CO_Hoya 

I picked up a new project last weekend (from the early 90's), and one of the first things I've noticed is that the water bottle bosses don't match.

Bosses on down tube:


Bosses on seat tube:


The threading is the same, but the fasteners for the flush-mount were also fancy to allow the heads to be flush with the seat tube:


Anyone seen this before?

All I can figure is that the expectation that many riders only carry one bottle led to the design choice. I'm also curious if the seat tube bosses would be weaker since they don't have a shoulder.

brockd15 

Never seen it before, but to me it looks home-made, like the previous owner just drilled and tapped holes.

CO_Hoya 

Definitely not. I can see the boss inside the seat tube when I look down it (with a flashlight in the BB shell).

Chombi

The seat tube bosses were home made by the PO. You cannot just tap the drilled holes as the tubing is too thin walled to accept any substantial amount of threads to hold the water bottle cage bolts securely. There has to be some special threaded inserts in there (I think they call them "Nutserts") that mates up with those bolts.

CO_Hoya 

But something like a nutsert or rivnut would be visible inside of the painted hole (paint is original and in excellent condition), and/or would need some sort of shoulder for structural integrity. As you say, the tubing itself is too thin to be tapped.

Torchy McFlux

Yes, I've seen this on production bikes in the late 80's/early 90's. It's a cosmetic thing, and the reasoning was that the secondary bottle bosses on the seattube were rarely used by many riders (at the time) so they used flush bosses to make them as unnoticeable as possible... or something. Anyway, it was a subtle trend for some manufacturers.

CO_Hoya 

That was my guess. Any reason to be concerned about the strength of those bosses?

Torchy McFlux

Nope. They're brazed on just like regular ones, it's just that the boss is mostly on the inside rather than the outside. If you bolt a cage to it, you're more likely to scratch the paint around it noticeably.

Chombi

I doubt if any bike manufacturer would handle their waterbottle bosses different from down tube to seat tube. Just does not make sense to do so cost-wise, and in terms of people wanting it not to be noticable, that does not make sense too, ad pretty much everyone would want to cover those holes with water bottle cage bolts, to keep moisture from going into the frame and also to protect the treads from corrosion damage.
If it seems like the paint covers any seams from threaded inserts inside the seat tube, the bike might have been repainted at one time and the re-finisher/frame builder(?) might have actually installed the second water bottle mount on the seat tube for the owner the same time it was refinished.....
There are many different types/styles of nutsert. I think I've seen some that do not have an outer flange visible on the surface for use in applications where you need a really flush surface to bolt you item to. That's what might have been used on your frame. Frankly, I would have used a shouldered nutsert just to make sure the bottle cage has something flat to bolt to so it's more stable.

CO_Hoya 

I'd probably scrounge some standoffs if I put a second cage on.

Thanks!

Torchy McFlux

If you really need to use those bosses, I'd just use one of those old bottle cages with a resin base:

miamijim

My thought was they are low profile to prevent interference with a pump...

jiangshi

Exactly.

Chombi

Uhmmm..... Per my experience, pump bodies usually do not contact the seat tube. They are usually a couple or of mm's apart from each other when the pump is mounted on the frame, Just the fact that the seat lug and the BB shell will push out the pump ends forward on the bike frame, you cannot expect the pump body to be contacting the seat tube, so I don't think a totally flush boss like those is not really needed.

JohnDThompson 

More likely, brazed on standard bosses, then filed off the flange to make it flush with the tube.

CO_Hoya 

I would agree with you, but for the fact that I'm fairly certain (or as certain as one can be in the C&V world) that this frame is as-delivered from the factory. I think Torchy McFlux nailed it, even though it makes little sense to me (or you).

Here are the flush holes with their original fasteners installed - should be well-sealed from moisture:


This would make a lot of sense, except for the fact that there's a pump peg on the head tube (that brown smug on the upper lug is dirt, not rust):



Thanks for the responses.

Chombi

I guess it's just a case of "different strokes for different folks" with the builder of that frame. Certainly not the "normal"/"usual" way to handle seat tube water bottle cage mounting bosses,........ but there it is!

buldogge

My (ex) '89 Pinarello Treviso had this exact set-up…I ran small hard rubber washers between the bottle cage and the frame…FWIW.

-Mark in St. Louis

RobbieTunes

I use hard rubber washers between all my frames and bottle cages.

As for the OP's bosses, he should finish the bike, market it to triathletes as having "aero" cage bosses, and double the price....

CO_Hoya 

Actually, hoping to make this one a keeper. We shall see . . .