Modern Vespa

Hi folks, quick question, just need to know whether a cosa pinched nut (the flanged type) should feel tight when threading onto the crank. The crank is new as is the nut but when I tried to thread it on it stopped a couple of threads in. Doesn't feel right. Is this normal or are the threads f**cked from new?

Yes it should go in tight. I use them on all Cosa II clutches...

I think there's a proper name for them but I don't know what it is... I believe the threads are slightly off standard - on purpose - so that when you put it on, the threads deform slightly and hold tight. They are meant to be single-use... I do re-use them but of course they are never quite the sane afetr the first time.

Welcome to MV!

Thanks fellas, I didn't like how the nut went tight and felt like the threads were damaged. So, there is a ''distortion'' in the threads or the body of the nut itself to ensure a tight fit and less likely to shake loose? The crank ins't installed yet so if I pull the crank in using this nut with a spacer/washer combo the nut wont be as good as it was? Maybe I should just warm the casing and throw the crank in the freezer then tap it in with a mallet?

It shouldn't be that tight, maybe it is damaged? Can you get hold of a regular nut, or an old one to pull the crank in? It's by far the preferred way of doing it to avoid damage.

Sorry I was late getting back to you. Yeah, I'm at a loss with this friggin nut situation. I'm not familiar with cosa cluthes or this nut and don't want to start tightening things up if it feels like the threads are fucked but I don't think they are. While not a fool proof method I ran a thread repair tool over the crank threads to see if they were ok as well as using a magnifier to get a good look at them and they're fine.By chance I found new cosa clutch nuts in a box (don't ask me why I have them because I don't know either lol) and they wont screw on all the way by hand either. They stop a couple of threads in. It looks like this is just the way these nuts fit. Maybe I'll order some m12 standard nuts from amazon and see if those fit better and I can use that to pull the crank in. It's definitely a new one as far as I'm concerned. Never had a nut NOT screw all the way down the threads easily by hand. You live and learn.

Yeah the standard nut test should tell you. Maybe order a standard bolt at the same time to check your clutch nuts? Not sure what thread pitch is on the crank, just check that first to order the right nuts.

The bottom of a flanged “pinched” lock nut for the Cosa II clutch should look normal like any nut, but the top should have a slightly distorted look, as if it’s been squeezed in a vice.

It should also go on the same way... it should start threading on easily at first, but quickly require a torque wrench, which is entirely the effect intended. It avoids the need for a lock washer.