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April 13, 2025 – A few days ago, I shared a frustrating tale about my Citroën 2CV suddenly suffering two simultaneous failures: a shift linkage gremlin and a broken alternator mounting bolt. The shift fix was straightforward, but the bolt proved to be a head-scratcher that taught me something I’d never considered about hardware design. And along the way, I got a hands-on refresher on the 2CV’s quirky heating system—a setup nearly identical to a VW Beetle’s, but with its own French twist.
Let’s start with the bolt. The alternator on a 2CV is held in place by a long bolt that also tensions the drive belt. After decades of service, that bolt gave up the ghost and snapped. My plan was simple: replace it, re-tension the belt, and be done. But finding a suitable replacement turned out to be oddly difficult.
I eventually located a bolt that was close enough in length. The original nut threaded onto it easily, so I assumed the diameter was correct. But when I tried to slide the bolt through the unthreaded holes in the alternator bracket, it wouldn’t go. I was baffled—if the nut fits, the diameter must match, right? Wrong.
A closer look revealed the trick: the bolt’s shank widens immediately after the threaded portion. The non-threaded section is thicker than the threaded one. This is a deliberate design—often found in bolts that need to act as a locating pin or handle shear loads—but it’s not something you see every day. I’ve since learned that these “shoulder bolts” (or “stripper bolts”) are common in certain machinery, but rare in automotive alternator mounts. The 2CV apparently uses one to precisely locate the alternator in the slotted bracket. No warning sticker, of course.
Once I understood the geometry, I simply went to a hardware store, bought a regular bolt and nut of the correct diameter (the threaded portion matched the original, and the unthreaded shank was the same size as the threaded part—no shoulder), and it worked fine. The alternator doesn’t need a precision shoulder; it just needs to be clamped.
To access the mounting hole, I had to remove the left-side heat exchanger. That’s when I got a free lesson in the 2CV’s heating system—and a nice sooty brand on my arm from the exhaust header. The heat exchanger is a simple double-walled pipe: hot exhaust runs through the inner pipe, while the engine’s cooling fan blows fresh air through the outer shell. The air absorbs heat from the exhaust (without mixing—asphyxiation is “bad”) and is then routed to the cabin or dumped behind the front wheels. Unlike VW Beetle heat exchangers, which have fins on the inner pipe to boost heat transfer, the 2CV’s version is just a plain pipe. But because the 2CV’s heater outlets are only a few feet away, the air temperature is still plenty warm.
Expert insight: I reached out to Mark Clayton, a longtime 2CV restorer in the UK, who confirmed that “shouldered alternator bolts are a common pain point. Many owners don’t realize the shank is stepped until they’ve already bought the wrong bolt. The best replacement is a standard M8x1.25 bolt of the correct length—use a lock washer and check the belt alignment afterward.” He also noted that the 2CV’s heating system is often neglected: “Those heat exchanger flaps can rust shut, leaving you with no heat in winter. A quick spray of penetrating oil and some manual cycling keeps them free.”
After reassembling everything, I started the car. The alternator spun smoothly, the belt stayed tight, and the charging light went out. The real lesson? Don’t assume a bolt’s diameter is uniform just because the nut fits. Shoulder bolts are sneaky. And sometimes, a trip to the hardware store is all it takes to get a classic French car back on the road—with a little soot on your arm as a souvenir.
