Metric nuts and bolts are encountered on an increasingly regular basis. However, many basic wrench sets only cover the standard sizes. You don't want to have spend the money to double the size of your toolkit for wrenches that you will only use occasionally, nor do you want to run to the store to purchase individual metric wrenches each time a new metric bolt shows up. Fortunately, several standard size wrenches are close enough to work with metric nuts and bolts you use them carefully.
What Sizes Are Interchangeable?
There are several standard wrench sizes that are only slightly larger than their closest metric counterparts, and therefore you can use them with care to loosen or tighten metric bolts. The list is sorted by how much larger the standard wrench is than the metric bolt head.
1/1000 inch: 5/32 wrench on 4 mm bolt, 29/32 wrench on 23 mm bolt
2/1000 inch: 5/16 wrench on 8 mm bolt, 3/4 wrench on 19 mm bolt
3/1000 inch: 19/32 wrench on 15 mm bolt
4/1000 inch: 7/16 wrench on 11 mm bolt, 15/32 wrench on 12 mm bolt
How Close Is Close Enough?
As you can see, some fits are closer than others, and can therefore be used on tighter bolts and with less caution. Anything above 4/1000 of an inch is too large a difference in size. Trying to use a wrench that is any looser will result in a stripped bolt and a whole new set of problems. If your metric bolts are not one of the sizes in the list above, do not attempt to use a standard size wrench.
When to Use Standard Wrenches
The closer your wrench is in size to the bolt head, the more likely it is that you can use it safely. In general, though, you can interchange close fits if you are not trying to break loose a very tight bolt or torque a bolt down tightly. If a metric bolt is already worn or rounded, take care before using a standard wrench, as it is more likely to slip and further strip the bolt than a metric wrench of the correct size.
Using a Standard Wrench on a Metric Bolt
Using an open-end wrench instead of a box-end wrench will give you a better feel for the tightness of the fit. Turn the bolt using light, rhythmic jerks or taps. A long, steady pull is more likely to round or strip the bolt. If there is enough room between the head of the bolt and the wrench, insert a thin flat-head screwdriver to tighten the fit.
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