impact driver question

[alex]

i am looking at getting a Milwaukee impact drive with 1300ft-lb nut busting torque. this is the high torque version. they also have a mid torque of 600ft-lb nut busting torque. i heard the mid torque would be sufficient enough and is also smaller so you would be able to get into smaller areas.

my question is, would you use 1300ft’lb torque on bolts or just nuts. what would be the likely hood of a bolt head shearing? would you recommend to just get a 600ft-lb version instead?

i am a DIY person and do all my repairs on my own with help of friends if needed. i dont own a shop or making a living fixing cars.

Topic

[Ian]

To understand these torque ratings it’s useful to first understand how an ugga-dugga gun works. Inside most good-quality guns you’ll find a pair of heavy steel “hammers” that are driven either by an electric motor or some sort of, well, basically air motor. The hammers spin around and strike the output shaft thereby imparting a force to it. What the torque readings on these guns generally indicates is that the gun can remove a fastener that’s been tightened to a particular torque value or tighten a fastener to a point that when checked with a torque wrench, the wrench will indicate that the torque value is X-number of foot-pounds. Another way to think about this is removing a bearing with say a 20-ton press vs. a BFH and a socket. They achieve the same result, but in different ways. While the press/torque wrench does it’s job in one big “push”, the hammer/gun does it’s job through lots of tapping and knocking. Tap, the bearing moves a bit more, tap, the bolt turns a bit more…

There’s some marketing wank involved with these ratings as well. For starters, to my knowledge, there’s no agreed upon test equipment or procedures amongst rattle-gun manufacturers, but you can bet dollars-to-donuts that they’re gonna use whatever gives them the best rating for that particular gun in a perfect laboratory setting. Thus, your mileage in the real world may vary. To make matters even worse, some guns are “biased” meaning that they can achieve a particular torque rating in one direction, but not the other. But of course, the manufacturer is gonna market the gun using whatever value is highest.

While weight, size, and price are important considerations, IMO more power=more betterer. Look at it this way, they’ll both (ostensibly) loosen a fastener that’s been torqued to 600 ft/lbs, but I gurantee you the bigger gun will it do it faster. As far as screwing up fasteners goes, I wouldn’t worry about it. At one point, I worked at a salvage yard that specialized in Jeeps. Most of what me and the other guys blew apart was 20+ years old, some of it was 40+ years old. Even in a drier climate like where I live, rust and corrosion will eventually take their toll. We always sold frames and body pieces seperately and some of those body-to-frame bolts were STUBBORN f***ers. Our SOP was to hit them with the ugga-dugga gun until they either came loose or broke clean in half from fatigue. I don’t think we ever broke just the head off of one. In fact, most of the heads that we did break off (we tried to avoid this obviously, but s*** happens sometimes) were on smaller fasteners that had too much torque applied to them in one big “push” with a breaker bar or a cheater pipe.

[Dave]

TLDR; Get the 600 ft-lb.

First, I suspect you are referring to Impact Wrenches, not drivers. A wrench is set up to accept a socket rather than a bit.

Second you did not mention your location or the model #s under consideration but the ft-lb figures you listed are dated, now there are 1400 ft-lb and 700 ft-lb, and some brushless models if those aren’t.

For once in a while, rare uses I might buy a brushed model as a closeout if cheaper, it would be powerful enough for the purposes of passenger automotive DIY jobs and you’d be unlikely to run down the battery one vehicle at a time. Production pro automotive repair on the other hand, if you use a tool more often it can justify the greater cost.

1300 ft-lbs is overkill for working on cars. Commercial trucks, bridges, etc is more the target market of such a tool. You will also need top shelf, expensive impact sockets to get anywhere near that without failures. Yes that much torque or even 600 ft-lbs will shear some fasteners off, but this is not necessarily a bad thing. With such stuck fasteners, you have to evaluate the value of the fastener (or rarity) vs your time. Do prep one with penetrant spray, do heat it with a torch or induction to help remove it, but ultimately a stuck fastener either has to come out or shear off then be beat out. If it takes that much torque then it takes that much. Some fasteners can’t be saved and some shouldn’t be if that corroded.

On the other hand, blind holes where you’d need an extractor if it shears off, I would not use an impact wrench at all on those until broken free as they are much more of a PITA to remove than the little extra effort to manually break them free THEN use an impact wrench or driver.

The 600 ft lb version or even lower if a smaller size, is going to be more versatile for passenger automotive uses. While we can’t guess how much rust will increase torque requirements, generally there is nothing on a passenger automobile requiring anywhere near 600 ft lbs. Getting a job done faster with higher torque, doesn’t really apply in this case. It’s not a production environment where you’re doing nothing other than unfastening dozens of fasteners in a row with no other activities in between. We’re talking a single-digit # of seconds difference per fastener on one automotive repair at a time.

The other issue is “accidents”. Suppose you were to put a nut on and accidentally left the 1300 ft lb monster on high mode. You might irreversibly damage the fastener, even if it seems to go on and hold, it could fail later in use of the vehicle or fail the next time someone tries to remove it. Monkeys at shops who use impact wrenches wrenches to put tires back on instead of finishing with a torque wrench, I’m looking at you. Several times I’ve had wheel lug studs break off during difficult removal because they were far overtorqued!

You don’t need 1300 ft-lbs and it’ll just be heavier, bulkier, and more expensive.

[Author]

Replies