''Pinched'' nut
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LML 150
Joined: 10 May 2019
Posts: 3
Location: UK
Fri May 10, 2019 2:47 am quote
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?
Molto Verboso
1979 P150X, 1983 P200E, 1987 T5, 1996 PX200E, 2011 Yamaha Fazer 600 S2
Joined: 02 Aug 2015
Posts: 1384
Location: Veria, Greece
Fri May 10, 2019 3:12 am quote
Yes it should go in tight. I use them on all Cosa II clutches...
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 6783
Location: Victoria, Australia
Fri May 10, 2019 3:19 am quote
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.

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LML 150
Joined: 10 May 2019
Posts: 3
Location: UK
Fri May 10, 2019 4:13 am quote
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?
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 6783
Location: Victoria, Australia
Fri May 10, 2019 3:39 pm quote
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.
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LML 150
Joined: 10 May 2019
Posts: 3
Location: UK
Fri May 10, 2019 4:59 pm quote
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.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 6783
Location: Victoria, Australia
Sat May 11, 2019 4:29 pm quote
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.
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1771

Sun May 12, 2019 11:37 am quote
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.
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