tjc4golf

Just got a used bike on craigslist with a Cannondale aluminum frame. One of the bottle cage rivnuts on the downtube was missing (just a hole in the frame where it used to be) and the other was messed up.

A couple of questions regarding the repair...

(1) Which kind of metal? I see rivnuts in aluminum, stainless, zinc plated carbon steel. Which is best to use on an aluminum frame?

(2) Would M6 rivnuts create any issues? I realize M5 is standard for water bottles. But the hole in the frame for the rivnut that had been removed prior to my ownership is 10mm diameter (IDK how this happened). The hole in the frame for the rivnut I removed is 8mm. I've looked into the size of rivnuts - sizes shown in chart below appear pretty standard - and believe an M5 would fall through the 10mm hole. So I'm thinking of doing one M5 and one M6 or making the 8mm hole slightly wider (8.9mm) and doing 2x M6.

dsbrantjr

If it were my bike I would go with 2x M6 in stainless. Better corrosion resistance than zinc plated, stronger and less likely to strip than aluminum. Both the same size works better for my OCD than one of each.
Edit: An alternative would be to insert 2x M5 Rivnuts slightly offset from the current locations and either cover or ignore the existing holes.

Miele Man

Using aluminium Rivnuts would remove any chance of dissimilar metal corrosion issues. I too would go with a larger Rivnut than the original M5 size.

Cheers

tjc4golf

Good call on the aluminum. Looked into that a little further - definitely don't want galvanic corrosion from a steel / aluminum combo.

Glad to hear it doesn't seem the M6 bolts will be an issue. I don't have any handy to test. Thought they might be too big for some bottle cages but I'm probably overthinking it.

Miele Man

I don't think you'll have too much trouble using a pair of M6 bolts to mount your bottle cage. then again, that'll depend on what your bottle cage is made from.

Good luck and cheers

dsbrantjr

Aluminum and stainless (technically "corrosion resistant") play well together, as a matter of fact stainless shims are often used as a buffer between dissimilar metals to prevent electrolytic corrosion. Unless you use aluminum screws, which I strongly discourage, you will have aluminum vs steel (hopefully you will choose stainless) anyway. Stainless Helicoils are routinely used in aluminum parts in military, aircraft and aerospace products.

Novalite

Galvanic corrosion is typical on bikes. just look around the valve hole in a rim, it hangs full of white powder - dissolved aluminium, by the stainless from the valve.

tjc4golf

Quick update. I successfully completed the rivnut repair using two M6 aluminum rivnuts.

Metallurgy: I ordered aluminum before seeing dsbrantjr's reply that suggested stainless would be fine. But I suppose aluminum rivnuts will not be a problem so long as I use stainless bolts.

Sizing: I bought an assortment of rivnuts on eBay for $8 thinking I might mix M5 and M6 but the M5 rivnut looked a little small for the smaller hole so I ended up using two M6 rivnuts. The small hole had to be widenend at two spots (it wasn't perfectly round) using a dremel.

Installation: I didn't use a rivnut tool. I had an M6 bolt and a larger nut lying around and was able to use this technique to install the rivnuts. Installation was easy and only took 5 minutes or so.

Summary: easy job. Thanks for all the help. I was probably over-thinking things since I've never done this before. If you haven't done it before then don't worry - it's super easy.

Finished pic below. Note: stainless steel bolts are in the mail - these are just temporary for a few days.

Reynolds 531 

Presta valve stems are very very very rarely stainless. They are brass, or if silver colored, are nickel plated brass. Hence the corrosion.