Shimano crank preload caps – alloy vs. plastic

My Deore cranks (and the 105 cranks on my gravel bike for that matter) came with the plastic preload bolt. I swapped it out for an aluminium one after my crank worked loose on a 3-day trip (thankfully towards the end of the last day) and I didn’t have the special tool with me and I mangled it improvising to get me home. I know what you’re thinking – they make them out of plastic so you can’t over-torque them, but the only reasons I swapped it were so I would always have a tool with me (alloy ones have a hex fitting) and it wouldn’t get mangled from removing/refitting the cranks.

I went to change my chainrings this week and the alloy cap was completely stuck in. Thought it must just be a bit gunged up. I rounded the hex fitting, which isn’t very deep. A couple of hours of hammering, chiselling, hacksawing, and swearing later, I got it, or what was left of it out. Even opening up the crank by putting the back of an old brake pad in the gap and pushing it apart with the pinch bolts didn’t make any difference. The thread was damaged at (i.e. it wasn’t cross-threaded) and it didn’t seem particularly corroded.

So, question(s) is (are), what happened to it? How? Should I get another alloy one?

I’ve used an alloy one for a few years with no problems, fortunately! I grease the threads and only really just nip it up.

My only thought is that you overtightened it, maybe?

It does sound like a painful situation!

It’s only an issue if you overtorque the pinch bolts, or strip the plastic thread. We’ve had this argument before, I’m not doing it again, but I always use alloy caps.

Up to you.

The plastic cap/bolt is there to provide the correct play in the bearings. There is the little safety lever to provide a location aid. If torqued up to the required setting then I’ve never known a crank work loose. Alloy or plastic cap shouldn’t make a difference, it’s like a stem cap, an adjustment mechanism. !!! And as an edit, loosen the two crank bolts before trying to remove the preload cap.

This argument will go on and on, but all I know is Saint cranks come with an alloy cap. That says to me that under certain load there is a chance they could work lose with the plastic one. Even though it isn’t there to hold the cranks on.

I’m using alloy on MTB and plastic on gravel bike at the moment but also torque them up properly….

Oh and in answer to your actual question, did you grease it?

A 5p works as a tool to tighten the plakky ones.

A butter knife can also be used in a pinch but that is better for at home rather than on the trails.

Just don’t use the best ones if you don’t want to annoy your OH

Plastic for me on the cranks they’re supplied with, cranks don’t/ shouldn’t come loose if they’re put on right in the first place – the pre-load cap is exactly that, not a crank arm retention mechanism.

How tight do people do them… The tool I have is a metal disc about 50mm circumference.

I used to just nip it up gently but a couple of times recently I’ve misinterpreted play on the cranks as worn bearings…

As the tool is designed to limit leverage, should I just be using it like a normal tool, i.e reassuringly tight without needing to grip it hard or make gggnnnnnnngggg! noises?!

If you think you need an alloy cap you are bike spannering wrong.

Shimano probably put one on the saint cranks because they wanted it to be gold and shiny and dhers at that time were magpies for anodised foolery.

I always use alloy ones, always greased, kcnc are nice and use the proper plastic shimano tool, just nipped up at a torque less than the 2nm my torque wrench starts at, I always torque the crank arm bolts to 12nm, the preload cap won’t move after that but when the bolts on the crank arm are undone the preload is or should be easy to undo, sometimes just by jamming my finger in it I can undo it, how you’ve got one stuck is baffling.

Alloy or plastic cap shouldn’t make a difference, it’s like a stem cap, an adjustment mechanism. !!! And as an edit, loosen the two crank bolts before trying to remove the preload cap.

This. The cap is not intended to retain the cranks, only to set a slight preload on the bearings. The pinch bolts are what retain the crank arm on the spindle. If the cranks are coming loose, it’s nothing to do with whether you have a plastic or alloy cap.

Never had a plastic one fall off/come loose since I started actually doing up my crank bolts properly to the correct torque. I use a dinner knife to tighten it, a 20p works well too.

the pre-load cap is exactly that, not a crank arm retention mechanism.

As many have said. The two allen bolts are tightened to 14nm so should not be coming loose and if they do the problem is not the pre-load cap…

Just fitted a GRX crankset and came with a plastic preload cap…I dug out an alloy one…preload is a load not a nip up…and nothing to do with loose crank arms

Thanks for the replies. I guess the important take-home is I’m not committing some cardinal sin by using an alloy one, so long as I don’t overtighten it. I’m well aware that it should only be lightly torqued, and would usually copper slip it, but I don’t remember when I last fitted it. Maybe I was wasn’t paying attention.

would usually copper slip it

You thought plastic vs alloy preload caps was a controversial topic! 😉

I’ve gone back to using regular grease to assemble threaded parts on the bike.

There’s a proper tool for this for a reason. It allows you to set the preload without overdoing it. The tool is a couple of ££. As some above said, it is a load, not a nip and once done up with the crank bolts to the right torque, it shouldn’t move.

I went to change my chainrings this week and the alloy cap was completely stuck in

Stupid suggestion so forgive me 🙂 but the last time I took my cranks off the plastic preload thingy was locked tight, couldn’t get the thing to move!

After gnashing my teeth for a while I realised I hadn’t undone the crank bolts. Once I’d done that the preload cap undid easily 🤦🏻‍♂️

I’ve got an alloy one on one bike, because I lost the plastic one in my spares pile.

It shouldn’t make a difference and I’ve only even snugged it up, never used a torque wrench on it. Depends on the BB too, Shimano needs a preload (i.e. snug it up) aftermarket ones I do it up then back it off as their bearings aren’t for preloading.

I suppose an alloy one could get deformed by the crank clamping on it, where a plastic one might rebound?

would usually copper slip it

Copaslip is a good idea when you’ve got similar metals and extremes of heat.

A bad idea when you’ve got disimilar metals as it’s conductive (and metallic itself) so will accelerate any galvanic reaction.

Better to use normal multipurpose grease or an anti sieze paste.