Sorry for reviving a dead (thread) but I couldn’t resist. While reading about the D4 Ti I came across this thread where some compared a wishlist BLF version of the S42 to the D4, then I realized my modded S42 was essentially that.
Basically I used the above/post mentioned S42 TA Narsil Driver from Lexel and got most of the features mentioned by others in this thread, an Emisar D4 like finished product. Some may ask, “why bother”, I wouldn’t have but I also couldn’t stand letting the S42’s potential go to waste by sitting in a draw unused and unloved (primarily due to the poor UI), plus modding is in the BLF spirit.
Changes to the S42 starting with the original pre-order model –
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Replaced original 18350 battery tube with longer one and replaced loose USB cover, corrected/supplied by BG
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Added a 18650 battery tube for longer runtime option as needed, purchased on BG
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Sanded down the sides of the original optic so that it drops down further onto emitters for more throwy focus
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Replaced existing S42 driver with Lexel’s TA Narsil driver (see post above)
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Elected to retain USB charging function (which is optional) by re-using original charging board
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Elected to retain lit side button as a “locate me” feature (which is optional) but also serves as an indicator for various other light functions such as battery voltage, FET/SOT power levels, lock-out indicator, etc.
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- Added a second smaller spring into tailcap to help with any added current demands
New Driver (top) used to replace the original board on bottom-right

Wiring completely removed from original boards before re-soldering to new driver board. Also unseen is partially desoldered MCU on underside of origal USB power board (on left)

Side switch with all onboard LEDs rewired

New FET+6 Driver Board installed with battery post (used instead of spring)

MCU Function indicated/locate me Switch LED

(Sorry in my excitement to test the UI and see if the side switch LED worked I forgot to take a photo of the completed driver assembly before installing.)
The light now has the excellent Tom E. / TK ramping firmware (similar to that as used in the D4) - smooth stepless ramping with direct access to turbo, last mode memory, battery voltage indicator, tactical turbo mode, etc. AND no more long presses for on/off!
Opening, disassembling and removing the old driver and parts and soldering and reassembling the new driver was the most complicated mod this novice has tackled to date, probably why it sat on my table untouched for weeks. I finally got the nerve to start when I saw member Edness’ completed project in the ‘Modded Today’ thread. In the end I was pleasantly rewarded with a fully functional version S42.Blf which is what it could have been all along (or a D4 wanna-be with USB charging! for those who prefer.)
My understanding is that a new version of the S42 is being considered for release with a TA driver, I don’t know what FW will be used but hopefully Narsil. For any others with an unloved original S42 that enjoy modding and can’t wait, Lexel’s TA Narsil driver is worth your attention.