Most heavy truck metal tanks are sourced by OE number — the part number identifies the exact specification and the supplier either has it or doesn't. Custom tanks work differently. There is no catalogue number, no standard spec sheet, and no shortcut past the confirmation process.
This guide explains what information a buyer needs to provide, how the custom tank confirmation process works, and what to verify at each stage before committing to bulk production.
When You Need a Custom Tank
Custom fabrication applies in four main situations:
OE discontinuation. The original manufacturer has stopped producing a specific tank and no aftermarket replacement exists. A drawing or sample of the original part is used to produce an equivalent.
Chassis modification or retrofit. The customer's chassis has been modified — extended wheelbase, added axle, non-standard body mounting — and the standard tank no longer fits the available space. A new tank is designed around the actual installation envelope.
Fleet standardisation. A fleet operator wants to consolidate to a single tank specification across multiple chassis variants for maintenance simplicity, or to increase capacity beyond the standard OEM option.
OEM supply. A vehicle manufacturer or bodybuilder requires tanks produced to their own drawing as a supplied component for new-vehicle assembly.
In all four cases, the starting point is the same: providing enough technical information for the manufacturer to understand exactly what needs to be built.
What to Provide at the Inquiry Stage
The more complete the information at the inquiry stage, the faster the confirmation process. The minimum required to start:
Option A — Drawing A dimensioned engineering drawing (2D or 3D, DXF/DWG/STEP/PDF) is the most reliable starting point. The drawing should specify:
- External dimensions and mounting hole positions
- Capacity (litres) or internal volume
- Wall thickness or material specification
- Port locations and thread specifications (drain, sender unit, breather, filler)
- Surface treatment / coating requirement
Option B — Sample or existing tank If no drawing exists, a physical sample of the original tank can be used for dimensional reverse-engineering. Provide the sample with all ports and fittings intact. Note any dimensions that are critical tolerances versus approximate fits.
Option C — Installation parameters If neither drawing nor sample is available, provide:
- Available installation space (length × width × height envelope)
- Required capacity
- Mounting method and available attachment points
- Port requirements (quantity, approximate position, thread type)
- Vehicle chassis make, model, and year (helps cross-reference existing OEM dimensions)
The more constraints are defined upfront, the fewer rounds of clarification are needed before a drawing can be produced.
The Confirmation Process
Custom tank projects follow a structured sequence. Skipping or compressing any stage introduces risk that typically surfaces during bulk production or in the field.
Stage 1: Technical Review and Quotation
The manufacturer reviews the provided information and identifies any ambiguities or structural requirements not addressed in the drawing (for example, a drawing that specifies dimensions but not wall thickness for the operating pressure). A quotation is provided based on the confirmed specification, material, quantity, and surface treatment.
At this stage: clarify anything that is unclear in the drawing. Changes after sample production cost significantly more than changes at the drawing stage.
Stage 2: Sample Production
One or more samples are produced to the confirmed drawing. Samples are inspected against the drawing dimensions, pressure-tested to the specified working pressure, and shipped to the buyer for physical fitment and approval.
What to check on receipt of samples:
- All external dimensions against the drawing (use callipers, not visual inspection)
- Mounting hole positions and thread specifications
- Port positions and thread engagement
- Pressure test result documentation
- Weld quality: uniform bead, no visible porosity, full penetration at end caps
- Surface treatment: coating uniformity, no bare spots at port edges or bracket welds
Do not approve samples that pass visual inspection but have unresolved dimensional deviations — even small ones. A 5 mm shift in a mounting hole position that seems acceptable on the sample becomes a systematic fitment problem across hundreds of production units.
Stage 3: Sample Approval and Drawing Lock
Once the buyer approves the sample, the drawing is locked. This becomes the production reference. Any change after this point — even a minor port relocation — requires a new sample cycle.
Approval should be provided in writing with the approved sample retained by both parties as a physical reference standard for bulk production inspection.
Stage 4: Bulk Production
Production proceeds against the locked drawing. Reputable manufacturers maintain first-article inspection records for each production batch — measuring a defined set of critical dimensions on the first units produced and documenting the results before continuing the run.
Request first-article inspection records and pressure test records for each batch. For large orders, consider specifying incoming inspection rights at the manufacturing facility before shipment.
Stage 5: Pre-Shipment Inspection
Before final shipment, verify:
- Batch quantity against purchase order
- Random dimensional check against approved sample
- Pressure test documentation (hydrostatic test at 1.5× working pressure for air reservoirs)
- Coating or surface treatment per specification
- Packaging adequate for transit without surface damage
For first-time bulk orders with a new supplier, an on-site pre-shipment inspection by a third party is worth the cost.
Common Mistakes in Custom Tank Procurement
Approving samples without measuring them. Visual approval of a sample that "looks right" is not an approval. Measure critical dimensions against the drawing before signing off.
Omitting port specifications. A drawing that shows port positions but not thread type and engagement depth will result in samples with ports that don't accept the customer's fittings. Specify thread type (M, BSP, NPT), nominal size, and whether a mating fitting is required.
No retained reference sample. Without a physical reference sample held by both parties, disputes about whether bulk production matches approved samples are impossible to resolve objectively.
Changing the specification after drawing lock. Even apparently minor changes — a slightly larger filler neck, a repositioned mounting bracket — require a new sample cycle if they affect fitment or structural integrity. Factor this into project timelines.
Ignoring coating at port edges. Bare metal at port edges and bracket welds is a common shortcut in lower-quality production. Specify that all cut edges and weld areas must be coated, and check this on samples.
Material and Standard Considerations
Custom tanks are subject to the same material and quality standards as catalogue products. For air reservoirs, this means QC/T 200-2015 compliance regardless of whether the tank is a standard or custom design. For fuel tanks, the relevant standards depend on the application and destination market.
Confirm with the manufacturer which standards the custom tank is produced to, and request the compliance documentation before placing the bulk order.
Guanda fabricates custom aluminium and steel tanks from drawing or sample — including air reservoirs, fuel tanks, ADR-compliant tanks, and welded bracket assemblies. View metal tank products and enquire →