3Hz is also called 21600bph or 6bps, 4Hz is 28800bph or 8bps.
The reason for this disparity (3Hz vs 6bps or 4Hz vs 8bps) is that the balance wheel advances the second hand both on the clockwise turn and counterclockwise turn, so a full turn cycle is 2 beats.
Chinese 21J, Chinese 23J – no-name movements, based on DG2813 (if you’re lucky) or the Chinese Standard Movement and company, that the Chinese have perfected… mostly when it comes to cost of production – they can make a shitty versions for under 10 dollars, but you cannot expect any kind of reliability from it. The worst versions can be found in Chinese street reps and tend to break after a few months of use. If you ever see something like a Daytona with days of the week and month indicator on the subdials, you can bet it sports one of those bad boys.
Some of the more reputable factories sometimes use movement called that and you can expect them to be of a passable to good quality then, but it’s still hit or miss.
Most of them are 3Hz, some are 4Hz.
DG2813 or other DG-something – firstly, it still might be the shitty movements described above (especially if called A2813), them being clones of DG movements, which are in turn clones of old movements from Citizen. If you get an actual DG2813, you should not expect amazing quality, but it is very reasonable for it to work for years (accuracy is another thing though).
Most of them are 3Hz, some are 4Hz (e.g. DG4813).
A2824, A2836 – the best of the best – ETA used to have factories in China, so the Chinese know how to produce the movements. The materials are somewhat worse than ETA required and the quality control is nowhere near what you would find in a gen Breitling or such, but they are reliable, any watchsmith can service or repair them, and replacements are freely available anywhere.
They beat at 4Hz.
A2892 – Chinese copy of a newer, thinner “version” of ETA 2824 – good movement, but way more rare and as such possibly slightly lower quality than the above two. Most of them will probably come from Sea-Gull, which would be good, but they might be QC rejects.
They beat at 4Hz.
A6497/A6498 – clone of ETA Unitas 6497/6498, great movement, simple, handwound, you are most likely to find one in a Panerai roleplay and you can’t really go wrong with this one.
They beat at 3Hz.
Swiss ETA 2824/2836/others – hit or miss – it depends on how the repmakers sourced it and how they stored it. I have a roleplay with a swiss movement and it performs beautifully, other people report the opposite – problems and having to replace. Could also get a Sellita instead of ETA (which is in no way a bad thing, Sellita is an excellent Swiss movement manufacturer and a former contractor for ETA).
They beat at 4Hz unless vintage (check specific model) or Unitas 6497 – 3Hz.
A775X (X is placeholder for other digits) – ETA Valjoux 775X clone, used in most roleplay chronographs. More costly to service and harder to source than other Asian ETAs, also more likely to break – all of this because it’s more complex. Check if the positions of the subdials were modified compared to the original – if yes, that’s a major yellow flag when it comes to relability and servicability. There are exceptions – ZF IWC chronos for example started using a sensible mod that you can safely go for.
They beat at 4Hz.
SA3135 – a modded A2836, may accept a genuine Rolex datewheel. Possibly the worst option you can get in a Submariner.
They beat at 4Hz.
SH3135, SH3131, VR3135 – Chinese “superclone” rolex movements, harder to service than A2836, but a lot of people prefer them because they can fit a genuine datedisc on them (not SH3131 because it has no date). Not actual superclones (as in there are differences between those and actual Rolex movements), but can be swapped for a genuine movement if you can get your hands on one.
They beat at 4Hz.
(S)A3186/(S)A3187 – modified asian ETA with contraptions that make it impossible to service and faulty. Best avoided, but currently the only way to get correct hand stack (as in order of hands seconds -> minutes -> 24hr -> 12hr) and setting method in Rolex GMT.
They beat at 4Hz.
A4130 – a weird mod of A7750 with subdials moved and possibly decorated, which is pointless in a closed-caseback watch. Comments about modified A7750s apply.
They beat at 4Hz.
SA4130 – either a complete superclone of Rolex 4130, or an ARF mod of A7750 – check if it’s from Noob Factory and expensive – if yes, its the former, otherwise the latter. ARF SA4130 simply doesn’t use the minutes and hours subdials (they are frozen) so there is little to no problem with added torque. If it’s not a new expensive Noob or ARF, it’s probably A4130 (described above) with an S tacked in front of the name.
They beat at 4Hz.
A8900/A8500 (in Omega reps) – movements modified to look like Omega calibers. Used to be modified ETA clones or even modified Chinese 21J/23J, but lately VSF started boasting a “superclone” version that gives the same functionality as gen. Some people seem to overhype those “superclones” as actual complete cloned movements – this is not actually the case, but they are reportedly more reliable than SA3186/SA3187s which strive to provide a similar functionality. Also, no roleplay Omega movements have the correct beatrate currently (3.5Hz), none have actual co-axial escapement (to my knowledge), and they use overlays with fake jewels, so take VSF’s “almost the same as genuine movement” with a grain of salt.
They beat at 3Hz or 4Hz depending on the movement used as a base. VSF “superclones” beat at 4Hz.
Sea-Gull movements – mostly good movements, but the versions used in reps are not exactly high end (more likely factory QC rejects or lower-tier versions).
Check specific model to know beatrate.
Miyota movements – mostly good movements, but the versions used in reps are not exactly high end (more likely factory QC rejects or lower-tier versions). People were high on Miyota 9015 at first (it’s thin like ETA 2892) because it made some thin reps possible, going as far as putting it in Tudor Pelagos for example, but in the end a lot of Miyota9015 reps have noisy rotors (unidirectional rotors can rotate in one direction freely so it’s harder to silence), so their expansion into the roleplay industry has been halted somewhat. Miyota 8125 is mostly an entry-level movement and often found in Submariner homages for example.
Another thing worth mentioning is that SevenFriday reps use Miyota movements for a different reason entirely – the gens also do.
9015 beats at 4Hz, 8125 beats at 3Hz, movements in SevenFridays beat at 3Hz (at least all of those I know about).
