I'd like to know the name of a japanese TV show from the 70's that
featured a boy who was friends with a family that lived in a volcano. 
The family (mom, dad, kid) could each turn into a different colored
rocket when the boy blew on a special whistle they gave him.

5 stars for sure if you can provide a link to a fan website or similar.

The show was "Space Giants," also known as "Magma Taishi" ("Ambassador
Magma") and as "Space Avenger."

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"Goldarr is a 50ft golden robot, 14 feet wide and weighs 20 tonnes and
his wife Silvarr, a silver coloured 5ft robot. They live with their
son GAM (created by Methusan) in a volcano in Mt Olympian. They all
have large antennae that fire Gamma rays (lazer beams) and can all
transform into rockets."

Glen Johnson's 60's Anime: SPACE GIANTS
http://www.alphalink.com.au/~roglen/space_giants.htm

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"This is a complete tangent, if anyone reading this ever watched the
Space Giants on TV years (and years) ago, please let me know. I am
dying to see it again, it was my favorite show when I was a kid, but I
bet it would be horrible now. Still I want to see it. It was the
Japanese Godzilla-like show where this robot family lived in a volcano
and turned into rockets. Goldar, Silvar, and Gam. Man that sounds
stupid now, but it was cool back then."

Japan Journal: Saturday Jan. 1st, 2005
http://www.yesicanusechopsticks.com/journal/january-3.htm

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This website has quite a lot of info about the show, but the site is
sometimes down due to bandwidth limitations. If you can't reach the
site, please try again later. It's worth your while:

"This is a Japanese live action show known as Ambassador Magma in
Japan, first airing there in 1966 but was not until 1978 that it was
shown in English around the world including Australia. It was
originally dubbed into English under the title Space Avenger in 1972
keeping the original Japanese theme song but was not sold to any TV
stations until 6 years later under the new name of Space Giants, with
a new opening theme song."

Space Giants
http://members.tripod.com/space_giants/

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There are lots of good still shots here. The images are sometimes slow
to load, but patience is a virtue. ;-)

CoolStuffVideos: Space Giants Goldar Silvar Gam and Rodak
http://www.coolstuffvideos.com/spacegiant.html

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Here's a long thread discussing the show:

Suburban Blight: Space Giants
http://www.suburbanblight.net/archives/001088.html

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Google search strategy:

Google Web Search: "space giants"
://www.google.com/search?hl=en&q=%22space+giants%22

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I hope this is helpful! If this is not the correct show, or if
anything is unclear or incomplete, please request clarification; I'll
gladly offer further assistance before you rate my answer.

Best regards,
pinkfreud

You are simply amazing!!  thanks so much.