I am reassembling my 1970 DT1 after many years apart.  I’m questioning which way the head gasket goes on, the thin area to the front and the thicker to the rear? And does it matter which side is up or down?  

The parts drawings are no help.  On my DT1CM I am going to install it this way.  There is a large gasket area to the front of the head that looks to correspond to the larger area on the head gasket.  My original (who knows after 52 years) was also a sandwiched style gasket, but looked symmetrical front to back and side to side.

 

By the way, nice paint work on your tins!  I guess that is what is shown in your avatar.

The head gasket will only align with the stud holes and the bore one way .
Your second photo appears to show the incorrect way .
The ridge on the gasket faces upwards .

Thick [wide] area to the front [exhaust side]. Can't remember which way up now after so many years but doubt it matters once all compressed up. In fact re-thinking, i'd put the flat side up so the head groove can bite on a flat surface to form a raised piece into the groove it has just out a little from combustion chamber. Can't find a pic of an inverted head easily.

Oh, haha, that ruins my theory while i'm typing Swoop. But gasket lines up flipped end for end & [but not crossways of course] but if you were half asleep fitting it the head could go on backwards & sit square over the cylinder--Just saying--or starting an argument lol. Happy to be proved wrong--no i'm not. Always thought the wider bit goes forward due to more heat that side being the exhaust side--theory & thik they have a E on that bit if its genuine.

Thanks all for the help!  Agree with you all that the the large part of the gasket goes to the front.  JayB, thanks for the compliment on the paint on the tins.  Wish I could take credit for it, but I only furnished the cash to get them done.  Marbles Motors in Texas gets the credit for the paint work. Beautiful stuff!

You mean thanks for the confusion haha. Took some pic just now of a couple i have hanging on the wall. Looks like i've used one which i guess is aftermarket & have the rolled over bit downwards as can see the ridige sticking up it's made from compressing into the head groove. But wait there's more--& think i've proved that to be wrong--because--the new original has the "E" stamped on the rolled side [as Swoop says] & i think Yamaha in their wisdom stamp things on the upside {rings & things]. Bottom line its not going to hurt anything if you did it wrong way up & its all together [in my opinion]. Good old DT's can handle it. Go faster if ya peeled the gasket apart & just fitted one thin layer--like i'd do that--or probably have.
Here's some pic i just took but forgot to insert so hit them to bring up bigger. Aftermarket looks poorly made & slightly offset to the bore from how i've had it compressed. Or holes are oversize & i wasn't careful fitting it & making sure it was as central as pos.

yesterday I decided to pull the head on my DT1E to set the timing, which was not as advanced as it should be.

Anyhow, the head gasket had an E and a small arrow stamped onto the wide part, telling me this larger gasket area should face the exhaust.  So after annealing it that's what I did.  Sorry I didn't snap a pic.  I think I gasket from a local S. CA gent who has a cache of Yamaha parts, he's a terrific person to deal with BTW.  vintagedirtandtrail.com/

Umm--i'd Not be annealing an original Yamy DT1 gasket which has asbestos or something trapped in the middle. If it's a solid copper one then 'go for it'. Just thinking the annealing process might ruin the center packing. You can see the packing just showing in the bolt holes in one or two of my pics. If you heated it cherry red then quenched it--hmm, i dunno, can't be good for the sandwich filling.

The thick wide area faces the exhaust exit

a