What is the metric equivalent of 31/2"

Assuming the nailer is in good order the problem is likely with the nails - the type, style, quality of nail you are using might not suit the nailer.
Some nails are poorly made and although I haven't personally measured them I suspect some nails purporting to be a particular gauge are either under or over the size. If they are imported, for instance, from a country using the metric system, they will probably be produced from sheet steel of a convenient metric dimension that is a near equivalent size, but not an exact match - 1.2 mm sheet is a tad smaller than 18 gauge while 1.3 mm is midway between 17 and 18 gauge.

Just a theory you understand.

Some nailers will be a little more roomy than others, especially those that have been worn in.
In addition it is equally possible the nailer was designed for metric nails but exporting to a country where metric nails would be unobtainable the manufacturer specifies a near equivalent, sometimes opting for the higher number and sometimes for the lower number - we see this a lot with imported clothes and shoes.

i cross referenced the spark plug (LD A7RTC)
and got the NGK equivalent:

SKU: #CR7HS
ITEM : # NGK7223
BRAND : NGK
Then got the specs off the NGK site

NGK 7223 CR7HS Nickel Spark Plug

It should be a 5/8" (16mm) spark plug socket.

Within the metric system 1 ml (millilitre) of volume is equivalent to 1 cc (cubic centimetre) of volume,
Therefore 200 ml = 200 cc
Trust this helps, regards Alan

The kilometre is unit of length in the metric system equivalent to one thousand metres.

1Km is equivalent to 0.6214 miles.

Hi jess49...

5,000 pounds is equivalent to 2.27 metric tons Please take time to rate me

8 cm is 0.08 m

It's usually a 3/8 ths wrench or the metric equivalent. If you're very careful you can use a vice grip pliers to loosen it.

Multiple your Kilogram total by 2.2 to get the pound equivalent.
http://www.metric-conversions.org/weight/kilograms-to-pounds.htm

Very wild guess. I replaced the bearings in my 2003, and was looking in my tool box. I came across a 1 1/4" socket that I purchased around the same time, but I realize that it should be the metric equivalent of this, so it probably did not fit perfectly. Metric conversion would be 31.75 mm.