Hi Guys.   Always lurking here for new tips & ideas.  Had a 1974 DT 360 for 20 plus years. Not ridden much, have a hoppy tinkering,not a lot of places to ride.  Decided to refresh her ,Bill Bune. Came back beautiful,had sent them 3rd over ,rings.  THEN go to install compression release-only screws in about 1/2 way. It’s a  glorified pipe, how hard can it be. Well I’m seeing some thread damage,try to keep to a minimum. My theory is it got dropped during shipping,and it is at a thin vulnerable spot-very top,very front. Probably could run a tap thru,don’t know thread size- measures 21.65 outside thread of decompressior. Visually can’t see damage outside or in?? Just lacks 1/2” or so of setting  just keep reaming on the wrench. Should have installed it on the bench,but who knew. It came out no problem. Thanks. long story-I know.   Going for a ride on one that runs

Is there a place close by that has nuts in metric? Like a Fastenal? You can find out what thread it is, can they order you the correct tap? Just an idea…

To measure a thread, put a ruler alongside the screw, bolt etc and count how many threads there are within, say, 10 millimetres and divide this into that 10 - this will give you the thread measurement.

If you can re-tap the thread, then maybe safest to remove the cylinder first, otherwise there's a danger of shards dropping into the engine.

I'm only going by memory which fails me regularly these days. Is there an unthreaded bit right at the start where the last part in is a step on the decomp to support the copper big washer & the step inserts in there. If so, has it got slightly squashed there & can you relieve it with a scraper. The thread by memory on the big decomps is the same as a yamy quad swing arm pivot bolt that inserts into the taper bearings. I cut one up to blank off my decomp hole once. Could get one of those & put a couple of grind marks longways & use as a thread chaser to clean it up if in fact the thread deep down is damaged. It's a bugger when things like that happen "that shouldn't". 22mm fine thread comes to mind but might be wrong.

Yamaha Service Bulletin from when they changed to the larger compression release says the thread is 22mm x 1.5 pitch

(Your memory is doing fine RT325 )

Thanks.  All Great Ideas.  Off to the hardware store tomorrow. Had to relax & take a breath.  Probably end up pull it back apart,do it on the bench.  sometimes when something goes too easy…   Yeah,it’s not buggared on very outside.  something wrong inside 3/4 way to end of thread. Thanks for the great support!!

Strange thing to happen to the thread let alone so far down in there.

May be better to gently try a thread chaser first before going to a tap.

Just a very random thought for an unusual happening but I wonder if it got bumped upsidedown so hard & unluckily in exactly the right place so it moved whatever blocks the drilling in the top fin. They drill the decomp hole all the way down then block it off then epoxy over the block off. Look in there for evidence of something random like that. On fact if ya feeling lucky & is 'that'  if you had a long strong piece of something to go up from the exhaust outlet it'll arrive at exactly 'that plug' to pump it away. That's if it is something solid ,& not just total epoxy in the top fin. Use a spoke cut off & ground flat maybe. Hope that's legible. On my phone. I'll correct it all later lol.

Alright here we are. Found a deal on eBay nearby for proper tap. After taking a day off I went back to it.   With the cylinder installed,working between the frame rails I couldn’t see what was happening. The ,when I get to the stepped up bit,it’s in fact hitting the outer edge hole. Being on the opposite side I’m working on I couldn’t see it happening. Better to disassemble so I can see better.   Now I’m reconsidering tap  I have to uncrooked the thread because it must be completely straight for outer flange to enter hole.  There is a bung up on outside edge