I’ve got a Sony wireless kit lav with 3.5mm plug screw lock connection. Is this compatible with Sennheiser G system? I have to film dancers and an inexpensive headset would be better than a lav. On ebay/Amazon say Sennheiser compatible...

The Sony and Sennheiser 3.5mm plug use a different wiring configuration so they are not x-compatible.
The Sennheiser Evolution series Tx uses the plug's Tip for Mic level. The Ring and Sleeve are tied together for ground/shielding.
For Line level input, the Ring terminal must be used as the hot. Tip and Sleeve for ground/shield.

As I recall, Sony uses the Ring terminal for both Mic and Line . . Off hand, I not know the tip connection wiring. Mic/line level switching is hardware or software based.

Wasn’t anticipating that. is there a safe way to test? I have a friend who has Sennheiser wireless so if I plug one of his lav mics into my Sony body transmitter it will either work or not but not damage anything.

Won't damage anything - so worth a try.

I was able to borrow a Sennheiser wireless lav mic. It didn’t work. The primary impediment was the ring lock on the Sennheiser prevents the full length of the plug to be inserted (see attached picture). Even if I was able to plug it in it still might not work because what Rick said being wired differently. I was happy to learn this before ordering the headset.

All of the molded-on locking plugs I have encountered cannot be used on a conventional non-locking 3.5mm input jack, because the locking ring does not retract enough. (as Pete's photo shows), OTOH, most of the solder-on locking plugs (Switchcraft TecNec, Neutrik) the lock ring retracts to be fully inserted finto a non-locking jack.
.. The Sony wiring config only works on a Sony Tx though. whereas the Sennheiser wiring config will work on most conventional mic inputs that has a 3.5mm jack.. If it is a stereo input it will only feed the left channel though.

Unfortunately there are very few headsets offered for Sony and they’re expensive $150+ this would explain why all high school theater productions use Sennheiser. Btw both are ring locks, the Sony retracts further but it’s disappointing Sony had to be different, the cost rules out getting a headset.

Some consumer stereo mics are unbalanced, on a 3.5mm plug, left=tip, right=ring, gnd=sleeve. I wonder what would happen if you plugged such a mic into the Sony wireless transmitter. Would the Sony use the R. channel of the stereo mic? This might at least verify the workings of the Sony, in regard to finding an appropriate headset.

Also, thinking way back in the PC world, mono mic inputs had audio=tip, bias=ring, gnd=sleeve. This allowed using a mono dynamic mic without applying bias to the mic element. Condenser mics made for this kind of computer simply tied tip and ring together, thus applying bias voltage and getting audio from the mic. Again, if you could find an old mic of this vintage, it might give you a clue about the Sony.

That doesn't exactly answer your question; it doesn't give you the make and model of a headset mic to use. (And in neither case would the test mic have a locking ring.) But based on the results of one of those experiments, it might point you in a useful direction. Of course it's a question of how much time you want to spend and how much tinkering you want to do.

Also, it's possible to buy a 3.5mm Y breakout cable, with a TRS plug and two TS jacks. The tip of one jack (left channel) connects to the tip of the plug. The tip of the second jack (right channel) connects to the ring of the plug. Using this adapter, you could plug a normal mono mic into the "right channel" jack, and have the signal appear on the ring of the plug, which would then go into the Sony transmitter. Again, a question of how much you want to tinker with extra adapters and cables ... it would be less than a clean & simple solution ... functional but maybe not practical.

Good suggestions if it wasn't a one off with just this client I would pursue it further. It's difficult to do any sort simple experimentation because the Sennheiser lock prevents it from being plugged into any normal TRS adapter, cable or plug. I image if I got Sennheiser compatible headset I could get it to work with the Sony by cutting off the plug and rewiring a new one in the correct configuration.

Now that I think of it I do have a 3.5mm non wireless lav mic, the Giant Squid which I got for Zoom H1. https://www.amazon.com/Giant-Squid-Omnidirectional-Mono-Microphone/dp/B00HWMG44U/din02c-20
I tried it and it didn't work with the Sony so that would confirm what you said that Sony doesn't follow the standard TRS configuration used in most mics.

Based on what Rick said in post #2, your mono Giant Squid would not be expected to work.

If you had a stereo Giant Squid mic, you could try the test I suggested. If the Sony picked up the audio from the R. channel of the mic, then you'd know Sony uses ring & sleeve, and all you'd need would be a stereo breakout adapter. (That successful test would also indicate that Sony apparently ignores the tip.)

I do see inexpensive head-worn mics on eBay, claimed to work with Sony, for about $20. That might be a good investment if only for a test. But they're coming from China with several months delay, so that probably won't help you with this project.

You could still get a stereo breakout cable within a few days, and try it with your mono Giant Squid mic. For that matter, since the breakout jacks are cable-mount, with no surrounding chassis, is there a slim chance your Senny plug would seat fully? It's only a $6 gamble.

. . .

Of course if you had a mic with a locking plug as shown in your photo, and if you never wanted to use that locking feature again, a Dremel tool could remove the ring.

Oscar SoundTech offers a headset mic for a very low price yet good quality:
http://www.oscarsoundtech.com/aboutus.html

And they can supply it with any termination / wiring you want, such as for Sony 3.5mm locking

The theatre industry use those Chinese ones remarkably often when they need what are effectively disposable headsets. Throwing. £250 DPA away is a major cost compared to the £25 Chinese one that sounds OK. Many in the UK buy these from distributors like CPC/Farnell for things like dance shows, kids shows, am dram groups where they get bent, wrenched, stood on etc. They're surprisingly good. On pro shows we have an A2 who looks after mics, but on the other shows they self-fit and share them. The booms get bent, covered in tape and get ripped off. They survive this behaviour pretty well for cheapies. A little gentle EQ and it's done.

B&H has some earset mics for under $20 with models for Sennheiser, Shure and Sony. They have 'usable' sound quality for non-critical work, but are not very rugged and would not last long in a pro environment.
The aformetioned OST headset mics (made in USA) are relatively low-cost for a pro quality mic and As I recall are around $125 (usd). OST uses the flat response TL40 element on these. The max SPL is a little low, so they may not be a good choice for horns and other high SPL sources

That's I think the point, Rick. They're not that rugged - BUT - the people and activities that destroy them often happen when you have your expensive favourites on people. They need a little EQ, but most headset or over ear types do anyway, and once they're balanced, it is perfectly possible to have decent audio from them. We still like DPA's, but before that we liked Countryman, and before them, it was MKE-2's I liked. They all sound different. I've a little list of mic destroyer names. One Broadway star is top of my list. He managed to wreck one a week, but due to his 'stature' he was OK to destroy DPAs. One other lower down the list, had his DPA swapped for a cheapie and nobody noticed.