There are two types of plastic that I have seen used as manifold to carburetor insulators.
All under carburetor ones I have seen are Phenolic composite and are a simple spacer design and can be in a number of thicknesses. Factory examples tend to be about 1/4" thick as this is thick enough to be reasonably effective and tough enough, but has minimal effect on bonnet clearance.
This phenolic composite is readily identifiable by the brown colour which is the natural colour of phenolic resins. These are normally wood flour filled, and cotton fabric reinforced. The colour and texture of the wood flour can normally be seen once you know that is what you are looking for, and the reinforcing fabric yarns can also be seen in places.
The only inlet manifold to head insulator I have seen is that used by GM on their 3.8 litre 90 deg V6. It incorporates the gasket and is in glass filled nylon, probably type 6.6 nylon, but type 6 should work and type 4.6 would be best, but 4.6 is probably not available in sheet form, and I suspect you do not want to spend $20,000 or $30,000 on a mould.
I would use phenolic composite sheet unless the ports are very close together like a SBC, then I would use glass filled nylon 6.6 or a maybe a Fiberglas sheet.
The change in height is simple high school geometry, so draw it up and work it out, or just fit the components together, use appropriate spacers made of whatever is convenient, then measure the result.
Regards
pat pprimmer@acay.com.au
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