Motorcycle Repair: disc brakes, cash flow problems, disc brakes

Question
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Followup To
Question -
I wonder if you could advise me on a few questions/issues to do with brake discs and calipers.My bike: Yamaha FZS 6oo Fazer.
The problem I have is a warped front disc (left hand)it is so warped that when I removed the caliper it was actually touching the fork. It has warped into a saucer (shallow) shape. This happened after recently having had my front tyre changed at the local dealers. It was preceded by a rubbing in the front brakes. Then I got the tyre changed but noticed a vibration in the handlebars at speed, so I took it back to the dealers to have them look at it. They re-balenced it for me although they said they couldn't see a problem with it.  After leaving the dealers the brakes were spongy but still just about working.  Then I lost all braking on the front and the brakes just got tighter and tighter. I thought the brakes may need an overhaul, it was then that I noticed the disc was badly awry.
A few questions:-
How can the disc become so badly warped?
I know I need to replace it, but do I have to replace both discs at the same time(my local dealers all say I should)?
I want to get the discs second hand from the breakers if possible(cash flow problems), is this advisable or not? If so what should I be looking out for in terms of disc condition/specifications etc?
Is there any advise you could give me regarding caliper overhaul? Do I absolutely have to replace the caliper seals?
Thanks very much for your time reading this (I wanted to get in as much detail as possible).  Please save my bike! Many thanks Gav.
Answer -
Hi Gav,

I suspect the disc was damaged during tire installation. We use a long pipe like tool to get the tire bead on the rim. The pipe or other tire tools could possibly cause the warped disc due to carelessness. If both discs were warped in the same locations, I would suspect a front end collision under heavy braking, common as with a crash.

You can buy used parts reliably. Before paying, measure the thickness of each disc in three places (consistancy), check for warping (flat surface), and they should have a smooth surface. See service manual for exact specs.

Calipers may not need overhauled unless leaking brake fluid, unless if you just want to clean them. If I had your eamil addy as requested, I could have returned repair information files.

Respectfully,
Mark Shively

Hi Mark,

Thanks for the advice. Everything has become much clearer now.  Sorry I thought that you got my E mail adress with the question(I'm new to this service!) it's gavin@cecil1215.fslife.co.uk.  I would be most greatful if you could send me the repair info files that you mentioned.
With regard to the discs, no collision occured so I guess that it was the dealers (although I doubt I could prove it). The disc was turned outwards all the way around not just warped in the one place and the other disc was fine, can this happen?
Just one more question,  how do I measure the discs? Do I need a special tool or can I just use a ruler?

Many thanks for your advice and attention to my problem.

Yours greatfully Gav

Answer
Hi Gav,

Measure discs with a micrometer.

Motorcycle shops make mistakes. That kind of mistake is hard to let go un noticed. You should inquire with the service manager or owner of the shop where the work was performed.

My family imigrated from UK in early 1900's. Manchester County area.

The files are en route. Email me directly with any questions.

Mark
mshively1@woh.rr.com