1560/1570 shortcut GMT conversion...

automatico · November 7, 2019

I've had a 'J$Wcom' 1655 case for 5 or 7 years that I got from a member but never used because I did not want to use my genuine GMT parts to convert a regular 1570/5 rlx date movement to GMT for a project as the GMT parts are getting expensive and very hard to get.

Also have a 'shortcut' GMT conversion that came with the case. The shortcut conversion gets a bad rap (usually because it deserves it) but there is more than one version of the conversion. The one I have is well made and has a finely finished minute wheel and 24 hour drive wheel with very good 24 hour wheel making it worth the trouble to give it a try.

The major defect with these conversions is the poorly fitting 24 hour wheel that is a sloppy fit over the original 12 hour wheel causing the 24H wheel to slip on the minute wheel drive gear or bind up and stall the movement. The other defect is these conversions were used with regular 12 hour date movement center wheel/CP and hour wheel jamming the 12H hand down too close to the 24H hand with very little clearance. The 24H hand is not too close to the dial because the 24H wheel tube is oem spec but the original 15xx date 12H wheel and CP are too short.

I made a bushing out of a stainless spring bar barrel to take up the slack between the 24H wheel ID and 12H wheel OD in addition to using a genuine 1570/75 GMT center wheel/CP/hour wheel combo to allow room for the 12H and minute hands. By using these genuine GMT parts, no one can tell it is a shortcut conversion when looking at it with the dial installed.

Another hassle with these things (besides locating/affording GMT parts) is getting the 24H hand, 12H hand, and date flip to all work together at 12 midnight. It is bad enough to get them all to correspond mechanically but it has the added problem of the date not flipping at the exact same time every night because although the date flip depends on 'precise' gearing to keep it on time, it also depends on spring tension and always will have a small amount of 'gear slack'.

You can install the hands at 12 midnight by slowly running the time around with the crown until the date flips and line them all up but after it runs 24 hours and all comes together again at 12 midnight...the date can flip early or late as much as 5 or 6 minutes.

Why?

Because of slack in the wheels and the 'date flipper' spring. You just have to get it as close as you can and live with it. There is also a difference between letting it run until the date flips to install the hands and running the hands around with the crown until it flips. There can be a minute of two (maybe more) difference between the two methods. I usually let them run until the date flips after setting the hands at 11pm or a little later by the crown.

This shortcut conversion movement has been running for a week now with no trouble and I will report on it again in a few months. The 12/24 hour hand alignment is Ok and the date flips within 2 or 3 minutes...close enough for me.

Miscellaneous:

All the standard 1575 date calendar parts and calendar disc spacer were used with this conversion. The standard calendar disc spacer is .8mm thick and a GMT spacer is 1.0mm thick.

A 1565/70/75 date movement is 6.3mm thick and a 1565/75 GMT is 6.5mm thick with all the difference being above the stem centerline. A genuine 1655 dial is 27.3mm in diameter. Ex II 1655 and GMT 1675 movements are identical.

The thicker GMT calendar disc spacer is not needed as the shortcut GMT conversion does not take up any additional space between the movement and dial because the longer 24H and 12H wheel tubes and CP pass through the dial center hole. A dial washer was used to keep the 24H wheel in mesh. The same 1570/5 'date flipper' is also used and this saves from having to use the high $$ oem GMT date flipper and calendar disc spacer.

The GMT calendar disc spacer (p/n 8037/7957 plus a few others) and calendar wheel aka 'date flipper' (p/n 8034) are near impossible to find today. Saw a used one for $965 on eBay a while back, # 264511039148. The special calendar wheel nut (p/n 8035) is also very hard to find.

The modified minute wheel on this particular kit has an 'axle' pressed in the minute wheel with the 24 hour drive gear made on the top of the axle so the original minute wheel stud (p/n 1888-1) has to be removed from the main plate (press fit). Now the 'axle' runs in the main plate and this should work Ok as it is a low stress part...time will tell. The minute wheel axle is lubed with KT22 grease where it turns in the main plate.

When looking for parts for rlx 1570/5 GMT movements, look for 1565 GMT parts because the 1565 was the first 15xx GMT movement and will include all the GMT parts.

There is no difference between any 1565/70/75 date movement and a 1560/1570/75 GMT movement except for the beat rate and GMT parts...the base movement is exactly the same. The 1565GMT runs at 18000 bph and the 1575GMT runs at 19800 bph.

The 1655 and 1675 GMT use the same crystal, 25-116. The 1675 GMT uses a 5.3mm crown and the 1655 uses a 6.0mm crown. A 1655 case is slightly thicker than a 1675 GMT but a replica GMT etc case can sometimes be used with a 116 crystal and oem spec aftmkt bezel to make a pretty good 1655 as long as the dial will fit and the crown guards look Ok. Crown guards are a common defect with replica 1675 GMT because they are usually cut for 6.0 or 7.0mm crowns and are not thick enough when looking at them from the front of the case. A not so good 1675 GMT case might make a pretty good 1655 case except for maybe being too thin and few will notice this small detail.

Every genuine 1655 I have seen had a hack movement...my project 1655 does not hack because the movement is older. Afaik the first hack movements were hit or miss in the late 1960s and 'officially' introduced in 1972. The first 1655 was around 1971. Many may disagree as usual.

Where can you get a 'shortcut' GMT kit?

I do not know and if anyone finds them please put it on here.

What have I learned?

Imho a 'freshened' swiss ETA 2846 with a China non adjustable 24H hand GMT conversion is a better $$ choice for most 1655/1675 projects. Budget 1655/1675 projects can get by with a DG 3804 as long as it is clean and has been properly oiled.

I stuck this one together because I already had a 1570/75 movement and the shortcut conversion.

'Freshened' = cleaned, oiled, and maybe a mainspring.

Pic of the $965 calendar wheel aka 'date flipper' p/n 8034 on eBay. Note the extra set of teeth to drive the 24H wheel, some sellers will claim a regular date flipper is for a GMT, the same claim is often made for regular 'date only' calendar disc spacers.

https://www.ebay.com/itm/VINTAGE-Rolex-GMT-Master-1675-Explorer-II-1655-Date-Wheel-Mounted-1570-RARE-Part/264511039148?hash=item3d961626ac:g:WQUAAOSwoGVds1HR

A low grade GMT conversion, note the loose 24H wheel. Mine was not this bad at all.

The lower picture is a correct GMT movement.

https://rwg.cc/topic/141303-cal-1570-gmt-tell-me-if-i-m-wrong/

Here is a movement being sold as a GMT that is not even a date movement much less a GMT and overpriced by $1200 or $1500:

https://www.ebay.com/itm/GENUINE-ROLEX-AUTOMATIC-MOVEMENT-Ref-1570-for-EXPLORER-II-1655-GMT-MASTER-1675/273552338514?hash=item3fb0fd6e52:g:waQAAOSwuJRb5f2p

Hope the buyer reads the fine print...

"GENUINE ROLEX GMT MASTER MOVEMENT Ref. 1570 for EXPLORER II 1655, and GMT MASTER 1675*.

*This IS the correct movement for the GMT MASTER, but needs additional parts to function as such. Sold as Is."