Blimey MartT you've been busy in the background & thanks for that. I'm looking at the pics above & i see the crank seal popped out!!. Is this OP's motor or just a sample pic.

Good eye RT325. 

Pretty sure that's his bike.  That's the pic I read the VIN from to confirm 1976. Hopefully when done with studies/work he'll be able to get back to us. 

Im unsure the year of the bike as it had no log
book or anything with it....ait.or the disc im unsure
I will measure them when im home....a friend 
orderd then in ebay for me.... so I will try and get the 
invoice from them...

Its my bike...forgive the stupid question but crank seal
popped out?? Doesn't sound good, can you explain if
I need to do anything with the crank seal?

Registration paperwork and titles are extremely unreliable for identifying model year so it doesn't matter if you have them or not.  I have at least 3 right now that were titled with the incorrect year.  Always check the VIN against the VIN tables.  Especially on a model like the 75-76 DT175 that had similar paint but some fairly major design changes. 

Measuring the disc thickness would be best.  And it's been a few years since I've been in one, but pretty sure 1976 pack starts with a friction and ends with a friction...  5 frictions and 4 steels. 

Pic of what RT325 noticed...  you might be able to gently coax the seal back into place...  best practice is to remove the gear and replace it with a new Yamaha part.  (not aftermarket)  If removing the gear, do not under any circumstances try to use a bar to hold against the outer clutch basket "tabs" to hold it while loosening the crank nut or you will almost certainly be looking for a new and possibly very hard to find clutch basket (marked with the 525 number).  It's a one-year only part I believe. 

 

Yep, look at the last pic as we see it & the crank seal behind the crank gear has moved out. Its probably JUST still sealing by the skin of its teeth as its come out on an angle. Better order a new one as it'll be a tighter fit in the case. Could just push this one back in if ya stuck & want to run it while waiting for new. Might be able to force it back in with a steel ruller on its flat behind the gear without removing gear.
Ahh Edit, Marks on the case while i'm typing haha. Thats good.

So if I understand correctly my bike is a 76..
so if I measured the friction plate and there 4mm
then I should get 3mm ones and hopefully jobs
Sorted... and the rubber that's out gentle push 
It in with a metal ruler and order a new one,?? 
is it a hard job to replace??

 
so this is the item I purchased its says 1974/1975
​​​​​so I brought it as I assumed it was 75... so do I take 
It that my bike is 1976?

All the answers I can give have been given as clearly as I can manage.  Maybe someone else can help better than I can?

Great model...  the best and most refined of the 175 twin shock in my opinion.  Hope she is up and running soon! 

If you go back and read the entire thread carefully, the experts here have confirmed that you have a 1976, so you probably need the thinner 3 mm friction plates, and you've got to seat that seal, or better, get a new one. If the bike has been running with the seal hanging out and askew like that, it's most likely damaged.

Do you have the original friction plates? Did you measure those?. If they measure around 3.6 to 3.8 mm or so, they were originally 4 mm, if they measure under 3 mm, they were originally 3 mm. By the way, measure the friction plates from the inside, because they form a rim on the outside as the steel plates wear them out.