mySQL and primary key

[jbv at souslelogo.com]

Hi list,

I have a mySQL table with 13500000 entries that weights
almost 20 Gb.
I need to create a primary key on the "id" column.
I have tried in phpMyAdmin as well as in a script with
"ALTER TABLE myTable ADD PRIMARY KEY(id)", but each time
I get a "504 Gateway Timeout" error, and no index is created.

What else can I try ?

Thank you in advance.
jbv