Rode my Cimatti grand prix xl around today for about an hour, ripping it at 50mph +, all nice and reliable like usual, and at the stop 30' from my house, it goes KLANK.
The bolt holding the stator loosened up, wedged itself into the hole in the magnet, and bent the crankshaft.
Cranks bent just slightly, imperceptible to the human eye, but after switching stators and mags, the new mag is off balance and rubs and makes ugly grindy noises.
Swapped it out already with a spare crank and it's all good.
I doubt the material of the screw has any significance. These holes have been worked for decades and a little split washer just wasn’t up to the task of holding the torque on.
> It looks like those points were ready to be replaced anyway . ;)
>
> Maybe the old crank was very slightly off just enough to work the bolt
> out ?
I dunno. It was the original factory bolt that holds the source coil to the stator. I've never touched it since I've owned the bike, and doubt anyone else ever had, either.
I've been running my cimatti full tilt since I got it about two years ago, after I rebuilt the lower end with new seals, bearings, and a polini kit, and the only minor problems have been a condenser and burnt plug from running a too cold heat range.
It's the stock, factory bolt that holds the coils on. It somehow managed to work itself loose after 44 years.
Never heard of anyone replacing them with titanium. And if it was titanium, it probably wouldn't have mattered. It wasn't a heat issue, it was just a freak occurrence.
Yeah I am pretty crazy about loctite on those lil buggers.
I don't know what the torque should be on a m4 flathead screw, or how you would set it if you knew... If you do them by hand you are almost certainly over the 'book' spec torque which is likely a few in-oz.
I used to design small stuff like this that did need a torque spec, we used inconel thread inserts that were self locking with beryllium-copper 6-32 screws. A failure was a 6-7 figure loss for the company so I had to develop a really picky process for installing them. Titanium was not an option, it is lightweight but not any stronger per size, the limiting factor is the threads into the aluminum and it risks galling/corrosion. The super tiny torque wrench we used was a couple Grand and I'm pretty sure nobody reading this owns one .. so yeah, loctite and send it!
I hate to be a nerd (just kidding I love to be a nerd) so I had to set this up in the shop.
I happen to have a spare V1 stator sitting on the bench, of all things. I got out my trusty snap-on torque-o-meter that reads from 0 to 10 ft lb and is actually calibrated to a standard at these types of torques. Having worked with these torque wrenches quite a bit I would not trust this one below two foot pounds, but I can just about guarantee you it's the most accurate torque wrench you can buy for under a grand. This one was actually thrown away at my old work, but it had previously been used to calibrate dynamometers to an NIST traceable standard, AKA it's pretty fucking accurate.
The way you would design a torque spec for something like this, would involve destructive testing where you would find the max torque then you would calculate the needed clamp load in the joint and depending on where the numbers line up, put a 15 to 20% safety Factor on the minimum torque needed to clamp the joint, not to exceed the maximum torque. This would require a bunch of holes drilled in an aluminum plate and breaking a bunch of screws which I couldn't do, so another good way to do it is to measure breakaway torque, which is the maximum torque recorded by the pointer before the screw begins to unscrew.
I had two screws on the stator that had never been loosened from the factory, so I checked them first by unscrewing them that's the first column. Then I used a handheld jis screwdriver to apply what I thought was a reasonable torque for some average schmuck in his garage. Finally I used the spec from Adams table of 16 in lb.
So, what this shows is that the best retention based on breakaway, is actually from the stock installed screws which may have had some sort of locking compound or may just be corroded in place after 40 years. The second best was hand tightened with a screwdriver, my gas is that you are putting almost 20-in lb in to the screwdriver just by hand if you do it the way that a reasonably skilled mechanic would do it, feeling for the resistance of the screw. Using the book torque, was way low as I expected. I can't say for certain, but given the amount of turning available after twerking it to book torque, it certainly feels like it would rattle out pretty quickly.
Tl; Dr: blue loctite and give er hell with the regular screwdriver. (edited)
Thanks for doing the nerdy part and figuring it out for us! I’ll have to show my grandfather in law who used to be a millwright before he retired. He will most likely love the real testing behind your work here!
Betcha can't figure out how much force it takes to jam-stop a magneto at, let's say, 5000 rpm, and how much destructive force is required to bend a crankshaft at the end of the taper.
> the torque wrench i use is the same one that snap-on sells, except
> without the snap-on logo.
> it’s CDI branded
I have a 1/4 cdi brand and I love it, got it on eBay cheap goes to 160 in/lbs or around 13 ft lbs I use it on all my heads and other little guys that don’t need to be super but I want “right by the book” because I love to be anal about it
