Seiko 'Gen 2' RAF Military watch 7T27B - 7A20 requires new movement

[Matt77]

Good day all,

I've recently re discovered my issued Seiko 'Gen 2' RAF Military watch 7T27B - 7A20 and upon inspection the movement is beyond repair due to battery leakage and corrosion. Does any one have a suggestion as to where I could source a new movement or replace it with a compatible one?

I am not a watch specialist and really appreciate your help

Regards

[AndyGSi]

Welcome to the forum.

Are you sure it's beyond repair? Photos would help.

[tIB]

2 hours ago, Matt77 said:

Good day all,

I've recently re discovered my issued Seiko 'Gen 2' RAF Military watch 7T27B - 7A20 and upon inspection the movement is beyond repair due to battery leakage and corrosion. Does any one have a suggestion as to where I could source a new movement or replace it with a compatible one?

I am not a watch specialist and really appreciate your help

Regards

I have done this very job!

I had a job finding a movement and in the end found a nos Hattori YM57a from a place in the States - it's a drop in but I had to switch the date disc to fit the correct black one and changed the rear plate over so it looks right.

I also had a bit of a job with the hour sub hand - theres a thread on it here somewhere. 

[Matt77]

18 hours ago, tIB said:

I have done this very job!

I had a job finding a movement and in the end found a nos Hattori YM57a from a place in the States - it's a drop in but I had to switch the date disc to fit the correct black one and changed the rear plate over so it looks right.

I also had a bit of a job with the hour sub hand - theres a thread on it here somewhere. 

Wow that’s great!

Could you point me in the direction of where you sourced the YM57a movement? I’m really struggling to get on in the UK. 
 

Thanks again

Matt

[AndyGSi]

1 hour ago, Matt77 said:

Could you point me in the direction of where you sourced the YM57a movement? I’m really struggling to get on in the UK. 

I don't think you'll find one in the UK.

The other option is a Y187 but they're also a rare movement.

Are you sure it's beyond repair? Photos would help.

[tIB]

12 hours ago, Matt77 said:

Wow that’s great!

Could you point me in the direction of where you sourced the YM57a movement? I’m really struggling to get on in the UK. 
 

Thanks again

Matt

Pretty sure it was this place: https://allwatchparts.com/Hattori-YM57-Watch-Movement_p_712.html

 

I should add it took some waking up once installed - my thread is here and the whole thing hasn't been particularly straightforward... I finally seem to have it all as should be though after quite a lot more messing than I imagined. 

https://www.watchrepairtalk.com/topic/30111-starting-a-7t27ym57a-chrono-fine-watch-not/

 

 

 

Edited February 3 by tIB

[Matt77]

Thanks tlB,

That’s great info and I’ll get one ordered.
 

im worried its a lot of hassle and it may go wrong. I will have to try and ask someone to fit the movement once I get it. Would any jeweller or watch repair center do it do you think? Or would it be best to try and find someone on the forum to fit it for me?

[AndyGSi]

48 minutes ago, Matt77 said:

Thanks tlB,

That’s great info and I’ll get one ordered.
 

im worried its a lot of hassle and it may go wrong. I will have to try and ask someone to fit the movement once I get it. Would any jeweller or watch repair center do it do you think? Or would it be best to try and find someone on the forum to fit it for me?

You're probably best trying to find a repairer willing to take on
the job before you go to the expense of buying the movement.

A lot of repairers won't work on quartz and even less would do a none like for like swap.

[Matt77]

On 2/4/2025 at 8:20 AM, AndyGSi said:

You're probably best trying to find a repairer willing to take on
the job before you go to the expense of buying the movement.

A lot of repairers won't work on quartz and even less would do a none like for like swap.

I think I will order the part anyway as I don’t want to sit with or throw out the watch. Hopefully I can get someone to complete the movement replacement for me. I will ask my local repairer if he will do it and if not I will have to try on the forum I suppose. 
 

Really appreciate all your help and comments.

If you know of any volunteers please let me know!

[AndyGSi]

@tIB Can you confirm what you had to do with the stem and crown on your swap.

[tIB]

9 hours ago, AndyGSi said:

@tIB Can you confirm what you had to do with the stem and crown on your swap.

I'm pretty sure I used the original crown and stem - there was a question mark over whether that was causing the starting up issue with the new movement I had irrc. (I'm not sure that the stem that came with the movement even fit when I tried it.)

 

If I remember right I did the following to get it running correctly: 

Changed rear plate, 

Changed date disc from white to black,

Changed front plate/date disc holder,

Changed hour subdial gear, 

Changed hour hand wheel after error by me.

 

I didn't expect to have to do the last three things, but it was process of elimination in getting the subdial hour hand working correctly. I also accidentally mangled that plastic hour hand wheel trying to fit an incorrect hand... Live and learn: fortunately I had the old movement to get all the parts over from.

9 hours ago, Matt77 said:

I think I will order the part anyway as I don’t want to sit with or throw out the watch. Hopefully I can get someone to complete the movement replacement for me. I will ask my local repairer if he will do it and if not I will have to try on the forum I suppose. 
 

Really appreciate all your help and comments.

If you know of any volunteers please let me know!

While I've done this I'm way too much of a novice* to offer to help really, though if you're nearby and willing to give it a crack yourself I can certainly lend a hand. Where are you based?

Richard Ashkram might be worth an ask - he definitely does Seiko and I think he does quartz. 

 

(* In that I take the risk in making a mess of my own buys but couldn't guarantee I wouldn't!)

[AndyGSi]

31 minutes ago, tIB said:

I'm pretty sure I used the original crown and stem

They're a different stems so that's probably why you had the problem, YM57 (351584) and 7T27 (351580).

[tIB]

Just now, AndyGSi said:

They're a different stems so that's probably why you had the problem, YM57 (351584) and 7T27 (351580).

I'm pretty sure I'm using the YM57 movement with the 7t27 crown/stem. I couldn't get the stem that arrived with the YM57 to fit it. 

[AndyGSi]

2 hours ago, tIB said:

I'm pretty sure I'm using the YM57 movement with the 7t27 crown/stem. I couldn't get the stem that arrived with the YM57 to fit it. 

I'll try and post a photo later but on the 351580 there's a split in the square section which depending how it lands could affect the operation.

 

Edited February 5 by AndyGSi

[Matt77]

Finally guys I have a photo. It doesn’t look corroded to me but I am no expert. I am intrigued about the whole on the back labelled “push”. What happens in that is pushed, what’s its function?

[tIB]

On 2/5/2025 at 8:38 PM, AndyGSi said:

I'll try and post a photo later but on the 351580 there's a split in the square section which depending how it lands could affect the operation.

 

Interesting - I haven't noticed any functional issues in mine, and 99% sure it's the old stem... the only thing I wonder about is if that was why I struggled to get it started up? 

1 hour ago, Matt77 said:

Finally guys I have a photo. It doesn’t look corroded to me but I am no expert. I am intrigued about the whole on the back labelled “push”. What happens in that is pushed, what’s its function?

The push is to release the stem. What happens with a battery in? There's a reset method on these where once the battery is in you have to short it... Can't remember what exactly now but I think it's in that thread of mine I posted. 

 

Edit: vague memory says you short the battery cover to where it's marked AC - have that confirmed though before you do it. 

Edited February 8 by tIB

[AndyGSi]

41 minutes ago, tIB said:

Edit: vague memory says you short the battery cover to where it's marked AC - have that confirmed though before you do it. 

Yes the battery +ve needs shorting to the AC contact to reset after a battery change.

@Matt77 Looking at your photos I can't see any obvious signs of corrosion.

I suggest you try a good quality battery (literature shows SR927SW but would go for a SR927W)
then reset the movement as above and see what you get.

Edited February 8 by AndyGSi

[tIB]

28 minutes ago, AndyGSi said:

Yes the battery +ve needs shorting to the AC contact to reset after a battery change.

@Matt77 Looking at your photos I can't see any obvious signs of corrosion.

I suggest you try a good quality battery (literature shows SR927SW but would go for a SR927W)
then reset the movement as above and see what you get.

The only bit I wondered about was the screw next to the battery - can't work out if that's just the light though?

[AndyGSi]

39 minutes ago, tIB said:

The only bit I wondered about was the screw next to the battery - can't work out if that's just the light though?

If you're talking about this black then I think it is corrosion but it's just the circuit cover.

I suppose it depends how much it extends under onto the board itself.

 

[Matt77]

10 hours ago, tIB said:

Interesting - I haven't noticed any functional issues in mine, and 99% sure it's the old stem... the only thing I wonder about is if that was why I struggled to get it started up? 

The push is to release the stem. What happens with a battery in? There's a reset method on these where once the battery is in you have to short it... Can't remember what exactly now but I think it's in that thread of mine I posted. 

 

Edit: vague memory says you short the battery cover to where it's marked AC - have that confirmed though before you do it. 

 

10 hours ago, AndyGSi said:

Yes the battery +ve needs shorting to the AC contact to reset after a battery change.

@Matt77 Looking at your photos I can't see any obvious signs of corrosion.

I suggest you try a good quality battery (literature shows SR927SW but would go for a SR927W)
then reset the movement as above and see what you get.

Thanks guys, I will try the battery you specified. You will have to tell me exactly how you short the battery to reset it. Will I damage it if I get it wrong?

[AndyGSi]

13 minutes ago, Matt77 said:

Thanks guys, I will try the battery you specified. You will have to tell me exactly how you short the battery to reset it. Will I damage it if I get it wrong?

You just need a pair of tweezers with a fine point, put one in the hole marked AC then touch the other on the battery for a couple of seconds.

[tIB]

FYI a load of NOS YM57 have gone up on eBay this week - no idea on seller/quality or anything and they aren't cheap.

[Matt77]

Right guys, got the new battery this morning and fitted it. I shorted the battery via the ac point and the battery but the watch failed to start unfortunately. I suppose I’ll have to wait for the new movement and get someone to fit it for me. 
 

thanks all for the help. 

[AndyGSi]

28 minutes ago, Matt77 said:

Right guys, got the new battery this morning and fitted it. I shorted the battery via the ac point and the battery but the watch failed to start unfortunately. I suppose I’ll have to wait for the new movement and get someone to fit it for me. 
 

thanks all for the help. 

So there's no signs of even a twitch from the seconds hand?

Have you tried the Chronograph?

4 hours ago, tIB said:

FYI a load of NOS YM57 have gone up on eBay this week - no idea on seller/quality or anything and they aren't cheap.

Interesting that they appear to have the 7T27 stem.

[tIB]

33 minutes ago, AndyGSi said:

So there's no signs of even a twitch from the seconds hand?

Have you tried the Chronograph?

Interesting that they appear to have the 7T27 stem.

Good spot! I dug mine out and, if it's even the correct one, it definitely isn't the 7t27 version.