Hi Geoff - For a simple answer, I'd start with one gasket @.015.  Real world, it depends on how the head was cut by the reworker.  Stock, the combustion chamber is a squish band, a nearly flat ring about .25 inch wide around the outer chamber edge, with a deep bowl in the center.  The piston comes close to the squish band at TDC, causing the mixture to rush to the bowl right at the instant of combustion.  This causes high turbulence and speeds the flame travel.  The combustion goes too quick, so it goes BAM instead of Whoosh.  For stunt use, we want slower combustion, so the engine comes on and off the 2-4 break smoothly.  So look and see if the head was cut to a true hemi, with almost no shoulder left at the edge, and one smooth radius all the way up to the plug, or if only the square shoulder at the edge of the squish band was cut to a radius.  More material removed means you have to use less deck height, to keep the compression ratio the same.  Typical numbers are .015 stock gasket, about .025 - .030 deck height for a 'shoulder raduis' hemi, and .005 - .010 gasket with .012 - .015 deck height for a real hemisphere.  Tom H.

Hi Geoff - For a simple answer, I'd start with one gasket @.015.  Real world, it depends on how the head was cut by the reworker.  Stock, the combustion chamber is a squish band, a nearly flat ring about .25 inch wide around the outer chamber edge, with a deep bowl in the center.  The piston comes close to the squish band at TDC, causing the mixture to rush to the bowl right at the instant of combustion.  This causes high turbulence and speeds the flame travel.  The combustion goes too quick, so it goes BAM instead of Whoosh.  For stunt use, we want slower combustion, so the engine comes on and off the 2-4 break smoothly.  So look and see if the head was cut to a true hemi, with almost no shoulder left at the edge, and one smooth radius all the way up to the plug, or if only the square shoulder at the edge of the squish band was cut to a radius.  More material removed means you have to use less deck height, to keep the compression ratio the same.  Typical numbers are .015 stock gasket, about .025 - .030 deck height for a 'shoulder raduis' hemi, and .005 - .010 gasket with .012 - .015 deck height for a real hemisphere.  Tom H.

Hi Tom  Geoff  All

The above while is true for some engines with some setups is not true of all engines, there are many stunt engines with a very smooth 4-2 transistion that have a large squish band. The Aero Tiger for one has a very smooth 4-2 and has  a  huge  squish band style head ,extreme to the point it may be  the  widest squish band head of any stunt engine out there.
 I have tried almost every head shape you can imagine on this engine and none work better than the  ultra large, wide  squish band head. The deck height on the AERO Tiger is set partially by the top lip thickness of the sleeve, since we make these that is  zero problems.
I have seen several of these engines ruined by cutting a hemi dome into the stock head, it just killed the performance, and did  nothing to improve the 4-2 transistion.
 There are also many other engines that have a really smooth and rapid 4-2. many of the older stunt engines such as the ST 46 and  HP 40 come to mind. proper deck height seems to be much more important than the size of its band. Many of the PA  engines have a wide squish band and can be setup to have a very smooth 4-2.
So as usual, there are many ways to get a great engine run, some involve cutting the compression ratio by cutting a very large hemi shape into the head, as Tom has described, others are a mixture of deck height / nitro percentage. not to mention the many other factors  such as prop load and venturie size.
Ya pays your money and take your chances, even if you ruin one it pays sometimes in the  ole  learning experince, and there is nothing wrong that a new part won't  cure



Regards
Randy

Thanks for the advice guys.  Notice that I am machining replacement parts.  If I don't like the outcome, I can revert to stock.  Also, as I am the other side of the Pacific and a mechanical engineer, I am interested to have a bit of a play myself—without ruining a perfectly good engine.

Thanks again, Geoff