JIS vs metric bolts on a classic Japanese car
A classic Japanese car was built with JIS hardware. It looks like metric, and the thread is the same, but the heads are often smaller. That is why a hardware-store bolt can be the wrong fit.
Head sizes are the giveaway
JIS bolts commonly use a smaller head than the metric bolts sold today. These are typical spanner sizes:
| Thread | JIS head | Common ISO or DIN head |
|---|---|---|
| M8 | 12 mm | 13 mm |
| M10 | 14 mm | 16 or 17 mm |
| M12 | 17 mm | 18 or 19 mm |
Put a 13 mm socket on a 12 mm head and it will round off. Fit a larger head and it may not clear the part next to it. Tight engine bays and brackets were designed around the smaller heads.
Strength marks
Most of the fasteners on these cars are plain, but some are stamped on the head with a 4 or a 7 to show strength, and the 7 mark is the stronger one. Putting a plain bolt where a 7 mark belongs is an easy mistake to make when everything is in one tub.
Finish and washers
Factory hardware had specific finishes, such as zinc with a yellow or clear chromate, and sometimes integrated washers. Matching them keeps the car looking right and keeps rust away.
What Boltright does about it
Each bag in the S30 Big Bolt Kit is matched to a measured car, using the correct head size, mark, finish and length. Where the exact factory part is no longer made, the parts lookup tells you what is different, such as head size, finish, washer or length.