Buick 300 heads on a 215?

[jim4mgs]

Hi! I'm new to the list but not to the hobby.  I put a 215 into a 1978 MGB some years ago and am now redoing the whole project...picked up a set of of 64 Buick 300 heads years ago and am now wondering about bolting them onto the 215 block.  I am aware that the larger combustion chamber size in the 300 heads will lead to a lessened CR but some seat-of-the-pants calculations indicate that using stock  215ci 10.25:1 pistons with these 300 heads would result in  a final CR of about 9:1.  Does anyone know if that is an accurate guess-timate?  Has anyone out there done this? Those 300 ports are much bigger the the 215 ports as are the valves...a very tempting substitution at first glance.

[mgb260]

I used 300  heads years ago on a 215. I used the hi compression pistons, and 8.5 to 9 to 1 is about right. You could use a compression calculator and figure 39 cc for 215 chamber vs 54 cc for the 300. .020 for steel shim vs  .040 for composition head gasket.  I would deck the block .030 and skim the heads to make sure they are flat to regain some compression. Here is a compression calculator:

http://www.csgnetwork.com/compcalc.html

[MGBV8]

The valves would be much less shrouded by using the bigger bore of the 3.9L.