If you’ve dragged home a bare VW Beetle (Type 1) engine case, you’re holding a magnesium alloy casting that’s light, strong for its era, and easy to ruin if you clean or handle it the wrong way. This guide shows technicians and DIY engine builders how to identify the case, clean it safely, inspect and measure it for serviceability, and decide—rebuild core, sell, or scrap—based on facts instead of folklore. We also include essential safety info on combustible metal hazards. Spoiler: do not burn magnesium.

1) Identify the Case: What You Have and Why It Matters

Most air-cooled VW Beetle engine cases are magnesium alloy, commonly marked AS41 or AS21. While both are rebuildable when within spec, identification influences desirability and how you approach cracks and heat-warp checks.

  • Material and marking:
    • AS41: Common late OEM magnesium alloy. Often stamped or cast near the distributor stand or generator/alternator pedestal.
    • AS21: Less common (notably Brazilian), also magnesium alloy.
    • Aluminum aftermarket: heavier, no AS41/AS21 marking, different ring when tapped, often thicker around main webs.
  • Case family and features:
    • Type 1 (Beetle/Type 1 engines): Two case halves split on the vertical plane, four-cylinder boxer.
    • Dual-relief vs. single-relief: Flip the case; dual-relief cases have two large slotted plugs on the bottom (pressure control and pressure relief). Dual-relief cases (’70–’71 onward) are generally preferred for better oiling control.
    • Stud size: 10 mm early; later 8 mm with factory case savers. 8 mm studs with case savers help reduce cracking near #3 cylinder.
    • Mount bosses: Some later cases include extra rear mount bosses for Bus applications; verify if needed.
  • Part numbering clues:
    • VW Type 1 part numbers start with 111, 113, etc. Porsche numbers look different (e.g., 901, 993, 986). Don’t confuse the two.
  • Previous machine work:
    • Look for stamps like “LB .020” or “+0.50” indicating the case was line-bored oversize.
    • Thrust cuts are often marked on the flywheel end (#1 main) as +1.0 mm (.040) or +2.0 mm (.080).

Why this matters: Rebuild bearing availability follows align-bore and thrust sizes. Oversize history and crack-prone areas determine whether the case is a solid core or a wall-hanger.

2) Safety First: Handling and Cleaning Magnesium Cases Without Drama

Magnesium is reactive, lightweight, and vulnerable to improper chemicals and heat. Treat it like the specialized material it is.

  • Personal protective equipment:
    • Nitrile gloves, eye protection, and a respirator when blasting or wire-brushing to avoid inhaling dust.
  • Don’t use:
    • Caustic hot tanks, oven cleaner, sodium hydroxide, strong alkaline aqueous solutions—these attack magnesium.
    • Strong acids or aggressive descalers.
    • Steel shot blasting—can embed and initiate corrosion; sparks from steel against magnesium chips are a fire risk.
  • Preferred cleaning methods:
    • Solvent-based degreaser labeled safe for magnesium (petroleum or citrus-based). Work in a ventilated area away from ignition sources.
    • Nylon or brass brushes. Avoid aggressive wire wheels on sealing surfaces and bearing saddles.
    • Media blasting: Walnut shell or plastic media at low pressure. Mask all bearing saddles, lifter bores, oil passages, and mating surfaces. If glass bead is used, keep pressure low and clean thoroughly afterward to prevent residue.
    • Ultrasonic cleaning: Only with chemistry approved for magnesium and with temperature controls.
  • Chip and dust control:
    • Collect grindings and chips in a clean, dry steel container with a tight lid; keep away from heat, sparks, or moisture.
  • Fire safety for combustible metals:
    • Have a Class D (combustible metal) extinguisher on hand if cutting, grinding, or generating fine chips. Sodium chloride- or graphite-based powders are typical for magnesium. Do not use water or CO₂ on burning magnesium.
    • Minimize fine dust and avoid high-heat tools that can ignite chips.

Key point: Burning magnesium produces extreme heat and bright flame. Never “test burn” parts. If magnesium ignites, only Class D media is appropriate—water makes it worse.

3) Core Inspection: What to Look For Before You Measure

After degreasing, inspect visually and dimensionally. A flashlight and borescope help find hidden damage.

  • Obvious damage:
    • Cracks radiating from the #3 cylinder area (flywheel end, right side on most cases), often due to heat and 10 mm studs without savers.
    • Cracks at the oil cooler stand or doghouse adapter area.
    • Cracks between head studs and cylinder spigots.
    • Main web damage: crushed or fretted bearing saddles, especially #2 center main.
    • Broken case ears, oil pickup boss damage, distributor bore cracking.
  • Thread integrity:
    • Head studs: look for pulled threads, previous case savers, mixed 8 mm/10 mm studs. Case savers are a plus if installed correctly.
    • Drain plug and relief plug threads: inspect for cross-threading, helicoils, or damage.
    • Oil pump mounting threads and flatness of the pump pocket.
  • Flatness and contact surfaces:
    • Case parting surfaces: check with a straightedge and feeler gauges across the main webs and across the spigots.
    • Oil pump pocket: check for scoring and flatness.
  • Oil passages and galleries:
    • Remove galley plugs if serviceable and brush galleries; confirm no corrosion or debris. Many builders convert to threaded plugs; if so, verify quality.
  • Lifter bores:
    • Look for scoring and bell-mouthing. Oval lifter bores lead to oil pressure loss and cam/lifter failure.

If you see multiple structural cracks (especially main web to cam tunnel, or severe #3 area cracking), consider the case a non-core for engine duty.

4) Measurements That Matter: Line Bore, Thrust, Spigots, Deck, and Bores

Use calibrated tools. Record every measurement; buyers and machinists will ask.

  • Main bearing saddles (align-bore):
    • Use a bore gauge sized for the main saddle diameters. VW Type 1 bearings are sized by align-bore oversize on the case: +0.25 mm (.010), +0.50 mm (.020), +0.75 mm (.030), +1.00 mm (.040).
    • If stamped “LB .020” or “+0.50,” verify all saddles match. Look for out-of-round and taper. Excessive align-bore history (beyond +.040) is usually a pass.
  • Thrust face at #1 main:
    • Measure thrust cut: common oversizes are +1.0 mm (.040) and +2.0 mm (.080). More than +.080 typically pushes the case out of practical rebuild territory for most builds.
    • Inspect for fretting and step wear where the bearing locates.
  • Case shuffle and deck parallelism:
    • With dowels in place, torque a few case through-bolts to spec without bearings. Blue the parting line to confirm contact. Excess gaps indicate warpage.
    • Deck height flatness and parallelism: Use a straightedge across cylinder spigots; compare left and right halves for step differences.
  • Cylinder registers (spigots):
    • Check for ovality and fretting from loose cylinders. Ensure bores are square to the case deck.
  • Lifter bores:
    • Measure ID and check roundness. Oversize or worn bores often need bushings; confirm your machine shop offers this service.
  • Oil pump pocket:
    • Measure for flatness and depth; excessive scoring can cause pressure loss. A light surface skim may be possible; don’t overcut.

Decision thresholds vary by build intent. For stock or mild engines, a case that’s within +.020 or +.030 align-bore and +.040 thrust, with straight decks and uncracked webs, is usually a viable core. Performance builds favor new aluminum cases to avoid inherited issues.

5) Rebuild, Sell, or Scrap: Make the Call

Use the findings above and be realistic about budget and intended power level.

  • Good core candidates:
    • Dual-relief AS41/AS21 with no structural cracks.
    • Align-bore not beyond +.040 and thrust not beyond +.080, with clean saddles and good bore geometry.
    • Threads intact or properly repaired with quality case savers.
  • Marginal cores (requires machine work):
    • Lifter bore bushings needed, minor spigot fretting, oil pump pocket cleanup, or one more align-bore (+.010 over current) by a qualified shop.
  • Non-cores (sell as art or scrap):
    • Main web cracks, severe #3 area cracking, heavy warpage, thrust face hammered beyond +.080, or multiple failed thread repairs.

Market reality: Clean, verified dual-relief magnesium cases can sell to rebuilders. Provide clear photos of saddles, thrust area, lifter bores, and all stampings. Note any align-bore/thrust sizes and whether case savers are installed. Expect modest value relative to machine work costs; pricing varies by region, but uncracked, measured cases move faster.

6) Don’t Burn It: Magnesium Fire Facts and Emergency Tips

Burning magnesium is not a “fun demo”—it’s a hazardous combustible metal fire that blinds, warps, and can’t be doused with water.

  • What happens if magnesium ignites:
    • White-hot flame with intense light; radiant heat can crack concrete and ignite nearby combustibles.
    • Water and CO₂ are ineffective and can intensify the reaction. Class D dry powder is the correct extinguishing agent.
  • Prevention in the shop:
    • Avoid generating fine chips and dust. Keep cutting cool and controlled (band saw with lubricant rather than abrasive cutoff wheels).
    • Collect chips dry in a closed steel container; keep away from moisture and ignition sources.
    • Keep a Class D extinguisher where you cut or machine magnesium and train staff on its use.
  • Health note:
    • Avoid inhalation of magnesium oxide dust/fume; use local exhaust and respiratory protection when needed.

Bottom line: Salvage or sell the case if it’s sound. Don’t turn valuable alloy into an uncontrolled fire.

Quick Checklist: VW Magnesium Case Triage

  • Confirm AS41/AS21 and dual-relief status.
  • Degrease with magnesium-safe solvent; no caustics.
  • Inspect: #3 area, main webs, oil cooler stand, head stud bosses, lifter bores.
  • Measure: align-bore size/roundness, thrust cut, deck flatness, spigots, lifter bores.
  • Verify threads: head studs, relief plugs, drain plug, oil pump.
  • Decide: rebuild, machine, sell as core, or scrap.
  • Equip Class D extinguisher if cutting or generating chips.

FAQs

Q1: Is an AS41 case better than AS21?
Both are magnesium and can build reliable stock/mild engines. AS41 is common and desirable; AS21 shows up in some Brazilian cases. For high-output builds, most builders choose new aluminum aftermarket cases for strength and stability.

Q2: What align-bore and thrust limits make sense before scrapping?
As a general rule for street builds: up to +.040 align-bore and +.080 thrust can be serviceable if geometry is correct and there are no cracks. Beyond those, or if saddles are pounded/out-of-round after prior cuts, the case is typically not worth more machine work.

Q3: Can I hot tank a magnesium case?
No if the solution is caustic. Caustic hot tanks attack magnesium. Use magnesium-safe solvent degreasers, controlled ultrasonic with approved chemistry, or low-pressure non-aggressive media blasting with thorough masking and cleaning.

Q4: What extinguisher do I need for magnesium?
Class D (combustible metals) with appropriate dry powder (e.g., sodium chloride or graphite-based). Water, foam, CO₂, and most ABC powders are not suitable for burning magnesium. Store and use chips with care to avoid ignition.

Q5: Are 8 mm or 10 mm head studs better?
Later 8 mm studs with factory case savers are preferred for reducing crack risk around #3 cylinder. If a 10 mm case is otherwise good, installing quality case savers is a solid upgrade. Ensure depth, alignment, and thread engagement are correct.

Q6: What should I include when selling a used case?
Clear photos of both halves, bearing saddles, thrust face, lifter bores, spigots, oil cooler area, and all stampings. List align-bore/thrust sizes, stud size, presence of case savers, dual- vs single-relief, and any repairs. Clean cases sell faster.

Recommended Tools

For the inspection and safety steps covered here, See our Measurement Tools and Inspection tools.

  • Bore gauges, micrometers, dial indicators, and straightedges for case measurement.
  • Borescopes, inspection lights, and thread repair tools for core evaluation.
  • Class D extinguishers and chip handling gear for magnesium work.