I installed a battery box from a '73 RT3 in my '71 RT1 to be able to install an AGM Battery (Motobatt MBT6N4). Filed 2 small notches in the sides so that the battery wires would clear. Fit right in with no other mods.

Tried to put the RT3 airbox and carb boot in as well so it would get better airflow and be able to use filter #308-14451-09 which is still available from Yamaha. The seat interferes with the top of the airbox by about 1/8". I plan to get another seat pan and modify it before installing a new foam and cover.

Just wanted to provide some feed back on the MotoBatt 6 Volt MBT6N4 which I have used with less than satisfying results in my 1968 DT1-250. I have used two of these batteries with the exact same results. Both terminated their lives at 12 months and 1 week, just out of the warranty period. No symptoms of battery failure prior to one cell shorting out and leaving both batteries with only 4.2 volt capacity. I did contact MotoBatt and their response was that there must be something wrong with my charging system. Yes the charging system on these older bikes is very dirty with spikes, as we all know that they are prone to blow lights. The MBT6N4 battery cost is $35 plus shipping and last only a year. I do not recommend Motobatt MBT6N4 for this installation. I have gone to a Sealed Lead Acid (SLA) Battery PS-640 @ 4.5amps 6V designed for Game Feeders or Kids Toys with a price range of $8.49 to $12.49. I have laid it on it's side to clear the seat pan as it is 3.94 inches tall and hits the seat pan. Note: I built my own battery box to fit the Motobatt battery which is much larger than either the standard battery or the SLA Battery. Even if the SLA battery only last one year the cost is three times less. I have purchase two of the SLA batteries two have one as a spare as they do have great shelf life.

Interesting. I wonder if the charging system is that much different on the '68. I have that battery in 2 of my '71s and 5 of my '73s. They all are over 4 years old. All of them still hold a charge. Also have voltage regulators in all of them.

I have a voltage regulator but just on the light circuit. The charging circuit is separate on the 68 DT1-250 and only goes through a diode.

Have you checked your diode . If it is faulty , and AC is getting back to the battery ,
that won't be doing the battery any good .

No did not think it was an issue. But that is an excellent idea and I will do it just to ensure the diode is working correctly.

Rebuilding a '68 DT1 and searched the site for battery suggestions and found this thread. Appreciate the comments. The '68 battery box (if one can call it that  ) is smaller than the one shown on the post. As I am trying to "restore" the battery needed to fit that small area. I was able to find one on (yup!) eBay by using the "PS640 SLA" term. 

Found a 5 amp hour AGM and have ordered that. Will report back with findings once I get the bike going.

Thanks, and regards. 

I had several motobat batteries fail and horrible customer service experiences. I make a point to share that experience to save others the heartache.  Yuasa old school for me  every time.

I also just installed Yuasa in a 68 dt1.
Good luck!!

What Yuasa # did you use? Was it the old style 'wet' lead/acid, or sealed?

I did get the SLA I one ordered off eBay, and it does clear the retaining strap, just.