JDM stands for Japanese Domestic Market: clubs made for sale in Japan. Some are models you will not find elsewhere. Others are Japan-spec versions of clubs from global brands. Much of what is written about them is opinion, so here is what you can actually check.
The head is often the smaller difference
When a model is sold in both markets, the difference is frequently the shaft rather than the head. Do not assume it either way, though. Compare the published specs for the exact model.
The shaft is usually the big one
Japan-spec stock shafts are often lighter and softer than their US counterparts. This is a tendency, not a rule. Some Japanese shafts play stiffer than you would expect, and there is no industry standard for flex in either market.
Reading the flex label
- R2: softer than a regular flex.
- SR: between R and S.
- M-37, M-40 and similar: PRGR codes that point to a swing speed.
Treat these as starting points. An R from one maker can play differently from an R from another.
Weight and length
Japan-spec clubs can be lighter overall, with a slightly lower swing weight. Some are also a little shorter. Check the figures on the listing rather than assuming they match the US version.
Who they suit
If you have a moderate swing speed and want an easier, lighter club, a Japan-spec stock shaft may be exactly right. If you swing fast, look at the shaft first. Many used JDM clubs have been refitted with heavier steel shafts such as Dynamic Gold or N.S. Pro, so a head you like can still come with a shaft that suits you.
Two things to check before you buy
Competition rules. Some Japanese high-rebound drivers are built outside the rules of golf. If you play in competitions, look the model up on the USGA conforming list.
Warranty. Do not assume a maker's warranty applies outside Japan, or to a used club. Ask the seller what happens if there is a problem.