Crankcase for BM-40 Piston Compressor

1. Purpose and operating principle

The BM-40/50 is a single-cylinder piston compressor from MZB (Zhejiang Meizhoubao) rated at 2.2 kW (3 HP), 2800 rpm, 8 bar and 200 L/min; it is built with a 40 L or 50 L receiver. The crankcase of such a pump carries the main loads of the crank mechanism, so its tightness, the integrity of the bearing seats and the cleanliness of the oil sump directly determine service life. The crankcase is replaced if the housing is cracked, the bearing seats or the flange are damaged, or oil leaks through the housing and sealing repairs no longer help.

2. Technical specifications

Data for the compressor this part is intended for (per the manufacturer’s page ru.chinamzb.com, model BM-40/50):

⚠️ Data for the part itself and some compressor data are not confirmed. No public manufacturer information was found for the crankcase: part / OEM number, material (cast iron or aluminum alloy), dimensions and weight, bearing and cylinder seat diameters and center distances, motor flange size, oil capacity, scope of supply (with or without cover, sight glass, breather or gaskets). Fill these in from the part itself (calipers, nameplate) or confirm with a manager or the supplier’s catalog. The manufacturer’s page also does not explicitly state the drive type (direct or belt), oil lubrication or oil change intervals: direct drive and an oil sump are assumed here from the catalog section and the 2800 rpm speed. “47 × 1” is read as one cylinder with a 47 mm diameter (to be confirmed). The units of the dimensions (probably centimeters) should also be clarified.

3. Design features

Compressor designation breakdown:

The receiver volume only defines the tank size, not the pump design, so a crankcase cannot be selected by the number “40” without checking dimensions.

What the crankcase should include and provide (the presence of each element on the specific part ⚠️ must be checked):

• a sealed oil cavity with a filler opening and an oil level indication (dipstick, sight glass or mark);
• seats for the crankshaft bearings and a flange for mounting the cylinder;
• a flange or fixing points for connection to the motor;
• faces for gaskets and covers, a breather or an opening connecting the crankcase cavity to the atmosphere.

4. Installation recommendations

• Drain the oil and inspect the assembly. Before replacing the crankcase, check the crankshaft, connecting rod, bearings and piston group for wear and scoring: a new crankcase with a worn mechanism will not last long.
• Clean the mating surfaces of old sealant, metal chips and oil film; there must be no nicks or deep scratches on the faces.
• Replace gaskets and seals with new ones. Reusing old gaskets is the main cause of leaks after repair.
• Bolt tightening torque — per the pump manufacturer’s data ⚠️ (not confirmed). Tighten in a crisscross pattern, evenly, without overtightening: threads in the housing strip easily.
• Fill with fresh oil to the mark and turn the shaft by hand before starting to make sure it does not seize.
• Break-in. After repair, carry out the first oil change earlier than the scheduled interval; take the exact intervals and oil grade from the BM-40 compressor manual ⚠️ (they are not stated on the manufacturer’s page).

5. Alternatives and comparison

There are no verified part numbers for an equivalent crankcase for the BM-40, so no cross-reference list is given — compatibility cannot be invented. Selection is made by dimensions and design:

Comparison with the FL-24 from the same MZB brand (per the product pages):

Power and flow differ by almost a factor of two, so the BM-40 and FL-24 crankcases should not be considered interchangeable without checking dimensions. The best way to select the part is to bring or send a photo of the old crankcase with its dimensions and the compressor nameplate.

6. Common mistakes

1. Selecting by receiver volume. People order a “crankcase for a 40 L compressor” without checking dimensions: compressors of this capacity from different brands have different pumps and different motor power.
2. Confusing the BM-40 and BM-50. It is the same model in two receiver sizes; check the nameplate for which one you have and that the pump matches (⚠️ confirm with a manager whether the crankcase is the same for both).
3. Fitting a new crankcase on a worn mechanism. Bearing play or scoring on the cylinder quickly destroys the new part too.
4. Old gaskets and uncleaned faces. After assembly oil leaks at the joint, and contamination gets into the sump.
5. Overtightening bolts in the aluminum housing. Threads strip and cracks appear around the mounting points.
6. Skipping the oil change after break-in. Break-in metal particles stay in the sump and accelerate wear of the bearings and piston group.

7. Applications

• Repair and maintenance of the MZB BM-40/50 piston compressor rated at 2.2 kW.
• Restoring the pump after a cracked housing, oil leaking through the housing, or damaged bearing seats.
• Service workshops, car repair shops, garages and small production sites where such a compressor is the main source of compressed air.

8. Expert opinion

“On a compressor like this the crankcase is replaced less often than the valves, rings or pressure switch, so first determine what has actually failed. If oil leaks at the joint, new gaskets and clean faces are often enough. A new crankcase is needed for a crack or ruined seats. Select by dimensions and the compressor nameplate, not by receiver volume, and don’t skimp on break-in: it is better to do the first oil change after repair early.”

Vitali Boluchevski, Lead Engineer, Acvatron SRL, 18 years of experience

9. FAQ

1. How do I know the BM-40 crankcase needs replacing? Signs: a cracked housing, a constant oil leak that new gaskets and seals do not cure, ruined bearing seats and knocking from the lower part of the pump. Before replacing it, inspect the crankshaft, connecting rod and bearings.

2. How do I select a crankcase for the BM-40? By dimensions: bearing seat diameters, cylinder position, motor flange, position of the filler opening and drain. Send a manager a photo of the old crankcase, its measurements and the compressor nameplate.

3. Will a BM-40 crankcase fit the BM-50, or one from the FL-24? The BM-40 and BM-50 are one model with different receivers; whether the crankcase is the same for both should be confirmed with a manager. An FL-24 crankcase does not fit without checking dimensions: the motor and flow of these compressors differ by almost a factor of two.

4. Do I need new gaskets when replacing the crankcase? Yes, replace them without fail. Old gaskets are the main cause of oil leaks after repair.

5. When should I change the oil after fitting a new crankcase? Do the first change earlier than the scheduled interval to remove break-in particles. Check the exact intervals and oil grade in the BM-40 manual or with a manager.