Aircraft gauges- what to look for, what to avoid. Never collected these before.

1st off- I know nothing about them other than I like the look.
Ok- I've always liked aircraft gauges but never had the time to check them out. I'm a newbie at this so I want the straight poop. What's hot, what's not. What can I expect to pay. Can cracked glass be replaced (with a hole drilled in it). The square mount ring- some I saw were metal, some looked like bakelite (and had hairline cracks through the mount holes.

Is there a standard size?
Are they 12 volt?
Are there different brands? What brand is better than another?
How many different types of these exist? A list would be nice.
What can I expect to pay?
Can the black backgrounds be restored or should I hold out for better examples?
Are they all in celcius?
Got pics?


Here's the deal. I found 5 of them at an antique store. 2 are the same- cylinder temp in celcius. One is in good condition, one has a loose cracked glass.
One is smaller, no square external mount ring & is for hydraulic pressure.
one is some sort of altitude gauge w/ an indication of what fuel pressure to run at what altitude. The face has some white damage at lower corner. I don't know if it would clean up. The 5th- I forget. The 4 all seem to have a mount ring made of bakelite.
He wants $20.00 each or he will give me all 5 for 50 bucks.
Worth it?
What condition should these be in for that price? I'm clueless.
If they are a deal- I'll go back & get them tomorrow or Monday & post pics.

Lets see yours.

 

Iv'e got a couple of dozen of these,would like to use a few on my next build.I think theres about 6 or so standard sizes.Almost all are 28 volt unless they're early ww1.There are many brands.prices vary on age&cond.WW2 fighter&bomber ones seem to be worth more,and late model usable jet ones.The main types are engine&flight controls&parameters.I am sure the backrouds are restoreable just like auto guages,mite be cheaper to find a better one.Most temps in celcius.The post ww2 ones with cannon plg/pin connectors would be hard to hook up.You mite be able to use the early ones with mechanical connections-like the fuel press ones have AN fittings,there are adapters for those.There are plenty of these on the internet-ebay 7 aircraft surplus sites.Last year I bid on a large box of all different ones on ebay&won.Some were just parts,some so so,&dozen really nice ww2&Korean era ones made it all worthwhile.One was a large gyro horizon indicator with cracked glass.I typed in "aircraft instrument glass" in google,there were a few hundred references but the very first one happened to be the exact replacement glass for this 1952 gyro ind.made by Jack&Heintz in the original envelope -for $4.00.If you like the way these lookseems like you should take the 5 for $50.Just my 2cents,hope this helps

 

The 50 buicks isn't a big deal- I think they look cool. I didn't want to buy them & then find out they go for a couple of bucks each. I don't mind paying, I just don't like being had.
Thanks for the info- really helps. I think I'll do some EBAY surfing to get a price feel.