Can you just use O-rings from a hardware store?
 

I was wanting to replace all the O-rings in my A/C system lines. Instead of trying to order them from the dealer or auto parts store could I just match up a set from the hardware store? They look the same but are way less.
Anyone do this with satisfactory results?

No, don't even think about it.

What about generic ones in the auto parts store?

If they are A/C o-rings, they should be fine. There is nothing "special" about the A/C system on a W123.

I bought an assortment for European cars that included everything to reseal my entire system for under $6. And I still had half the assortment left over.

Here you are:
http://www.ackits.com/merchant.mvc?Screen=PROD&Store_Code=01&Product_Code=CP3025&Category_Code=vehiclespecific

That's the reason I was wondering since I was thinking the same thing that there is nothing special about the A/C system.

My local hardware is very good.

But they don't carry metric o-rings.

I think the difference between metric and SAE wouldn't be that much of a problem here.

Get the proper seals from the dealer. Why cheap out on something that is as difficult to change as A/C seals? The green ones that they will sell you are good for R12 or R134a.

What is it about an a/c seal that it needs to come from the dealer? Does a Mercedes use a different refrigerant or oil than any other car? Do you need to make sure that you only buy "Mercedes" R-12?

I don't have any idea what the dealer would charge for them but I was thinking it would be many times more than what I could find in a hardware store. Also my closest dealer is an hour away.
A/C seals are A/C seals. Also the compressor is the same model used in GM cars so I see no need to look specifically for MB parts here.

Agreed. But hardware o-rings are typically for plumbing applications. But if your hardware store sells automotive A/C seals, that's great. If they don't, just about any auto parts store should be able to fix you up.

You've got to have some sort of auto store near you....the A/C seals they sell will be better suited to the job than those from a hardware store....

I've picked up a couple of variety packs of nitrile AC system seals at places like Schuck's and Autozone in the past. They're metric, of course. A $5 packs gives you enough to do the entire system on most cars with some left over. I've had no trouble with them at all in either R-12 or R-134a systems.

That sounds like a cheap and easy way to go.

One way, not the other...
 

I'm ramping up to convert back Gray Ghost to R-12. In doing research it seems hoses and O-rings are different for each due to the molecular structure of each refrigerant. R-12 has fat molecules; R-134a's are smaller in diameter and, in a cheaply converted system, can leak through the OEM seals and hoses. That's why some references reccomend using "barrier" hoses as the molecular make up is less permeable.

In summary (I think) OEM hoses and original rubber O-rings (usually black) are OK for R-12 but will leak R-134a. Barrier hoses and the green metric O-rings, since they are designed for the smaller R134a molecules, will also work for R-12.

Recommend Diesel Giant's photo essay on DIY AC system purge. In it he recommends getting the O-rings from NAPA (just take in all the old and match them up) and to dip each ring in that special AC system lube oil.

Good luck.