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I just want to know if I put a B20a5 head on my B21a1 Block would I get a compression ratio bump?

I only sway threads for B21 heads on B20a blocks.

Thanks in advance.
 

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OT HOMO
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I just want to know if I put a B20a5 head on my B21a1 Block would I get a compression ratio bump?

I only sway threads for B21 heads on B20a blocks.

Thanks in advance.
think of it this way the b21 is a 83 mm bore with a head that was made for a 81mm bore so it would increase compression...
 

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There was some discussion about this in the past couple of years. I remember reading several threads about doing this swap. It may take you a while, but you should be able to find them if you search through some older threads. But yeah, it definitely will bump the compression a little.
 

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the exact compression ratio of a b20a5 head on a b21 block is 9.94:1.
with the practically guaranteed minor amount of carbon buildup, basically just think of it as being 10.0:1 compression.
stock b20 compression is 9.0:1 and stock b21 compression is 9.4:1

just remember that by doing that swap you will need to use 89 octane or higher at all times.
and you need to use a b21 head gasket still anytime either the block or head is from a b21, otherwise it wont seal if you use a b20 HG with the smaller diameter
 

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This might seem redundant but if you were to bore out a B20 to a B21's bore size which head gasket would you use in that case? B21 I'm assuming?
 
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