1951 BMW R 51/2 Specifications

Name: 1951 BMW R 51/2
Brand: BMW

All-Round • Specs last verified 31 October 2026

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1951 BMW R 51/2 Overview

For 1951, BMW offers the R 51/2 as an all-round motorcycle with 494 cc on tap. The 1951 BMW R 51/2 is built around a Four stroke, two cylinder horizontally opposed Boxer, pushrod operated 2 valves per cylinder engine, a compression ratio of 6.4:1. Out on the road, the R 51/2 delivers 18 kW @ 5800 rpm of peak power. In everyday use, the 1951 R 51/2 tips the scales at 185 kg and carries a 14-litre fuel tank. Every technical detail, from suspension travel to tyre sizes, is covered in the full 1951 BMW R 51/2 spec sheet below.

1951 BMW R 51/2 compared with other bikes in the All-Round category

Measured against other bikes in the All-Round category in our database (up to 146 models, one entry per model), the 1951 BMW R 51/2 comes out as follows.

With 97 W/kg, the R 51/2 has the 36th lowest power-to-weight ratio of 136 bikes in the All-Round category, 40% below the category average of 161 W/kg.
Power output of 18 kW is lower than that of 96 of the other 145 bikes in the All-Round category.
Narrowing the field to the 11 models within five model years of it, the power-to-weight ratio of 97 W/kg is lower than that of 5 of the other 10.

1951 R 51/2 - Pros & Cons

Strengths

Capable power output: 18 kW @ 5800 rpm of peak output puts the R 51/2 in good company among its rivals.
Low-maintenance final drive: Shaft final drive is about as close to maintenance-free as motorcycle drivetrains get.

Considerations

Carburetor fuelling: Carburetion is more hands-on than fuel injection - plan on periodic jetting and cleaning as part of routine ownership.
Drum rear brake: A drum rear brake is simple and durable, but expect less bite and more fade under hard, repeated use than a disc setup.

Wear Points & Servicing Notes

Heat management in traffic

Air cooling on the R 51/2 keeps the mechanical layout simple, but heat management in slow traffic needs more attention than on a liquid-cooled bike - plan routes and gear accordingly in warmer conditions.

Carburetor maintenance

Being carbureted, the R 51/2 is more sensitive to stale fuel and altitude changes than a fuel-injected bike - a carb clean and rejet restores crisp throttle response after any extended lay-up.

Suspension condition on used examples

Suspension duties are handled by Telescopic fork up front and Spring with coil compression springs and rubber deflection stops at the rear. Weeping fork seals aren't a dealbreaker on a used bike, but they're a fair basis for negotiating price.

Final drive maintenance

Shaft final drive on the R 51/2 removes chain and belt maintenance entirely - the trade-off is a little extra unsprung weight and periodic gear-oil changes.

Frame & chassis check

The Closed double loop steel tubular frame chassis holds up well with normal use; on a used bike it's still worth checking the head-stock and swingarm pivot area for impact damage or corrosion.

R 51/2 - Buying & Maintenance Guide

What to check before buying

A pre-purchase check on the 1951 BMW R 51/2 with its 494 cc all-round should cover the same ground every time: service history, drivetrain condition, and any oil seepage around the casings. This model runs shaft final drive, so ask for proof the shaft-drive gear oil has been changed on schedule - it's easy to overlook since there's no chain to draw attention to it. Now 75 years old, this all-round is as likely to show age-related wear (perished rubber, seized fasteners, degraded seals) as mileage-related wear. Start it from cold if you can - a well-kept R 51/2 settles to a smooth idle within seconds.

Oil and filter changes

Oil changes are the cheapest insurance you can buy for the R 51/2's 494 cc four stroke, two cylinder horizontally opposed boxer, pushrod operated 2 valves per cylinder. - keep to the manufacturer's interval rather than stretching it. The sump holds 2 l, so budget for that volume of quality motorcycle-specific engine oil. With a 6.4:1 compression ratio, running the octane grade specified in the manual matters more than on a lower-compression engine. Use the viscosity grade the manual specifies, not whatever's on sale.

Carburetor upkeep

An R 51/2 that's sat unused is almost always a carb-cleaning job before it runs right again - float bowls and pilot jets first, since they clog before anything else.

1951 BMW R 51/2 - Frequently Asked Questions

What engine does the 1951 BMW R 51/2 have?

The 1951 BMW R 51/2 uses a Four stroke, two cylinder horizontally opposed Boxer, pushrod operated 2 valves per cylinder. engine with 494 cc of displacement. It is air cooled.

How much does the 1951 BMW R 51/2 weigh?

The 1951 BMW R 51/2 weighs 185 kg. This is the manufacturer's wet figure.

How much power does the 1951 BMW R 51/2 make?

The 1951 BMW R 51/2 produces 18 kW @ 5800 rpm of power.

What is the fuel tank capacity of the 1951 BMW R 51/2?

The 1951 BMW R 51/2 has a 14-litre fuel tank. At around 5-6 litres per 100 km, that is roughly 233-280 km between stops.

What brakes does the 1951 BMW R 51/2 have?

The 1951 BMW R 51/2 uses ∅200mm Drum, duplex full hub at the front and ∅200mm Drum, simplex full hub at the rear.

What tyre sizes does the 1951 BMW R 51/2 use?

The 1951 BMW R 51/2 runs 3.50 - 19" Metzeler front and 3.50 - 19" Metzeler rear tyres. A good selection of sport, touring and all-weather rubber is available in these sizes.

What gearbox does the 1951 BMW R 51/2 have?

The 1951 BMW R 51/2 is fitted with a 4 Speed transmission.

How much oil does the 1951 BMW R 51/2 take?

The 1951 BMW R 51/2 takes 2 l of engine oil at a full oil and filter change. Always verify against the dipstick or sight glass after filling.

Key Specifications

Engine

Four stroke, two cylinder horizontally opposed Boxer, pushrod operated 2 valves per cylinder.

Power

18 kW / 24 hp @ 5800 rpm

Torque

Not recorded

Top Speed

Not recorded

Weight

185 kg / 407 lbs

Fuel Capacity

14 liters / 3.60 US gal

Full 1951 BMW R 51/2 Specifications

Year 1950 - 51

Make BMW

Model R 51/2

Make Model BMW R 51/2

Oil Capacity (l) 2.0 l

Oil Capacity (Quarts) 2.1 qt

Fuel Capacity 14 liters / 3.60 US gal

Wet Weight 185 kg / 407 lbs

Max Power 18 kW / 24 hp @ 5800 rpm

Stroke 68.0 mm

Displacement 494 cc / 30.1 cu in.

Engine Detail Four stroke, two cylinder horizontally opposed Boxer, pushrod operated 2 valves per cylinder.

Bore x Stroke 68 x 68 mm

Compression Ratio 6.4:1

Cooling System Air cooled

Fuel Induction 2 x Bing Spezial 1/22/29 and 1/22/30 carburetors

Ignition Bosch coil with manual advance by the handlebar lever

Starter Kick

Clutch Single plate, dry

Final Drive Shaft

Gear Ratios 1st 3.60 / 2nd 2.28 / 3rd 1.70 / 4th 1.30:1

Transmission / Drive 4 Speed

Fork Tube Size (mm) 36 mm

Fork Tube Size (in.) 1.42 in

Frame Type Closed double loop steel tubular frame

Suspension - Front Telescopic fork

Suspension - Rear Spring with coil compression springs and rubber deflection stops

Tachometer None

Brakes - Front ∅200mm Drum, duplex full hub

Brakes - Rear ∅200mm Drum, simplex full hub

Tyres - Front 3.50 - 19" Metzeler

Tyres - Rear 3.50 - 19" Metzeler

Specifications are compiled from manufacturer documentation and cross-referenced technical sources, and corrected on an ongoing basis. Spot an error? Let us know.

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