Several months ago we looked at the MB O6600H coach – Mercedes first rear-engined bus. It was a successful design, but due to its traditional body-on-frame construction was somewhat heavy and the separate chassis limited underfloor storage. Mercedes fixed these problems with the O321H, a similar-sized model, but constructed with a semi-monocoque stress-skin body. Built in two lengths and used in both urban transit and intercity roles, it went on to become one the company’s best selling buses of all time.
The Setra S8 was MB’s main competitor in the early to mid-1950’s and with its semi-monocoque body, was lighter, used a smaller engine, and subsequently had lower operating costs than the O6600H.
As a result, in 1954, Mercedes brought put the O321H – O for Ominbus, 321 being its company designator, and H for Heckmotor (rear). Dimensions were 9.2 meters long, 2.5 meters wide – with a seating capacity of 36 in nine rows.
The engine was a longitudinally rear-mounted OM 321 MB diesel, a 5.1 litre OHV six cylinder that put out 120 hp and 220 ft lbs of torque. Transmission was a Fichtel & Sachs five speed manual.
Operators asked MB for a larger model so in 1956 the O321HL (L=Lang or long) was introduced that was lengthened to 10.6 meters increasing seating to 45.
Styling remained the same for most of the model run, with a larger front windshield debuting in 1957 and longer panoramic style side windows that curved into the roof in 1961.
A larger 5.6 liter OM 322 engine came out in 1962, which increased hp to 138 and torque at 260 ft lbs.
The HL urban transit model could seat 36, with another 38 standing. The large “36” on the roof of the red bus, from Hamburg, designates the Route Number.
Assembly was at MB’s Mannheim plant which turned out fully constructed coaches and those in knocked down kit form for assembly in other markets. Some 11,000 kits were sent to MB’s Brazil Division for final assembly in-country.
They were a familiar sight throughout Europe and South America in the 50’s, 60’s, and 70’s.
Total production when the line ended in 1970 was 29,586 which makes it one of MB’s most successful buses to date.
It’s replacement was the O302 introduced in 1965.