B4 Collision — Structural Repair — practice test

High-yield B4 topics

The material that shows up year after year. If you're short on time, start here.

• Three-dimensional measurement: centerline (side-to-side symmetry), datum (the height reference plane), zero plane (the fore/aft origin). A tram gauge measures point-to-point; centering gauges show twist, sag, mash, and sidesway by how the pins hang relative to each other. Sight down four gauges: two low at one end = sag, gauges not parallel = twist, gauges hanging off the centerpins = sidesway.
• Steel classification and what you may NOT do to it. Mild steel and conventional HSS can generally be straightened within heat limits (commonly capped near 1200-1400 F for a very short duration). UHSS, martensitic, boron, and hot-stamped parts are replace-only: no heat, no cold straightening. If an answer offers 'heat and pull' on a B-pillar reinforcement, that is the distractor.
• Anchoring and pull sequence. Minimum four-point anchoring at the pinchwelds on a unibody, plus extra anchors so the pull does not drag the anchor points. Pull in reverse order of damage - the last thing to bend is the first thing to come back. Stress-relieve with hammer-and-dolly while the panel is under tension, and overpull slightly for springback.
• Sectioning is OEM-dictated, not tech-dictated. Know front rail rules, never section through crush zones/convolutions, holes, reinforcements, or suspension and airbag sensor mounting points. Know the joint types: butt joint with insert/backer, offset (lap) joint, staggered butt. Full-body sectioning at the B-pillar or roof rail follows the OEM procedure only.
• Welding: GMAW on steel with ER70S-6 wire and 75/25 argon-CO2. STRSW where the OEM specifies factory spot welds; MIG brazing with silicon bronze where the OEM calls it out to limit heat into UHSS and preserve coatings. Know the destructive weld coupon test - twist or pull the plug weld until it tears a nugget out of the base metal. Know porosity causes: no shielding gas, drafty shop, dirty or coated metal, excessive stickout.
• Stationary glass: full-cut vs. partial-cut urethane removal, leave a 1-2 mm bed of original urethane as a base, prime any bare metal and the glass frit with the correct primer, and respect the adhesive maker's safe drive-away time. The windshield is structural - it supports the roof in a rollover and backs the passenger airbag.

Where techs lose points on B4

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• 'Compare it to the other side' vs. 'compare it to the chart.' Symmetry checks find asymmetry, but they lie when both sides moved. On a dead-center hit, or any question where both rails are damaged, the tram gauge answer is wrong - you need the dimension chart. Techs pick the tram because that is what they grab at work.
• Assuming heat is always on the table. The obvious answer to a bent rail is heat and pull. On modern structure with boron or hot-stamped parts, the correct answer is replace at factory seams: no heat, no cold straightening. If the question names a part in the safety cage, lean toward replacement.
• Datum vs. zero plane. Datum is a horizontal reference plane below the vehicle used for HEIGHT. Zero plane is the fore/aft origin used for LENGTH. Both are imaginary, both sound alike on paper, and ASE writes look-alike distractors that swap the two.
• Kink vs. bend. A bend is often repairable; a kink - a sharp change, roughly 90 degrees or more over a short distance, with collapsed or torn metal - gets replaced. And do not call a cracked paint line a kink. The tell is a tight radius and visibly collapsed metal, not just a broken finish.

A study plan that works for B4

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-4: Measurement and damage analysis. Get datum, centerline, and zero plane cold. Hang four centering gauges under any car in the shop and read them until sag/twist/mash/sidesway is instant. Pull a real dimension chart and measure a straight car so you know what 'in spec' actually looks like.
2. Days 5-8: Steel classification, anchoring, pulling. Memorize straightenable vs. replace-only and the heat rule for each. Drill the sequence until it is automatic: anchor, measure, pull last damage first, stress-relieve, overpull, re-measure.
3. Days 9-12: Welding and sectioning. Read the OEM sectioning procedures for two vehicles you actually work on and note what they forbid. Burn a coupon and destructive-test it so the nugget question is muscle memory. Cover STRSW and MIG brazing and when each is specified.
4. Days 13-14: Stationary glass plus timed mixed practice. Glass is a small block and easy points - urethane bed, primer, safe drive-away time. Then run full-length practice sets under time and review only what you missed and what you guessed right.

Sample B4 questions

Straight from the bank — answers highlighted, with the explanation underneath.