The Sunon SP68 is not a worthy VH31 replacement
Sweep second movements are cool. You can get sweep second from a tuning fork watch, a typical mechanical watch, an electronic watch, or a sweep second quartz watch, in decending order of price.
There are 2 brands of sweep seconds quartz movements that are obtainable freshly new nowadays: TMI VH31 and Sunon SP25/28/68 series.
The Reddit r/SeikoMods community where poor folks show off their first/second/third/nth build to serve up their confirmation bias had revealed a trend of using the VH31 quartz sweep second movement to replace the automatic NH3X series of movements, even so far as to creating a set of VH31 movement rings to fit them onto NH3X-compatible watch cases, in order to save money on the movements.
Adding various Chinese microbrands making homage watches with the VH31, the sweep second movement which once costs ¥20 ~ ¥30 RMB now costs ¥80+ and is trending unobtainium.
I have 2 VH31 bought before the shortage, and I am pleased with its performance.
Until one day I found out about the Chinese sweep second movement series SP25/28/68 by Sunon largely unknown to the westerners, sold at prices varying from ¥2 ~ ¥6.
I have tried both and I think they serve their own respective niche that can hardly overlap. I will explain.
When placing the two movements next to each other, it's immediately apparent that they have different form factor.
VH31 is bigger and thicker than SP68 and having a size nearly the same as a Miyota 2105, therefore it could fit into a case and case ring designed for Miyota 2105. However its stem height (from base of dial to the middle of the stem) is off by 0.5mm requiring the dial to either be slightly thicker or a plastic spacer is needed.
Meanwhile the SP68 follows exactly the form factor of a Miyota 2105, the world's most popular quartz movement. Meaning it will fit into any of the billions of watches with a Miyota 2035 movement without any workaround needed. Its date/day siblings SP28D and SP28DD are interchangeable with Miyota 2105 and 2115.
In fact, some did a PC21S -> SP68 movement swap on YouTube — in the most brutalist way imaginable as a watch maker, I might add.
By the way, the TMI PC21S movement on everyone's favorite Daiso Mili Watch has exactly the same dimension as the Miyota 2035, meaning it's easy to mod the Daiso Mili Watch to using an SP68. In fact, someone from Japan wrote a blog post about this.
When it comes to watch hands, VH31's hour/minute/second post dimensions are slightly larger than those of the 2035/SP68/PC21S. This means if someone forces a hour hand for 2035 onto the hour post of an VH31, the tube of the hour hand may cut into VH31's plastic hour hand post, causing permanent damage.
Good news is the hands of TMI VJ/VX/VH series are compatible with a VH31. Since TMI is making PC21S with 2035-compatible hands, why can't they just use the same sizes for the V- series?
SP68, just like its external dimensions, share the same hour/minute/second post size as 2035 and PC21S, meaning there are exponentially more hands available, and for cheap.
So far it sounds like SP68 is on the lead in the compatibility department. Of course, that's because the intention has been to swap out a PC21S or 2035 from a cheap watch into a SP68. Or even a 2105/2115 for that matter.
But then guess why the many Chinese micro-brands are using VH31 instead of the SP68 on AliExpress to sell to r/ChineseWatches Reddit-ers showcasing their latest impulsive purchases cheap only from the part of the world with VAT and sales tax, meanwhile driving up the price and causing shortage in this part of the world?
I can name 3: accuracy, battery life, and build quality.
I have had my fake Shanghai 7120 running on VH31 for nearly a year and it'd only been off by a minute. The technical document claims an accuracy of ±15 seconds/month.
The accruacy of SP68 is ±45 seconds/month, 3× the range of an VH31.
Since both movements are sweeping at 4 ticks/second, they inevitably consume more power than the 1 tick/second PC21S and 2035. While I do not have data for SP68, the VH31 claims to have ~2 years of battery life.
My guess is SP68 would require a battery change long before VH31 does.
But if someone knows enough to swap a movement, changing battery should just be a piece of cake.
In terms of build quality, VH31 just works. It could even take some abuse and still work just fine, quoting my experience of partially spltting the hour post in half.
When I pull the crown out, the second hand immediately stops. And when pushing the crown back, the minute hands stays where it is.
Not sure much for the SP68. I had once dealt with a lemon SP28D with a free rotating second hand post, rendering the sweep second, its biggest merit, unusable.
When I pull the crown out, the minute hand freely rotates, making hand setting a big trouble. And when I push the minute hand back, it snaps to a particular position (probably snapping to the nearest gear tooth) that doesn't necessarily line up with the minute markers.
On one of my SP68, the second hand would rock back and forth within a particular quadrant of the cycle.
These issues are shouting "Chinese quality" louder than anything else.
The Sunon SP68/SP28D/SP28DD cannot be a drop-in replacement of the TMI VH31. They serve different niches — cheap sweep-second upgrade at the cost of quality and accuracy versus bang for the buck quality micro-brands.
VH31 is 3× ~ 15× more expensive than Miyota 2035/TMI PC21S/Sunon SP68/SP28D/SP28DD. The incompatibility of parts justifies higher quality parts be made for the VH31.
And when one finally put the watch on the wrist, an SP68 can never be nearly as good as VH31 in terms of user experience, when SP68 fails to even get the basic minute hand alignment right (without meticulous hand setting process).
Therefore, the Sunon SP68 is not a worthy VH31 replacement. They mostly likely will happily occupy their own market segment with little overlap.
Thank you for bearing with me this whole time.
I have to admit, I am everything I criticize about the Reddit people of r/SeikoMods and r/ChineseWatches, showing off my builds on this website and going for even cheaper sweep second movement than VH31 so that I can save money on the next build.
Guess why I have to make a read-only website?
Also I hope nobody go snatching up a bunch of SP68/SP28D/SP28DD, driving up the price and causing shortage after reading this. That'd make me an influencer.