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I have square taper shimano sugino chainset (triple)with 8sp cassette. Can I change to a compact double (34/50)?
Will I need to change the chain/front derailleur?
yes. It'll have to be for the same length BB as yours though
no. You'll have to loose a link or 2.
yes no maybe... it's not right, but it'll push the chain up and down.
Your shifters might not trim correctly, but there's usually a bodge in there if you can spend the time fiddling with limit screws and mech heights
Cheers that's really helpful. 
Does that mean I can buy compact chainsets advertised as for 9sp?
yep. will work fine. Glad to help!
Chainline will be an issue, but if you're dropping the BB and going with outboard bearings then not an issue.
You may need a new front mech (a normal "double" mech) as triple mechs can foul the inner ring. Might get away with this if it's band on and also because of the teeny inner ring.
Shifters will be fine.
Sorry didn't follow most of that. chainline/outboard bearings/band on - all completely new to me!!
If dropping the BB means getting a new bottom bracket - I'm trying to replace the fewest parts possible. But want to be able use compact properly.
Get [url= http://www.fatbirds.co.uk/detail.asp/sku=BGCHS017 ]this[/url] and [url= http://www.fatbirds.co.uk/detail.asp/sku=BGBB005/Shimano_Tiagra_Bottom_Bracket_Cups ]this[/url]. Job done. Chainsets are totally reverse compatible, you can use an 8, 9 or 10 speed chainset with your 8 speed set up.
Front mech may not be perfect, but you should be able to get it to work.
Thanks. All top tips. Can I replace BB with any type of BB as long as it's 68mm??
I have looked this up on web and forums but every component is another can of worms!
BB needs to be the right type for the chainset - i.e. sq taper, HT, HTII etc, and also fit your bb shell (68mm as you say)
All road BBs are 68mm, except Italian threaded frames which are 70mm.
The important dimension is the axle length, which must be matched to the chainset on older style set ups. If you're talking about outboard bearing designs, like the one I linked to, then you can buy any road BB you want (but not MTB ones).
I should go to a helpful shop. I think you may end up buying things that don't work together properly otherwise.
This is all gold dust. Thanks everyone.
I have looked up chainline/outboard bearings etc on tinterweb. Now I think the penny has dropped. My BB is a sq taper 68x113mm, which is longer than the 68x110mm which seems to be standard for compacts. So this would make the chainline wonky causing it to rub the gears?
I'll just make sure I get the recommended BB for whatever compact I buy (as long as its 68mm) or one that comes with a BB.
Still don't get the deal with external/outboard bearings but never mind! Will defo get a shop to fit it for me.
113 may well be fine - I'd risk it!