Looking on CMSNL, I reckon that the "-X0" part suffix is incorrect - it should be "-90" (see links). "-X0" doesn't have the lights switch.

1T98292090: Lever Holder Assy (r.h) Yamaha - buy the 1T9-82920-90 at CMSNL

1T982920X0: Lever Holder Assy Yamaha - buy the 1T9-82920-X0 at CMSNL

CMSNL doesn't list any fitments for the "-90", which is probably correct, as most of their parts lists for bikes of that period are for North American models and the "-90" is likely for UK/European models.

 

Nice catch. USA lists have a completely different part number. It looks to me like in 1978 Yamaha removed the lights "on-off" switch from the DT100 model. (I think 1978 was the first year of the Federally-required "always on" headlight law for newly manufactured motorcycles sold in the USA)

"infamous sealed beam unit... It looks brand new. That headlamp shell is exactly the same as the DT175mx unit.  I may fit the conventional DT175mx reflector and bulb in its place."

TK

Yes, probably not original to the bike - not sure that UK bikes came with sealed beam units.

I've been operating under the impression that with a "1T9" prefix, it's likely an import from the USA?

Years ago a member here posted a UK VIN list that he had put a lot of time and effort into researching...  that list shows the UK variant of the DT100 having the "1N1" prefix.  I'm not sure of the accuracy of the information but it my understanding he got his information from Yamaha UK and other credible sources/experts. 

yamaha-enduros.com/index.php/uk-vin-information

(The Yamaha model code list shows "1N1" as a DT100 model specifically for "ENGLAND" which concurs with the above)

 

To be clear.  The bike is a US import.

Found it.  CMSNL lists fiches for US bikes.  It did not list the 1977.  However actually entering "DT100 1977" a new fiche layer opens - DT100 US and Canada.

 The pic of the switch below is what I have seen before on CMSNL and threw me off the scent.



There is another entry without a pic earlier on.  Just a different part number (1T9W829230). It says (US) in the notes.  It also says fits only the one model. 

Went back to look closer at the other entry without the light switch ( 1T98291001) and note says Canada.  Because this had the pic, I completely failed to see the US variant.

So the switch in question was only ever fitted to the 1977 US variant.

Thanks for helping me track it down guys.

OK, thanks TK - my bad (again!  ).

The missus being at work, I had an hour or two to do a bit more on the bike.

First thing I did was pull the sealed beam unit. 
Since I decided to fit a UK standard reflector and bulb (exactly the same as my DT175MX unit), I thought I would box the sealed beam unit and keep it safe for someone who needed one as it looked pretty new.  I gather they are getting quite rare. 
Even still had a little green dot paper sticker on the glass like a quality control sticker.
Alas a quick resistance test across the terminals revealed that both filaments were blown.  Squinting through the glass confirmed it.  Damn.

Three hours later and we are down to a bare rolling chassis with a far better understanding of what state the bike is actually in.

And it has its problems, but overall I am pretty happy with it.  I have no inclination to make this a concours factory fresh bike and it suits this purpose well.

So tank removed and about to start pulling off all the electrical bits first.  It is readily apparent that someone has been "into" the loom.  Further investigation required.


Bike is complete, even the original flasher can is present and correct.  Nothing seized and no bolts snapped.


First real horror.  Original air filter by the look and with a broken cage.  Luckily I seem to remember genuine Yamaha RT100 filters identical to this came with a new cage.


The thing I am noticing is that very few of the fittings have lost their plating and gone rusty.  A quick rub to get the thin film of dust / mud off and the plating is certainly good enough to refit.  The patina in this condition adds to the bike, not detracts from it. This is almost unheard of on a unmolested 46 year old UK bike.


Again, condition is surprisingly good.  I have had two year old bikes used daily through the UK winter, with our salt ridden roads in worse condition.


Horn, once again in impressive condition.  I tried it across the battery to see if it worked.  It did not.  Boo.  What a swizz.  
It was clear the points had fused together inside, just a click as the solenoid traveled back.  I screwed the tone screw right in, gave it a good clattering on the bench, then took the tone screw right out and gave it a good clattering again in the hope that I broke the bond between the contacts.  Screwed the tone screw in about halfway and tried.  Nothing.  The screw was preventing the solenoid pulling back the diaphragm.
Screwed it out a full turn and PEEEP.  We have a working horn. Yaay!  It would have been disappointing to have had to fit a new one with no patina.


Around three hours later we have a bare rolling chassis and a good idea as to what it needs.  Not much apparently.  Of course the general safety stuff I have on order already, cables, wheelbearings, brake shoes ect.  Then we have the oil pump to rebuild, the exhaust to burn out, the chassis to bead blast and repaint.  I measured the frame and it will fit in the blast cabinet at work with ease, so no need for daft stuff like having to blast it with door ajar.  The door will close easily and the cabinet will seal and develop negative pressure inside.  I can breathe easy with no need for a mask.


Lid lifted off and we are on standard bore size.  


And a quick run around with scotchbrite to clear the carbon ring away round the top of the bore to assess condition.  Bore is in excellent condition with absolutely no wear ridge at the top, the original hone pattern is still faintly visible, and the few scores it has cannot be felt with a fingernail.  A minute or two with an expanding hone will see those gone and a good hone pattern restored.  I intend to strip it of its hardware, take it to work, give it a beadblast to ready it for paint, hone and a good clean out.  I will not even bother to put my bore gauge down it.  New rings and gaskets later and Robert is your father's brother. A nice fresh top end.






 

A place near Basingstoke sells new fork tubes for TY175, and they are far cheaper than re-chroming. Might be the same as DT ones?

Found a set of genuine std Yam rings on fleabay.  They are surprisingly thin on the ground and the vendor priced accordingly.  I made a more sensible offer and new rings are on the way.
 

Had another couple of hours this morning.  Frame is now bare and ready to take in to work next week for beadblasting.
Nothing really surprising to report. 
It is what it is, an inexpensive 46 year old bike that had a brief but tough life, probably involving kids riding and maintaining, before being parked in someone's garage for years.

First we remove the rear wheel.  The rear sprocket carrier axle nut was loose and had been for a very long time, judging by the chain puller.
We also appear to have had new wheelbearings at some point.  Unfortunately tin shielded Z ones.  Blaaargh!




Brake shoes looked OK although badly worn at first glance, but a curious, light probe with a flat blade screwdriver in the bonding line produced delamination with very little pressure applied. 
This is why you should always change old shoes that have been stood many years, even if they appear fine.
The tyre is cracked and is the original IRC Trials tyre that was on the bike from the dealer showroom.  Front is also original, but also unserviceable.


And we come to the front end.  The forks are badly bent as is really apparent in this pic, but I can straighten them.  Again, front tyre is original fitment.
Speedo worm drive was found to be seized up, so will need attending to.


And as expected, the steering head bearings are totally shot.  The locknut had been tightened down very tight to take the slop out, probably by the seller.   Those silver flecks actually are metal flakes from the spalling bearings and tracks..