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What's the proper torque for a 3/4" long, 5/16" diameter, 26 tpi crank bolt?
Crank Bolt Torque?
- Thread starter moribundman
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I forgot to mention the bolt has a 14 mm hex flange head. You know the kind, like a hex head with an attached washer with teeth on the back. The bolt head says "12 9," which "probably means 12.9, which indicates a high tensile strength bolt. The bolt is black (plated or with an oil finish?). I do know that the crank axle into which the bolt goes screws is made of cromoly steel.
If this is indeed a grade 12.9 bolt, then it should never be reused. A grade 12.9 bolt is supposed to be torqued to 90% of its proof load. The question is, what is that proof load?
If this is indeed a grade 12.9 bolt, then it should never be reused. A grade 12.9 bolt is supposed to be torqued to 90% of its proof load. The question is, what is that proof load?
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What kind of bike/brand of crank,,good rule of thumb is Lubricate all surfaces with at least common engine oil,,then when crank is put on tap lightly with a rubber mallet then screw in bolt,use a good hex wrench,L shape ball end preferred,then firmly tighten ,say firmly,till the wrench starts to hurt your hand,make sure the bike is properly on a good surface,not suspended on ropes or a stand as this caused flex,tight is the word.but not too tight,about 50 ft lbs is plenty..BL
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The bike is a Sussex. It's a fairly new bike, but a bike that's been in production for probably over ten years. It's put together with parts that seem dated and frequently odd. No brand name is on the crank.
Well, I've thought this over. The crank really is held in place by being jammed on the tapered axle ends. The bolt does nothing but prevent the crank from slipping off the tapered ends. I would "think" that torque isn't all that important. It has just to be enough for the bolt not coming loose. And that's why it's so odd that the original bolt is a high tensile strength (12.9) bolt. It's just unnecessary, but going by the rules, every bolt is made with a proper torque spec in mind. So I went to the bike shop and checked regular crank bolts, which are M8x1.0 Allen flange bolts of unknown grade. I also saw some M8 outer hex (14mm head) flange bolts and plain old hex keys with a loose washer.
Nobody can tell me the proper spec. The bike mechanic torques them "good'n tight" with his calibrated elbow. I'm now using a regular, modern M8 Hex flange bolt with blue threadlocker and use 35 Nm (26 ft-lbs). To me, 50 ft-lbs (68 Nm) seems way too much for an M8, regardless of grade.
The bolt in the front is the original bolt. The bolt in the background has the loose washer, which in my opinion is preferable. The bolt on the right is what many current bikes use. I ended up getting that type, but with an integrated plastic cover, mostly because I'd rather carry a 8 mm Allen than an 8 mm hex socket. It also looks pretty.
You can see that bolt installed in the picture below:
I'll find out if it comes loose...
Well, I've thought this over. The crank really is held in place by being jammed on the tapered axle ends. The bolt does nothing but prevent the crank from slipping off the tapered ends. I would "think" that torque isn't all that important. It has just to be enough for the bolt not coming loose. And that's why it's so odd that the original bolt is a high tensile strength (12.9) bolt. It's just unnecessary, but going by the rules, every bolt is made with a proper torque spec in mind. So I went to the bike shop and checked regular crank bolts, which are M8x1.0 Allen flange bolts of unknown grade. I also saw some M8 outer hex (14mm head) flange bolts and plain old hex keys with a loose washer.
Nobody can tell me the proper spec. The bike mechanic torques them "good'n tight" with his calibrated elbow. I'm now using a regular, modern M8 Hex flange bolt with blue threadlocker and use 35 Nm (26 ft-lbs). To me, 50 ft-lbs (68 Nm) seems way too much for an M8, regardless of grade.
The bolt in the front is the original bolt. The bolt in the background has the loose washer, which in my opinion is preferable. The bolt on the right is what many current bikes use. I ended up getting that type, but with an integrated plastic cover, mostly because I'd rather carry a 8 mm Allen than an 8 mm hex socket. It also looks pretty.
I'll find out if it comes loose...
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Mori, not much help, but my Leonard Zinn Roadbike repair manual says not to use any lube on the axle mount or crank mount surface for fear of allowing the crank to slide too far onto the axle and "stretching the tapered crank hole (same if you over tighten).
He lists 305 to 435 INCH pounds for cranks and for Campy, 283 to 336 INCH pounds.
I would try to call the bike manufacturer for specs. Or, have you tried Roadbike Review or Bicycling web sites?
He lists 305 to 435 INCH pounds for cranks and for Campy, 283 to 336 INCH pounds.
I would try to call the bike manufacturer for specs. Or, have you tried Roadbike Review or Bicycling web sites?
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I don't use any lube on the crank mount for the reasons you state. Your torque specs are not that far off mine.
I've talked to a few bike mechanics, and none of them could tell me what torque they were using. None knew about bolt grades and proof-load. They knew less about that than me, and I know basically only that I know nothing about bolts.
I don't want to call Taiwan. I'm going with the empirical method instead.
The biggest mystery to me is that the manufacturer would use a bolt that requires torquing to 90% proof-load, which in turn will likely stretch the crank hole too much. I suspect the new crank bolts will work fine, but it will take 100 miles or so to find out.
I've talked to a few bike mechanics, and none of them could tell me what torque they were using. None knew about bolt grades and proof-load. They knew less about that than me, and I know basically only that I know nothing about bolts.
I don't want to call Taiwan. I'm going with the empirical method instead.
The biggest mystery to me is that the manufacturer would use a bolt that requires torquing to 90% proof-load, which in turn will likely stretch the crank hole too much. I suspect the new crank bolts will work fine, but it will take 100 miles or so to find out.
I'd go with 37 lb-ft on that type of bolt (Shimano recommends 26-37 lb-ft). Shimano prints technical specs on their website, and the Alivio-level crank still uses that taper design. I wouldn't worry about the properties of the bolt, as long as it's strong enough to not break. As far as I know, a bolt only needs to be torqued to a certain load if the specific application requires it. If an application calls for 50 lb-ft with a grade 5 bolt, then a grade 8 bolt can be used at 50 lb-ft in the same application. At both industrial sites I've worked at, to prevent the possibility of using a lower-grade bolt in any application, we stocked nothing below grade 8.
When I was fixing bicycles, for Shimano cranks I always just put a little grease on the thread and gave it a good pull with a 3/8" drive ratchet. With no-name cranks made of softer aluminum, stay away from the grease or you'll just stretch the hole. They never really tighten up the way better cranks do, you just have to call it good enough at some point. The aluminum used for RaceFace Turbine LP cranks used to be so hard that I'd grease the bolt and even the tapered face of the crank and axle to prevent creaking. Even with all that grease, the point at which it was tight and would move no more was very well defined. Unfortunately, our little Canadian RaceFace company got bought out and now sells a lot of Taiwan stuff! Same thing for Synchros.
When I was fixing bicycles, for Shimano cranks I always just put a little grease on the thread and gave it a good pull with a 3/8" drive ratchet. With no-name cranks made of softer aluminum, stay away from the grease or you'll just stretch the hole. They never really tighten up the way better cranks do, you just have to call it good enough at some point. The aluminum used for RaceFace Turbine LP cranks used to be so hard that I'd grease the bolt and even the tapered face of the crank and axle to prevent creaking. Even with all that grease, the point at which it was tight and would move no more was very well defined. Unfortunately, our little Canadian RaceFace company got bought out and now sells a lot of Taiwan stuff! Same thing for Synchros.
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Well, I'm not sure. It may well be that this type must stretch to yield in order to not come loose. I never torqued that bolt more than I thought the crank could handle, and the bolt kept coming loose despite the use of thread locker. I seriously suspect that the bike importer/assembIer has used some unsuitable parts. I've put about 60 miles on the bike since swapping out the bolts, and so far so good (no creaking or noticeable play).
The bolt is really not what's supposed to keep the crank on the axle mount. Once the crank has been "pulled tight" onto the tapered axle mount, the crank "should" stay on, even if the bolt were to fall out. The bolt really has to be only strong enough to pull the crank onto the tapered mount. And the bolt should remain in place.
That class 12.9 bolt on a bicycle is nuts. I didn't even know they went over 10.9 (equivalent to grade 8, I believe). You should donate those bolts to the space shuttle project or to a local nuclear plant.
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I should put them up on ebay.
It's probably a counterfeit bolt anyway, and actually equivalent to grade 5!
But if you're talking boilers or pressure piping, counterfeit bolts are not funny.
just as a reference a plain 5/16-24 grade 8 bolt 75% yield torque is 30 ft-lbs
an 8mm class 10.9 is 40 Nm
an 8mm class 10.9 is 40 Nm
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Anyway, it's 300 miles later and the cranks bolts have remained in place.
Does the Park tools website have the info you need? It does for me so far.
It has been a GoX send.
It has been a GoX send.
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I think you can type Godsend.
I have actually figured out what caused the problem. I only had fito remove with a file some ridges that I had previously failed to notice in the square crank holes. The problem wasn't caused by improper crank bolt torque, it was caused by improper fit of parts that need to match exactly. Now the cranks snug up properly on the axle, which before they didn't. I simply had failed to realize that. The crank bolt torque ( I use 35 Nm now) is not that critical. What's critical is that the axle fits snugly into the square hole and that the crank bolt gets tightened after a while, because the aluminum will give in a little initially and the crank needs to be snugged up. All is good now. In the future I will get nothing but a bike with a keyed crank. Anything else is really outdated.
I have currently about 7k miles on my shaft drive bike. I haven't had to relube the drive since figuring out what grease works well (Red Line CV-2). I did modify the transmission housing, though. I added a Zerk fitting and I sealed the housing with silicone gaskets. I put enough grease in there so that the gears will always pick up a bit grease from the bottom of the housing. That way I do not have to use the tacky additive bruce381 so kindly let me have. I'm planning a grease change at 10k miles.
The Park Tools website is very useful. The Harris Cyclery site is even better.
I have actually figured out what caused the problem. I only had fito remove with a file some ridges that I had previously failed to notice in the square crank holes. The problem wasn't caused by improper crank bolt torque, it was caused by improper fit of parts that need to match exactly. Now the cranks snug up properly on the axle, which before they didn't. I simply had failed to realize that. The crank bolt torque ( I use 35 Nm now) is not that critical. What's critical is that the axle fits snugly into the square hole and that the crank bolt gets tightened after a while, because the aluminum will give in a little initially and the crank needs to be snugged up. All is good now. In the future I will get nothing but a bike with a keyed crank. Anything else is really outdated.
I have currently about 7k miles on my shaft drive bike. I haven't had to relube the drive since figuring out what grease works well (Red Line CV-2). I did modify the transmission housing, though. I added a Zerk fitting and I sealed the housing with silicone gaskets. I put enough grease in there so that the gears will always pick up a bit grease from the bottom of the housing. That way I do not have to use the tacky additive bruce381 so kindly let me have. I'm planning a grease change at 10k miles.
The Park Tools website is very useful. The Harris Cyclery site is even better.
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Mori, you're correct; the Harris Cyclery website rocks. I recently needed a new 7 speed cassette for my mid 90's Trek road bike. Where else are you going to find a BRAND NEW Shimano 11/26 7-speed cassette? Answer - Nowhere but Harris Cyclery!!!
Rock on boys!
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