Look For The 10 And The 20

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B_: If and when I rebuild my 307ci, I want to know if it is possible to safely bore it out to 4.000" or 4.030"? If I do this, can I use 327ci heads and pistons? Do the stock 307 heads have smaller valves to clear the smaller bore?

JD: No and yes. To bore a 307 to 4.000" you would need to bore it .125". This would leave you with paper thin walls (which would crack soon) or water jackets where your bores used to be. If you want a 4" bore, use a '68 or later 327 or 350 block with your crank to make a 327. Make sure to rebalance the crank as the 327 pistons will be heavier. As for valve size, they are small; not just to clear the bores, but because the 307 was never intended to be a performance engine, hence the 2-barrel.

S_: Before you do anything with a 307, check to see if the iron it's cast from is any good. Look for a "10" and "20" cast into the front of the block under the timing cover. If you don't see both a "10" and "20", the block is cast from low grade iron and should be considered a paperweight. The 307's had small valves because they were an economy engine, not because they wouldn't fit. The 2.02/1.60 valves should clear a 283/307 bore, but not a 305 bore unless you are using Trick Flow heads. The 262/267 engines are truly hopeless in the "big valve" department.

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Archivist: Tom Wilson


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