Triumph Mystery Frame
What the Heck?

Triumph Mystery Frame a...mystery

At first glance, the Triumph Mystery Frame doesn't seem like much of a mystery at all. It's obvious! It's a 1971-or-later Triumph 650 or 750 Twin oil-bearing frame, right? Wrong! Oh, let us count the ways...
ABOVE: Triumph Mystery Frame, detail on the neck.

All Oil-in-Frame Triumphs have the oil filler cap behind the gas tank, under the nose of the seat. When originally designed, the geniuses at BSA's lavish tech center, Umberslage Hall, the plan was to fill the entire 3-inch diameter backbone with oil. But for some reason, this led to oil foaming, so they abandoned the entire top section of the backbone, moved the oil filler from in front of the tank to behind the tank, solving the foaming problem, but cutting the oil supply in half. Always a problem on OIF Triumph twins. So, is our Triumph Mystery Frame in fact some early factory prototype, before they figured out it didn't work? Or did some shade tree mechanic do what the bright minds of BSA couldn't do? Either way, everything about this frame doesn't look stock. Take a close look at that big plate-steel gusset reinforcing the neck. It was obviously cut out of flat plate, whereas the '79 Triumph frame, below, uses a stamped sheetmetal piece that is sculpted to fit. Also the top end engine mount is much farther forward on the Mystery Frame than on a stock Triumph. While some of the welds in the Mystery Frame look pretty good, it looks like they had to build it up to close gaps in places like the top of the steering head (above), where the welding bead is over a half-inch wide. A telltale clue that this may not be an exotic factory item.

BELOW: My 1979 Triumph T140D project bike, detail on the neck.
ABOVE: Triumph Mystery Frame - engine cradle

A fairly beefy bracket has been added to the right frame tube (left on the photo above) that takes a very odd angle. This would place it under the gearbox cover. What the heck is it for? Below you can see that there is no such appendage on a stock frame (my '79 T140D).

If you examine the engine cradle above, you will see no accommodation for a center stand. And it wasn't cut off because there are no scabs. Now look at the above frame tube in the lower-right of the photo. There should be a side-stand gusset there, but there isn't one. And it appears as though there never was.

BELOW: My 1979 T140D frame, detail on engine cradle.
ABOVE & BELOW: Triumph Mystery Frame, detail on oil sump.

Check out the way they plumbed the oil pickup at the bottom of the oil 'tank'. On a stock Triumph, it would come off of a cast cover plate on the bottom of the sump. Here it has been replaced by a heavy steel plate with only a drain plug in it. Our Triumph Mystery Frame gets its oil from both sides of the sump. Why did they do it this way?

Just when you think this is a one-off custom frame, you see that stamped-steel reservoir at the bottom of the oil tank tube, running side-to-side between the frame rails. This is stock Triumph Oil-in-Frame stuff, all the way. There was some extensive welding on the left frame rail that looks almost like they cut that section out of a stock Triumph Frame, then used it as just part of our Mystery Frame.
NOTE: This is perhaps a better angle to see that there have never been any centerstand or sidestand brackets on this frame.
ABOVE: Oil sump on our Mystery Frame.

BELOW: Oil sump on my '79 T140D project bike. As you can see, this is a single 3/8" spigot. The Mystery Frame has two outlets and they're smaller in diameter, likely 5/16".
ABOVE & BELOW: Ever seen an OIF Triumph with notches in the seat post like this? There's another on just like it on the other side. Obviously, they're there to clear the air cleaners on a post-1978 twin-carb Bonneville head. By this time, the splayed carbs that make Bonnevilles look so sexy were gone in favor of the crappy Amal Mark IIs which were set close together and parallel, for some reason that had to do with emissions. But, I've never seen a stock frame with these notches in it. I have a 1979 T140D Bonneville sitting in my shop right now, along with my 1973 Bonneville
Yellow Bike custom cafe racer, and neither has these notches in it. Did they come later? You can see a large welded seam where the vertical seat post meets the backbone. Perhaps the seat post-portion along with the sump, and a small chunk of the left frame tube were grafted from a stock frame. Have any of you ever seen an oil-bearing frame with notches like this?

What the hell is that big ugly round patch job in the middle of the frame? If you examine the Above & Below photos carefully, what it looks like is that they used a stock Triumph OIF seat post and sump, then grafted in a new backbone, which is capped at the rear (below), and obviously cut open inside to join the two internal volumes that hold the oil.
ABOVE: Mystery Frame, detail under seat nose.

This isn't even close. Triumph used a heavy-gauge of steel plate, and made it fairly small so as not to interfere with under-seat access. Our Mystery Frame Fabricators chose a much thinner, but larger piece of sheet metal to do the same job. Note the slide-mount on the Mystery Frame above for a typical aftermarket race seat back in the day. This is beginning to look more like a custom race frame every minute. But what kind of racing? And how much of it is stock?
ABOVE: Mystery Frame, detail around swing arm pivot.

Theres' lotsa stuff going on here. A heavy bolt-through gusset on the left engine cradle, some very nice welds on a well-cut piece of plate steel for the engine brackets/swing arm pivot bolt. But what is that bracket behind it? The one with the two 'wings' around the bolt hole, is it for a kick stand?

The numbers that are stamped into the frame don't make any sense. The neck is stamped with D056786 which does not correspond to any OIF Triumph twin (1971-or-later).

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