Hmmm... My school's shop has a .00001" flat granite table....

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

I do not think they would be happy with you lapping on it; and a plate of glass is easier to take home

I use a plate of glass and 600 grit lapping compound I bought from a Rock Shop supply seller on Ebay. (They use the compound to tumble Gemstones and Rocks to smooth and polish them; as well as to lap flat surfaces on them.)
If I was in a hurry I'm sure I could have found a local place to buy the compound from.

One of the members balked at my use of 600 grit; but we used 500 grit on a Cast Iron hand lapping plate and the Lapping Machine at the Fuel Injection Shop I used to work in.
I have rebuilt 4 sets of Injectors using the Glass Plate with no problems with leaks.
I would like a regular lapping plate but keep getting out-bid on Ebay or the shipping of the heavy plate is too expensive.

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84 300D, 82 Volvo 244Gl Diesel