Volvo Penta · Inboard diesel

Volvo Penta MD21

The MD21 is a four-cylinder, four-stroke diesel of 2.11 litres (90 × 83 mm) based on the Indenor XDP 4/90, with swirl chambers, wet replaceable liners, a CAV rotary injection pump, glow plugs and fresh-water cooling through a heat exchanger. Volvo Penta’s 1968 book rates the MD21A at 75 hp (SAE) at 4500 rpm; later leaflets give the MD21A and MD21B 45 kW (61 hp) at the propeller shaft at the same speed. It went into motor boats and larger sailing yachts with the RB, Borg-Warner Velvet Drive or MS3/MS3B reverse gear, as the MD21B/110S with saildrive, and as the AQD21A and AQD21B with Aquamatic 250 and 280 sterndrives. Volvo Penta’s dated literature runs from 1968 (MD21A book) to 1982 (MD21B bulletin); several parts changed at engine-number breaks, so the notes say which version each number fits.

Manuals & documents

The links go straight to Volvo Penta. We host no manuals or drawings — the manufacturer’s copy is the current one.

Wear parts & part numbers

PartOEM numberAlso fitsNote
Impeller kit (with gasket)219513501,2,3,4replaces 875575—For sea-water pump 831178: all MD21B and AQD21B, and MD21A/AQD21A from engine no. 103387. Volvo Penta lists 875575 as replaced by 21951350. Earlier MD21A engines have a different pump and take kit 875811 (next row).
Impeller kit (with gasket)8758115,6—Only early MD21A/AQD21A up to engine no. 103386, with sea-water pump 826891. The kit includes gasket 826244. Volvo Penta’s catalogue shows no current replacement number.
Raw water pump8311782,3,1—MD21B/AQD21B and MD21A/AQD21A from engine no. 103387. Service kits for this pump: wear kit 21951380 and shaft kit 21951435.
Raw water pump8268915,6—Early MD21A/AQD21A up to engine no. 103386. Repair kit for this pump: 875369.
Engine oil filter215495447,8replaces 3581621—Late-type filter for the MD21B and AQD21B; Volvo Penta replaces 3581621 with 21549544.
Engine oil filter8293909—Late-type filter for the MD21A and AQD21A.
Engine oil filter4184329,7—Listed as the early type in both the MD21A and the MD21B breakdown. Compare it with the filter on your engine before ordering.
Fuel filter (engine)23662810,11,12—Insert for the CAV fine filter on MD21A and MD21B (filter assemblies 875542 and 829764; 829764 replaced 875542). Sealing rings 1695001/1695000 and rubber ring 236629 belong to it.
Fuel pre-filter82966613,12,14—Insert for water separator 833971 (MD21B, and MD21A with the late-type fuel system).
Fuel lift pump826550 · 82973715,16—826550 is the complete feed pump, which replaced the old repair kit 826217; 829737 is its strainer kit with gasket 829300. The same on MD21A and MD21B.
Alternator V-belt82920717,18,19—Alternator belt, the same number in the MD21A and MD21B alternator breakdowns.
Anode (engine)83892920,21,22replaces 800476—Zinc plug in the heat exchanger; Volvo Penta lists 800476 as replaced by 838929. The MD21A heat-exchanger breakdown shows three, the MD21B breakdown one, so count the plugs on your engine.
Glow plug829857 · 84064323—Four per engine, 12 V: 829857 on the MD21A and on the MD21B up to engine no. 3130871/216681, 840643 on the MD21B from 3130872/216682. For 24 V systems: 870681.
Fuel injector826857 · 84064424,12—Four per engine. 826857 (nozzle 826574 included) on the MD21A and earlier MD21B; the MD21B from engine no. 3130872/216682 has 840644.
Gasket set875567 · 87538825—Decarbonising (cylinder head) gasket kit: 875567 from engine no. 3119180, 875388 before. Complete overhaul kit: 875387. All three are listed for MD21A, MD21B, AQD21A and AQD21B.
Water pump repair kit87537226,27,28—Repair kit for the fresh-water circulation pump 826870: shaft, bearings, seal washers, impeller and gasket. Listed for MD21A, MD21B, AQD21A and AQD21B.
Exhaust mixing elbow81835929,30—Exhaust pipe elbow, listed in the induction and exhaust manifold breakdowns of MD21A and MD21B.

Numbers checked against the sources below on 12 October 2026. Always confirm against your serial number and exact type before ordering.

Service intervals

TaskInterval
Engine oil changeMD21B book: every 100 h or at least once a season, first change after 20 h. MD21A book (1968): every 50 h or once a season, first after 20 h31,32
Oil filterMD21B book: every 200 h (every second oil change), first after 20 h. MD21A book (1968): every 100 h or once a season31,32
Fuel filterChange the fine filter and clean the feed-pump strainer every 100 h or at least once a season (both books)31,32
Fuel pre-filterDrain water from the water separator daily, before starting (MD21B book)31
Raw water impellerCheck, and replace if needed, every 100 h or at least once a season (MD21B book)31
Sea water strainerClean every 50 h (MD21B book)31
V-belt checkMD21B book: check the tension every 14 days (about 10 mm deflection under thumb pressure); inspect the belts every 100 h and replace worn ones. MD21A book (1968): every 50 h31,32
AnodeZinc plugs in the heat exchanger: check every 50 h (both books) and replace when more than half is gone31,32
Air filterClean the intake air silencer at least once a season (MD21B book)31
Valve clearanceMD21B book: every 100 h or at least once a season, by a Volvo Penta workshop; MD21A book (1968): every 50 h. Always reset after retightening the cylinder head bolts31,32
InjectorsMD21B book: have opening pressure, spray pattern and leakage tested every 600 h; MD21A book (1968): check once each season31,32
Glow plugsTake out and clean off the carbon once a season (MD21B book)31
Retighten cylinder head boltsNew or overhauled engine: retighten after 20–50 h, then once a year (MD21B book)31
Gearbox oil changeMD21B book: MS3, Borg-Warner and RB every 200 h. MD21A book (1968): RB and Borg-Warner every 100 h or once a season31,32
Drive oil changeAquamatic 280 drive (AQD21B): every 200 h (MD21B book)31
Rubber seal (drive to hull)110S saildrive: replace the flexible sealing ring every fifth year (MD21B book)31

“MD21B book” is Volvo Penta’s instruction book for the AQD21B/280, MD21B and MD21B/110S; “MD21A book” is the 1968 operator’s manual for the MD21A and MD29A, which sets shorter intervals in places. Hours or season, whichever comes first.

Specifications

VersionsMD21A and MD21B inboards with RB, Borg-Warner or MS3 reverse gear (MD21B also MS3B); MD21B/110S with saildrive; AQD21A with Aquamatic 250 and AQD21B with Aquamatic 280 sterndrive31,33,34,35
Cylinders4 in line, four-stroke diesel with swirl chambers and overhead valves; wet replaceable liners; engine designation Indenor XDP 4/9031
Displacement2.11 l (129 cu in)31
Bore × stroke90 × 83 mm31
PowerMD21A, 1968 book: 75 hp (SAE) at 4500 rpm, 51 hp at 3000 rpm for work boats. MD21B and later MD21A leaflets: 45 kW (61 hp) at the propeller shaft at 4500 rpm (DIN 6270 B), 33 kW (45 hp) at 3000 rpm for work boats; 47 kW at the flywheel (MD21B book)32,33,36,31
Maximum torque13.2 kgm (about 95 lb ft) at 2000 rpm (MD21A, 1968 book)32
Rated speedMax. 4500 rpm for pleasure boats, 3000 rpm for work boats; cruise at least 200 rpm below the maximum reached; unloaded engine max. 4850 rpm31
Idle speed650 rpm31
InjectionCAV Roto Diesel DPA rotary pump; CAV fine filter DES 583-6100; injector holders RKB 35 S 5118 with nozzles RDN-12SD-651731
Injection timingInjection begins 19° BTDC31
Injection pressure130 ± 5 kp/cm²31
CoolingFresh-water cooling with heat exchanger, belt-driven circulation pump and thermostat; the sea-water pump with neoprene impeller also cools the oil cooler and the exhaust manifold31
Thermostat opens at68–72 °C31
Coolant capacityFresh-water system including heat exchanger: about 9.5 l (MD21A, 1968 book), about 11 l (MD21B, AQD21B)32,31
Engine oil capacityAbout 5.5 l including filter (varies with the installation angle)31
Engine oil gradeMD21B book: API CD, SAE 30 (SAE 20/20W below 0 °C). MD21A book (1968): API “Service DM”, SAE 30 or 20W-4031,32
GearboxVolvo Penta RB, mechanical, 1.91:1; Borg-Warner Velvet Drive, hydraulic, 2:1 or 2.9:1 (1968 book: 1.91:1, 2.1:1 or 2.91:1); Volvo Penta MS3 1.91/1.92:1 or 3.0:1 with 8° down-angle; MS3B 1.93:1 or 2.91:1; 110S saildrive 2.15:1; Aquamatic 250 (AQD21A) or 280 (AQD21B) sterndrive31,32,34
Gearbox oilRB about 0.5 l and MS3 about 1.2 l of engine oil; Borg-Warner about 3.0 l of ATF type A31
Drive oil capacityAquamatic 280 drive: 2.6 l of engine oil; 110S saildrive: 2.3 l (API CD, SAE 20W/30)31
Valve clearance (cold)Inlet 0.15 mm, exhaust 0.25 mm31,32
Tightening torquesCylinder head bolts 6.5–7.5 kpm (engine cold); injector nuts 1.5–2.5 kpm; glow plugs 4–5 kpm31
Alternator12 V, 35 A (420 W) brushless alternator; the 1982 MD21B/MS3B bulletin lists 50 A (600 W), the 1968 MD21A book a 240 W DC generator36,34,32
Starter motor1.3 kW (1.8 hp)36
WeightWith gear, per the MD21B book: about 290 kg (RB), 300 kg (MS3), 330 kg (Borg-Warner), 325 kg (110S saildrive); AQD21B with 280 drive about 330 kg. The leaflets quote 275 kg (RB), 285 kg (MS3/MS3B) and 310 kg (Borg-Warner)31,36

Good to know

  • MD21A engines changed sea-water pump at engine no. 103386/103387, and the two pumps take different impellers: up to 103386 pump 826891 with impeller kit 875811, from 103387 (and on every MD21B) pump 831178 with kit 21951350. Read the engine number on the plate or compare the pump before you order.3,5,1
  • The Borg-Warner gearbox runs on automatic transmission fluid (type A), while the RB and MS3 gears take the same engine oil as the engine. Check which gear you have before topping up.31
  • The V-belts also drive the fresh-water circulation pump, so a slack or oily belt shows up as rising temperature as well as poor charging. Volvo Penta wants about 10 mm deflection under thumb pressure between the pulleys.31
  • Check the zinc plugs in the heat exchanger every 50 hours: scrape off the deposits, replace a plug once half of it is gone, and make sure it has bare-metal contact when refitted.32
  • After draining the water separator or changing the fine filter, bleed the system at the vent screw on the filter head, pumping with the hand primer on the feed pump until the fuel runs out without bubbles.31
  • Keep a spare impeller and spare glow plugs on board, as the 1968 book recommends, and take the impeller out of the sea-water pump for winter lay-up. The MD21B book also has you take the glow plugs out once a season and clean off the carbon.32,31

Fitted in

Boats whose builder lists it as standard or optional equipment:

  • Hallberg-Rassy 352 — Volvo Penta MD21B (Standard from 1978 until Volvo Penta stopped making the engine in late 1984: four cylinders, 45 shaft hp in the brochure, single-lever control on the pedestal)
  • Hallberg-Rassy 42E & 42F — Volvo Penta MD21 (42E: the first of the three engines the yard lists for the 42E; it gives neither the version (MD21A or MD21B) nor the years)

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Sources

  1. volvopenta.com/shop/0/part-sections/54145435
  2. volvopenta.com/shop/0/part-sections/54147064
  3. volvopenta.com/shop/0/part-sections/54145432
  4. volvopenta.com/shop/0/parts/21951350
  5. volvopenta.com/shop/0/part-sections/54145495
  6. volvopenta.com/shop/0/part-sections/54145433
  7. volvopenta.com/shop/0/part-sections/54147054
  8. volvopenta.com/shop/0/parts/21549544
  9. volvopenta.com/shop/0/part-sections/54145360
  10. volvopenta.com/shop/0/part-sections/54147080
  11. volvopenta.com/shop/0/part-sections/54147079
  12. volvopenta.com/shop/0/part-sections/54145519
  13. volvopenta.com/shop/0/part-sections/54147081
  14. volvopenta.com/shop/0/parts/829666
  15. volvopenta.com/shop/0/part-sections/54147083
  16. volvopenta.com/shop/0/part-sections/54145523
  17. volvopenta.com/shop/0/part-sections/54147086
  18. volvopenta.com/shop/0/part-sections/54145597
  19. volvopenta.com/shop/0/parts/829207
  20. volvopenta.com/shop/0/part-sections/54145493
  21. volvopenta.com/shop/0/part-sections/54147076
  22. volvopenta.com/shop/0/parts/838929
  23. volvopenta.com/shop/0/part-sections/54147098
  24. volvopenta.com/shop/0/part-sections/54147082
  25. volvopenta.com/shop/0/part-sections/54147184
  26. volvopenta.com/shop/0/part-sections/54147185
  27. volvopenta.com/shop/0/part-sections/54147077
  28. volvopenta.com/shop/0/part-sections/54145494
  29. volvopenta.com/shop/0/part-sections/54147069
  30. volvopenta.com/shop/0/part-sections/54145465
  31. pubs.volvopenta.com/publications/7723924
  32. pubs.volvopenta.com/publications/7722350
  33. pubs.volvopenta.com/publications/7765222
  34. pubs.volvopenta.com/publications/47700044
  35. pubs.volvopenta.com/publications/7722370
  36. pubs.volvopenta.com/publications/7765223

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