Seiko 7A38 - by the numbers

Time permitting, later in the year, this could well be something I tackle myself. But I've got a couple of other projects I'd like to do first.

To get the background, you'll need to have read, and completely taken in much of what I wrote in the Jean Lassale Thalassa thread:

http://www.seiko7a38.com/apps/forums/topics/show/7614141-jean-lassale-thalassa-quartz-chronograph-the-missing-7axx

.... and read that in conjuction with Simon's excellent set of (ever so slightly ) posts about the 7A34 and other movements:

http://www.seiko7a38.com/apps/forums/topics/show/7408813-are-7a28-and-7a38-modules-interchangeable-indeed-are-all-7axx-movements-

The challenge is to build yourself a 'poor man's 7A74 using Seiko 7Axx components - The ULTIMATE Seiko 7Axx Franken !! 

Those of you who've already read the thread on the Jean Lassale Thalassa chronograph will know that the 7A74A movement which it uses, is effectively just a modifed 7A34A movement fitted with a 7A38's date ring (oh, and a re-branded gold anodized back-plate etc.)

 So, to build a 7A74 movement Franken, one would perhaps need to follow some sort of a recipe or guidelines, wouldn't you ? 

Take one working 7A34 movement. Remove the 7A34 date ring and replace it with a 7A38 date ring (of your choice of colour). 

Take one perfectly good (or gash) 7A28 dial and carefully drill (and / or file) a hole, either round or oblong in it, at the 4:30 position.

Ensure it properly aligns with the numbers on the date ring. Fit your carefully modded dial with naturally a non-original set of hands.

Identify and select a suitable 7A38 or 7A48 case and dial ring (either Tachymeter or Minute) to house your franken creation. 

Fit that with a non-original (ideally Seiko) bracelet or strap ....

Display proudly to the forum. 

Any budding tinkerers amongst you fancy having a go themselves ? The only really tricky bit will be getting the date aperture right. 

I had a little tinker at the weekend Paul.

My 7A34 movement has been sitting un-cased since I popped a 7A38 into the case it came in, and I have a few 7A28's sitting around in various states so ................

Here's the 7A34 movement and date ring:

and here's the date ring swapped for a 7A38 one:

in fact, you'll see I just overlaid the 7A34 date ring to show its relative numeral orientations.

The one we're interested in for a '7A74' is the 29 - note the orientation on the blue 7A34 date ring compared to that above.

Next we need a 7A28 dial:

and a back shot:

and the dial in place on the above movement:

now, where's my Dremmel ............................

tbc  - when I feel a bit braver 

(a round hole should be relatively easy - but I think this deserves a rectangle, which means a suitably small file)

Kudos to you, Simon, for taking up the challenge so quickly, while I was merely toying with the idea, and .....

Thanks for confirming what we already knew: that the 7A74 movement was nothing more than a 7A34 fitted with a 7A38 date ring. 

(a round hole should be relatively easy - but I think this deserves a rectangle, which means a suitably small file)

A round hole may actually be the easier and more practical way to go. 

I did get as far as marking up where to cut a 4:30 date window on the back of a gash 7A38 dial, at the weekend - but no further. 

For one thing, it's tiny - I measured it at 2.30mm wide x 1.80mm deep. Even I don't have a suitably shaped jewellers file that small.

It's not just a matter of getting the edges parallel and corners square, but also getting the 45° alignment spot on, for it to look right.

Another thing to consider is that the sub-dials of the Jean Lassale Thalassa 7A74 are a smaller diameter than most Seiko 7A's:

So by attempting to file a rectangular date window, you'll be working closer towards the 7A28's subdials. 

Ignoring that odd escutcheon thingummy around it, the 12 o'clock date window on a 7A34-7010 / -7019 is actually round:

As indeed are the 4 o'clock date windows on some Citizen Cal. 3510 powered chrono's, such as this Pryngeps:

However, both of them appear to be using date rings with rather more condensed number fonts than those of the 7A38 ....

which might necessitate a slightly larger diameter round hole for the double-digit dates to be fully visible. Food for thought. 

I had a thought about this on the way to work this morning. I'll make a card template of the dial, and then 'test' the correct position of a date window.

This will prevent ruining a dial with a badly positioned hole 

I'm going to go for a rectangular window - the round window doesn't look right.

Maybe this bracelet is the right one for your challenge

http://www.tungchoywatch.com/product_info.php/cPath/140_163/products_id/2320

Nice try Andre  .... but I'm sorry - there simply aren't enough component parts in that bracelet to qualify. 

In fact, that bracelet looks a far better copy of the Breitling 'Rouleaux' bracelet than it does the Thalassa's.

See: http://forums.watchuseek.com/f29/breitling-rouleaux-bracelet-bullet-18k-gold-steel-485426.html