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Risks of using an impact wrench on drain plugs

Posted by jerry_nj (My Page) on
Sun, Feb 12, 12 at 10:02

I used my electric impact wrench to take off the fill and drain plugs on the rear differential of my 2004 Forester. I changed the GL-5 lubricant.

I used the impact wrench because the subject plugs didn't come loose with my use of a 1/2" manual ratchet socket driver. The car has about 55K miles, this is the first time these have plugs have been taken off.

The torque wrench "zipped" the plugs right out. I used the 1/2" manual ratchet drive to put the plugs back in, I didn't bother to use a torque wrench.

I have torque limited extensions for my 1/2" impact wrench, but I think they over tighten... another point. Yet another point/question, as the Forester is Japanese it must be metric, yet the 1/2" drive fit okay (could be a small bit under size) into the recesses on the subject plugs. If the plug recesses are 13 mm, a 1/2" should work, but isn't the best fit.

After posting my experience on the differential oil change on a Subaru Impreza forum I got one response telling me to never use a impact wrench on these (or similar?) plugs. Keep in mind, I used the impact only to remove, not reinstall.

Back to other experience, I do the same when dealing with wheel lug nuts. When I tried the 65 foot pounds torque limiting extension on the impact wrench the lugs are a bit overtightened, I believe. Proper use of a impact wrench to tighten may be an "art". I hope the tire shop's air impact (mine is a low end electric) wrenches are much better calibrated on tightening torque.

Back to reason for post: it there a high risk of damage when using an impact wrench to remove items such as differential drain/fill plugs?


Follow-Up Postings:

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RE: Risks of using an impact wrench on drain plugs

Impacting a stuck thread is a good idea in my mind. Been told it works well on stuck spark plugs. Stripping plug threads is not good. Friend over tightened oil plug and cracked pan. What if gun is set on tighten by mistake?


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RE: Risks of using an impact wrench on drain plugs

Right, I understand there is a risk of over-tightening, thus the torque wrench...which I almost never use even on manual tightening. I have started using a torque wrench on spark plugs, I purchased a 3/8" drive with a click limit indicator. I don't know how it is accuracy- wize but it sure is easier to use than a dial torque wrench. I've had a 1/2" dial torque wrench for over 30 years, it is fine for wheel nuts, but even there I have to work to get myself in a position to generate 65 foot pounds and still be able to see the indicator.

My electric impact wrench has the extra risk of the direction being set by the way the trigger is "rocked". Push down on the top, nut out, push down on the bottom, nut on. I always give it a spin in the air to be sure I've not gotten mixed up before I put it on a nut/bolt/plug.

Well, my thinking remains taking "things" off is fine, putting them on may result in an over-torque situation, and possible disaster.


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RE: Risks of using an impact wrench on drain plugs

If you had to resort to an impact driver to get the plugs out of the differential, those were in too tight. You should have been able to remove the plugs using a long handle breaker bar (1/2 inch drive in many cases). A ratcheting socket driver is the wrong tool for loosening really tight bolts and plugs.

Differential drain plugs have tapered pipe threads, not regular screw threads. These seal by bottoming the thread crests, same as water piping. For the differentials, these do not have to be tightened to high torques. They should be tightened only enough to stop leaking and not loosen to fall out.

Excessive tightening torque causes metal distortion and wear on the threads. Do this often enough, and the hole will be enlarged.

Oil pan bolts are a different matter. These have regular threads and seal using a captured elastomer ring under the head of the bolt. (Older classic cars may have plugs with a gasket.) The bolt heads may be on the soft side and easily rounded. These should be tightened enough to stay in place. Often, the elastomeric seal will expand and in time, it will require more torque to remove than what was used to install. When the self jamming effect begins to reduce, replace the bolt. These bolts are available at automotive parts stores. If the corners of the head begins to round off, replace the bolt. Its a good idea to keep a spare bolt in your garage or vehicle.


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RE: Risks of using an impact wrench on drain plugs

Thanks, lots of great information, should be of value to many readers of this thread.

Just for discussion, I decided to drain the rear differential with the car on the garage floor, I didn't even raise it on low ramps, so I had more difficulty developing much torque with my outreached arm I just looked up the torque, surprised to find it is 30 ft-lbs. I carefully watch all fluid changes, oil included, by putting a clean piece of cardboard under the drain plug area and coming back in a couple of hours and look for any drips - of course I wipe the plug area well before putting starting the drip test. This was my first rear differential change and the only other person to have their "hands" on my Forester is the dealer, and never for service. But when they get the car in they do a general check over and come up with $500 + recommendations for fluid/grease change. I always figured that was a good deal, they are looking for more profitable work and should be good at finding real problems too. Now here's one for you, given what I just said, I yesterday pulled the glove box to gain access to the cabin air filter. I figured it would be time to change as it is recommended at 30K and the car has 55K miles. Wow, open the filter housing, no filter! Now where did it go? Anyone's guess, but the only two I can come up with is 1) factory error, 2) dealer took it out and forgot (?) to put it back during one of their look for work efforts. The car has been to the dealer twice, once for tires and once for a recall.

The '04 Forester has a "crush" washer on the oil drain plug, and when not changed/replaced at least for a few (I wasn't counting) drains it may start to drip very slightly. I was in Advance Auto yesterday to buy ATF to drain/fill the auto transmission (just the pan, no power flush)and asked after purchasing some new crush washers. They had some, per their computer, but had trouble finding them. They must not be a big seller. I have never replaced an oil drain plug washer in any other car and I've kept one of those to 200,000 miles. There may be an oil drain plug replacement with a better washer arrangement. I suppose the auto transmission drain plug is the same. But as this is the first time it is off, I'll just reuse it.


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RE: Risks of using an impact wrench on drain plugs

Hope you didn't lie on the bare floor using that electric wrench.

I use a 9v battery screwdriver for a speed wrench.

If you use a power wrench often enough you will inevitably crank it the rong way. Might be a long time between but it will happen.


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RE: Risks of using an impact wrench on drain plugs

My electric outlets in the garage are "ground fault" protected, I should be able to stick my finer in one side and stand in water and not be killed...but who wants to test this theory?

On the "if it can be done wrong, it will be" theory sounds like the voice of experience. I've probably done it. I have torque limiting extensions (god knows why, or at least I don't now that I've tried them) maybe it would give some protection to always use the 65# one, that's as low as they go, best I can remember sitting here in the warm house. I have not yet stripped a thread (to my knowledge) with the torque wrench, but it has seen very little service. The reason I bought it was I needed to pull the flywheel on a brigs and straten vanguard 2 cylinder engine. I could either use a strap wrench or a impact. The impact zipped it right off, I may have had some drag on the flywheel, but I think the engine compression was enough inertial drag to case a real whack to hit the nut. I used the torque limiters to put the nut back on. The engine has a hundred hours or so on it since.. seems fine.


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RE: Risks of using an impact wrench on drain plugs

My electric outlets in the garage are "ground fault" protected, I should be able to stick my finer in one side and stand in water and not be killed...but who wants to test this theory?

In a forum long, long ago was a man, an engineer by training, who did exactly that to demonstrate the effectiveness of a GFCI receptacle. He was in his late 70's or early 80's at the time so I guess those darned things work.

Frankly, I wouldn't want to deliberately test one either. ;-)


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