaped ends tacked to the flange on the inside of the forming box section. Put the whole assembly upside down on a flat bench and work down the inside of the section tacking the top plate to the flanges. Once you reach a radiused corner you turn the assembly over and fold the top section over tightly and tack on the outside. Once you have formed the radiused section you turn it over and continue working in the inside tacking the top plate to the next section of flange inside the crossmember, then repeat until the top plate is fully formed and tacked.

Seam weld the entire assembly by starting on the inside and welding the top section to all the flanges. Carefully weld the insides of the corners where there is no flange, turn over and seam weld the outside. Fit the bottom plate and seam weld, drill any drain holes or holes for services – install.

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Tee Nut, also known as a Blind Nut

Many people have issues with captive nuts on older vehicles and we have two simple solutions to rectifying these. The solution is the pronged ‘Tee nut’ (also known as a T-nut or Blind Nut), for when you bolt together your new bed, it is the pronged Tee nut which holds your bolt. These come in M6 or M8 thread sizes as the popular and cheapest sizes, but you can get M5 to M10 thread sizes.

Take your angle grinder and slitting disc and carefully cut off the prongs, grind flush if necessary. Drill a hole in your chassis/body in the exact position of your captive nut to take the centre or tubular section of your Tee nut, countersink if necessary, insert nut, weld in and grind the face flush. Allow to cool and run either an M6 or M8 tap down the thread to clean out. These nuts are available via Toolstation and any good fixings supplier.

For heavier duty captive nuts, or to replace nuts which have stopped being captive and spinning we use another method, this is the holesaw and connector nut approach.

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Connector nut – also available in stainless steel

Connector nuts are basically standard sized nuts, just a lot longer. They are designed for connecting lengths of threaded bar together when they are used as hangers for industrial equipment and come in M6, M8, M10, and M12 as standard sizes although they are available in other sizes at a price. For a new installation we decide upon our thread size and mark out the chassis and centre pop the hole and extend our markings beyond the diameter of the holesaw, centre pop one of the datum lined on the inside and outside of the cut for reference upon installation. Drill the hole and this gives you a core of steel from the chassis in the holesaw, remove and hold, drill out the centre hole formed by the holesaw pilot drill in your slug, which is usually 6mm to the desired size to clear your bolt size. Normally I use 7mm for M6 bolts, 10mm for M8 bolts, 12mm for M10 bolts, and 13mm for M12 bolts. These can also be sourced via Toolstation.

On the opposite side to your centre pop mark on your datum line you clean using your resin bonded disc, clean one end and sided of your connector nut as most are BZP (Bright Zinc Plated) coated, align the nut squarely on your slug and tack weld, turn over and ensure the hole in the connector nut is aligned squarely to the hole in your slug. Once you’re happy with the alignment you seam weld the connector nut to your slug and allow to cool. Place the slug back into the hole and align the centre pop mark and the datum lines, hold with magnets and seam weld back in; you now have one captive nut with plenty of thread. Allow to cool and grind the weld flush, run a tap through the connector nut as the heat when welding often closes them slightly, so screwing in a bolt can damage the thread.

To replace a captive nut which is damaged we use basically the same procedure, for 6mm threads we can leave the 6mm pilot drill in, but for 8, 10, and 12mm threads we have several options. My preferred option is to drill a bolt of the same size thread as the hole in the lathe at 6mm down the centre, screw this into the nut until finger tight and grease your pilot drill and insert into the 6mm hole down the centre of your bolt and cut your slug out of your chassis. Recover the slug and grind off the nut and any captivation, replace with a connector nut, and proceed as above. You will now have a new, fully welded blind nut which will never come loose again, and with plenty of thread.

If you find the connector nut is too long then cut a bit off after welding to your slug.

If your connector nut is in a prone area and fills with dirt then simply obtain a grub screw of the same thread, screw in a few threads on the end inside the chassis, tack weld in position before installing the slug back into the chassis; and you have a fully sealed blind nut.

Some links to view some of the listed items.

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