What are the best internet sites to obtain statistics on automotive
parts market in United States ?  I am looking for very specific data,
such as number, or value, of «brake piston» (or «calliper piston»)
sold (or made or imported) in USA,  As it is easy to know number used
by North American car manufacturers, I am not interested by data on
OEM market, I want data on total market (OEM and aftermarket), or on
aftermarket purpose only.

It appears you need a copy of the American Aftermarket Industry
Association Aftermarket Factbook, which costs $150 ($75 for AAIA
members). You can purchase it at:
<https://www.aftermarket.org/secure/shop.asp>.

You can look at the table of contents for free from the above link. I
would recommend calling the AAIA at (301) 654-6664 to find out if they
have a publication that meets your specific needs.

Some more general information is available for free on the Internet:

AAIA, "The U.S. Motor Vehicle Aftermarket"
<http://www.apaa.org/Masthead/About_AAIA/market.asp>

US Business Reporter, "Industry Overview"
<http://www.activemedia-guide.com/auto_parts_industry.htm>

Automotive Week magazine (not much online for free, but perhaps a good
contact for more information)
<www.auto-week.com>

Search strategy:

<"Automotive News" parts manufacturers>
<://www.google.ca/search?q=%22Automotive+News%22+parts+manufacturers&ie=UTF8&oe=UTF8&hl=en&meta=>

<auto parts industry pistons>
<://www.google.ca/search?hl=en&ie=UTF8&oe=UTF8&q=auto+parts+industry+pistons&meta=>

<auto aftermarket statistics>
<://www.google.ca/search?q=auto+aftermarket+statistics&hl=en&ie=UTF8&oe=UTF8>

This file provides some official statistical information regarding the
trade in automotive parts
(http://www.ita.doc.gov/td/auto/update06-01.exe).

Thsi excel file provides information on
http://www.eautoportal.com/eap/data/gov-stats/TradeBalance-Jan2002.xls
on import/export.

Hope that would help a little