The first generation of Reynolds 753 was only available with metric diameter tubes. The early Raleigh SBDU frames from the 1970s used metric 753 tubing.

About 1982 Reynolds introduced second generation 753 which was produced in both metric and Imperial (inch) diameters. They were available in 2 versions: 753r (Road) and lighter 753t (Track).

Metric diameter 753 tubing takes 27.0mm diameter seatposts. Sometimes you may see 26.8mm seatposts because the seat tube isn't round.

Inch size seat tubes take 27.4mm seatposts but again you may find a 27.2mm seatpost in an inch size 753 frame.

Supposedly some larger 753 frames had thicker seat tubes and used 1 size smaller seatposts. Also, Reynolds supposedly started supplying 531 fork blades with 753 tube sets.

My 1988 Peugeot Chorus with metric 753r tubing with a 27.0mm seatpost. The metric Reynolds 753r stickers are in French:



My 1984 Holdsworth 753r with inch size tubes. It takes a 27.4mm seatpost:



I think that any Merckx frames made of 753 produced after 1982 would have inch size tubing.

The "Z" stamped into the OP's BB shell indicates 1985 or early 196.

My 1985 Corsa with a Z stamp. 6C means 56cm Corsa.



Here's a link to a Merckx serial number guide and timeline:

Eddy Merckx Serial Numbers

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