Omega 321 parts: every part and its number, and how to tell a real one

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The Omega calibre 321 is the column-wheel chronograph that ran every Speedmaster from 1957 to the 145.012. Taken down to the plate, it comes apart in 106 steps: 65 parts and 41 screw steps, each with its number in Omega’s GF 620-4 spare-parts catalogue. Here is every part, in the order a watchmaker takes them off.

Strip a 321 on The Bench

Quick answers

• How many parts? 65 parts and 38 different screw numbers, from the dial side down to the barrel.
• How many are 321 parts? 21. The other 43 carry 320 numbers, because the catalogue gives the calibre 320 as the 321’s base, and the escape wheel is 220.1305.
• The 1968 changes: parts revised in 1968 carry .1 on the end of the number. Buy against the number, not just the calibre.
• Is yours a real 321? Column wheel, five ticks a second, 321 on the balance cock, and a reference that left the factory with one. The full checks are in our calibre 321 guide.

Every part of the 321, in bench order

Dial side first, then the escapement, the chronograph works, and last the train and barrel. Screws are left out of the table; there are 41 screw steps using 38 screw numbers, and each one is on The Bench beside the part it holds.

Part numbers: GF 620-4, calibre 321 and 320 folios, as transcribed into the Watch Breaker parts database. Bench order: Omega’s technical guide for the 861 where it covers the same works, otherwise normal bench order. Numbers are Omega’s; no catalogue pages are reproduced.

The 21 parts that make it a 321

Most of the 321 is shared with its base calibre, the 320. The parts that carry a 321 number are the hour recorder on the dial side, the balance cock marked with the calibre number, the chronograph and minute-recording runners, and the train: the barrel, the centre wheel with cannon pinion, the fourth wheel and the ¾-plate bridge 321.1002.

Two details collectors ask about. The column wheel, 320.1715, is a 320 number, so a column wheel alone does not prove a 321; it does rule out an 861, which has none. And the balance cock 321.1006 is where the catalogue prints the calibre marking, which is the quickest way to tell a 321 from the near-identical 27 CHRO C12 T1, marked on the plate instead.

What changed in 1968

• Hour recorder stop lever 321.1780.1, with its shoulder removed, is fitted together with the hour hammer 321.1783.1. Don’t mix one old and one new.
• Hour recorder bridge 321.1775.1 carries the new bushing 6180.
• Barrel arbor 321.1204.1 replaced the earlier arbor.
• For the hour hammer’s operating lever, cases 2917, 2998, 2999, 14360 and 14364 take 321.1784a.

How to tell a real one

A swapped movement is the most common problem in a vintage Speedmaster. Every Speedmaster up to the 145.012 left the factory with a 321; the 145.012 is correct with a 321 or an 861. Check four things: the column wheel, the beat (18,000 an hour, five ticks a second, against 21,600 for an 861), 321 on the balance cock, and the copper finish. Our calibre 321 guide walks through each one, with every 321 reference.

Learn it before you open one

The 321 is on The Bench, free in your browser. Take it apart in the order above, every part with its number on screen, then build it back up. In Practise mode it asks what comes off next before it moves. The 321 is also one of the three calibres in Season 1, with the 565 and the 861.

Play the 321 Bench

Questions