I have a 292 block that looks clean. I do not know if it has milled at any time in the past. What is the limit for milling the head surface and where should it be measured to determine if it is within limits? 

The nominal deck height is 9.775. Ted Eaton routinely surfaces the block decks for a final deck height of 9.75. If using composite head gaskets, like Best, you should have .030-.050 to work with between the block & heads before intake manifold alignment becomes a problem. If you have non-posted ECZ-G heads, Ford recommended limiting surfacing to .010; others have said that they're safe up to .025. My non-posted ECZ-G heads have been surfaced .013. For each .006, the chamber size is reduced 1cc. If you don't have non-posted ECZ-G heads, you can safely go more than .025. Keep in mind that the original cast pistons were about .020 below deck height; combined with the original shim gaskets (.025), the quench was .045. If you are using aftermarket cast pistons, they could be as much as .035 below deck; combine that with the compressed thickness of Best gaskets (.046), your quench would be .081, which is excessive. Zero-decking the block would provide quench of .046 with the Best gaskets. Ted Eaton has said that he hasn't experienced any difference in performance with a quench up to .060. 

Is there a minimum thickness for the deck itself to measure with sonic tester? going to 9.75 is only .025 off when you list stock can be .035 making it not even zero deck. 
Seth

Good question. Zero-decking the block is not uncommon, but the compression/pin height of the pistons being used needs to be taken into consideration, as well as rod length and stroke. The nominal compression/pin height for the original cast pistons is 1.768 for 292s & 312s. On the other hand, my Autotec forged pistons have a compression height of 1.780. Ted, John, or Joe probably can provide a nominal number for the block-deck thickness, but no two blocks may be exactly the same. 

If you look on the exhaust side, near the spark-plug holes, you'll find pads the height of which can be measured to determine whether the heads were ever surfaced after engine assembly. The pads should measure 0.999-1.001, if the heads have not been milled. 

The Y block has successfully been milled to 9.740" deck height and lived quite well.  The heads measure 1.000-1.011" if uncut at the pads on each end.  Joe-JDC

JDC

You have to consider that anything that is mass-produced will not be exact. 

Thank you for the help. Deck height looks okay based on the 9.7 I measured. 

But is it square? If it isn't square or the surface smooth, you might have head gasket sealing problems. Using a composite gasket, like Best, will help, but it will also increase the quench, possibly to an unacceptable limit. 

The decks are reasonably thick but the intake manifold fitting properly to the cylinder heads becomes a problem if milling the block decks and/or cylinder heads excessively.  Also of concern are head bolt lengths as the threaded holes within the decks do get shorter as the decks and heads are milled.

I have cut the decks as much as 0.065” without any issues on a pump gas friendly 500+HP performance Y.  Anything more than 0.035” total cut of the heads and the decks can require some machining at the intake side of the head so that the intake can fit properly to the heads.  I always prefer to cut the intake side of the head rather than the intake manifold itself as cutting the intake manifold makes it specific to using with excessively milled heads.  By cutting the intake side of the heads, any stock and/or non-machined intake manifold can be used without concerns about fitment.

In the above case where 0.065” was removed from the decks, the intake side of each head was milled 0.042”.  When going with composition head gaskets, you do get 0.035” leeway on not doing any intake side of the head milling simply due to the thicker composition gaskets raising the heads versus using the thinner OEM steel shim gaskets.  In the above case, the decks were milled an additional 0.030 and that number multiplied by 1.4 gives the 0.042” amount in which to mill the intake side of the heads.

Lorena, Texas (South of Waco)