19 May 2023
A rare rear bulkhead is located, but although the panels are straight, Trevor has some serious corrosion damage to repair
Need to know
Time: 6 hours
Difficulty: 3 out of 5 stars
Models: Defender 90, 110, 130.
Tools needed: Angle grinder with 115mm cutting disc. Jig saw with aluminium cutting blade. Drill and 5mm drill bit. Air rivet gun.
Parts & costs: YRM Metal Solutions: Rear of cab support bracket, product 389, £83.94; 2x body mounting HCPU, product 161B, £23 each; 2x body mounting HCPU, product 161C. £23 each; pair of reinforcing brackets, truck cab base, product 363, £52; pair of reinforcing mounting brackets, truck cab base, product 362, £50; seat belt mount kit with fixings kit, product 175A, £132.
• Wear protective gloves and safety boots when handling heavy objects
• Wear eye protection when drilling and using power tools
• Wear eye and face protection and thick gloves when cutting metals with angle grinder
Contacts: YRM Metal Solutions Ltd, yrmit.co.uk. Tel: 01388 488150.
A chassis cab, also known as a cab chassis or half truck, is a type of vehicle construction often found in medium- and heavy-duty commercial vehicles. Instead of supplying a finished vehicle with a pre-assembled van body, flat-bed or pick-up body, the manufacturer sells the customer a vehicle that is simply a finished cab and open chassis rails. The owner can then handbuild on whatever type of rear body is required. For example: a tipping body, fire appliance equipment or a cherry picker can be fitted to the chassis cab vehicle.
Over the years, Land Rover manufactured chassis cab Defenders, and earlier utility Land Rovers for many end-user purposes – and for the company’s Special Vehicles division to produce finished commercial vehicles.
The chassis cab format was also used by Land Rover to build both single and double cab High Capacity Pick Up (HCPU) trucks. This chassis cab type construction used a rear bulkhead to enclose the separate finished cab from the HCPU’s rear tub. This rear bulkhead is very similar in construction to the forward end of the rear body tub of a Defender 90 or 110 utility (hard top, pick-up or soft top).
Good straight rear bulkheads of this type are less easy to source than the complete tubs, because fewer were manufactured. The good news is that if one is located and needs repairs, the similar construction means that the same repair parts for a full tub can be used on a chassis cab rear bulkhead.
Corrosion damage to the rear bulkhead
The rear bulkhead for this 110 HCPU project was sourced from a donor vehicle that had remarkably good body panels throughout. Indeed, all except the main bulkhead were reusable on this project or elsewhere. However, just as the main bulkhead was unsalvageable because of advanced corrosion of the steel construction, the rear bulkhead had suffered much corrosion damage. This damage was not rusted steel, but rather electrolytic corrosion of the aluminium caused by the contact of two dissimilar metals (steel and aluminium). This type of corrosion is commonly found on the full rear body tubs of Defender 90 and 110 models, and consequently the availability of repair parts and panels is excellent.
At first glance: The truck cab rear bulkhead unit appears to be very straight, with little physical panel damage or abuse. It is a fairly difficult part to source.
Added strength: Although very similar to the front of a 90 or 110 rear body tub, the rear bulkhead has a pair of long steel reinforcing brackets fitted (black).
Nasty mess: On both sides of the rear bulkhead, the lower section of the B-post/door strike-plate is badly damaged through electrolytic corrosion.
Hidden issues: On removing the outer seat belt mounting brackets, it can be seen that the aluminium underneath is now very thin. A stud has also sheared off.
Peculiar to chassis cabs: At the back of the rear bulkhead unit, the reinforcing bracket on one side is missing and the other is very rusty. The mounting brackets are absent.
Another safety issue: There is very little material remaining where the rear bulkhead attaches to the chassis via the steel body mounting brackets
More powdery aluminium: One of two chassis mounting brackets on each side spreads load through the black brackets (shown earlier). Here, loose powdery corrosion is scraped and wire brushed off.
The old with new: There are two special brackets under the bulkhead that connect the body mounting bracket with the strengthening brackets. Badly rusted, they’ll be replaced by fresh galvanised brackets.
Repairing the mounting points
Front of tub and seatbelt mount replacement has been described before, although this rear bulkhead of the chassis cab has some important additional features related to how loads are spread through the rear of the cab, and to connection of seatbelt mounting points to the chassis. It is a requirement that every seatbelt mounting location has a solid steel connection directly to the vehicle’s chassis – simply bolting to the aluminium body is not sufficient. YRM Metal Solutions’ replacement and repair parts for the rear bulkhead have made the type of repairs seen here much easier.
Highly effective solution: Two-piece galvanised front of tub/seatbelt mount repair sections will replace rusty seatbelt brackets and strengthen body to chassis mounting points.
Correct position ensured: The studs on the repair panels are fed through the bulkhead from below, and the flange nuts temporarily fitted to hold them in place.
Bolted in: At the four inner seat belt mounting positions, bolts are fitted through to the repair panels to further ensure the correct location of the repair panels.
A minor snag: The underside strengthening brackets will not fit in position because of the flanges on the repair panel, which themselves are adding strength.
Measure twice: The brackets are easily trimmed in two places with the angle grinder to fit within the repair panels, and the cut edge will not compromise the galvanised protection.
Building strength back in: The load-bearing brackets are refitted in place above the underside brackets and the two are bolted together, sandwiching the bulkhead floor and the repair panels.
Other side: The brackets’ lower bolts are fitted temporarily, to be removed later because this is one of the mounting points for the four body to chassis brackets.
Rusty, removed: This support bracket running across the lower rear of the bulkhead, provides chassis connection points for the inner seat belt mounts. It’s rusty so removed.
New, galvanised: This replacement support bracket replicates the strength of the original part, with the bolt holes exactly matching those in the bulkhead so it can be bolted on.
Hold it in position: The new bracket is bolted to the rear bulkhead at floor level only, at this stage, as there is horizontal drilling through to do for the front of tub repair brackets.
Large bolt kit: The drilling process begins, where a large number of 8mm holes are drilled through the bulkhead in both horizontal and vertical planes.
Massively strong: The bolt-down is complete, with approximately 48 M8 x 30 stainless steel bolt sets fitted through the rear bulkhead’s repair and strengthening panels.
To see Trevor's previous work on this Defender project vehicle, go to our Technical section here.
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