Classic & Vintage - rear rack braze-on bolt size on 82 univega

I have an 82 univega viva touring on which I am attempting to install a rear rack, and I'm trying to locate the proper bolt size. they would appear to be 6mm diameter but 6mm-1.0 bolts only turn a little and then wedge, so it's probably a different TPI. (is there such a thing as an M6-0.8?) either that, or the pitch is wrong. 5mm is most certainly wrong as they drop right through.

based on another univega which has braze-ons which are untapped, I would guess that these bikes didn't come pre-tapped and that it was left to the shop or customer to do that part. (in which case they might not even be metric) so I'm not sure if there's any standard, but I'm hoping someone might know a more common size to try next.

I'd tap the hole, if it's full of paint and crude, I can never get a bolt to fit. My local hardware stores have metric taps for ~$5 each.

Normally would be 5mm x .8

FWIW...My wife's Nuovo Sport mixte was all tapped...all 5mm x.8 as well.

-Mark in St. Louis

I have an 82 univega viva touring on which I am attempting to install a rear rack, and I'm trying to locate the proper bolt size. they would appear to be 6mm diameter but 6mm-1.0 bolts only turn a little and then wedge, so it's probably a different TPI. (is there such a thing as an M6-0.8?) either that, or the pitch is wrong. 5mm is most certainly wrong as they drop right through.

based on another univega which has braze-ons which are untapped, I would guess that these bikes didn't come pre-tapped and that it was left to the shop or customer to do that part. (in which case they might not even be metric) so I'm not sure if there's any standard, but I'm hoping someone might know a more common size to try next.

It may have originally been intended to be a bolt with a nut on the other side, and not to be tapped at all. In which case a 5mm might actually be the right size.

turns out the holes had been tapped to M6-1.0mm, just not perfectly. a little grease and the proper tool and the bolts went in without too much effort. I didn't want to force a bolt in on incorrect threads, but after noticing that the same M6-1.0 bolt went in much better in a different hole I was less worried.

I'm guessing the previous owner tapped it at 6mm instead of the more common 5mm with the idea of allowing the use of stronger bolts.


It may have originally been intended to be a bolt with a nut on the other side, and not to be tapped at all. In which case a 5mm might actually be the right size.
that would not work on these frames, as even the smallest nut I could imagine would get in the way of the chain at the hub. plus, on the other univega with the untapped braze-ons, a 5mm bolt won't even fit through the holes, which is why I believe these frames came untapped but were intended to be tapped by the shop to whatever the customer wanted installed. If a nut on the inside end would have worked, I could have certainly used a 5mm on the 6mm tapped holes, assuming I didn't care about saving the threading.

my next hurdle is attaching the rack (a Topeak Explorer) to the seat stays. the mounting bracket arms are far too long, but I believe I can use a drilled metal strip creatively bent around that will work. I just need to make another trip to the hardware store.