Reason for large valve relief versus small valve relief, why they made them different sizes.

1964SuperStocker

Well Known Member
Supporting Member 3
Just to poke fun :pokeand further edu-ma-kate the masses. Why is there different size of valve reliefs in different blocks? I would chalk it up to whom ever was doing the notching on the block that day. Any input would be appreciated. I haven't sized them myself but one is nearly twice as big as the other. Lets not get into how that will drop the compression by 1 or two. More interested in why they did it to different blocks. These blocks were cast 3 years apart.
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Fathead Racing

Well Known Member
Supporting Member 7
Interesting. I have only one truck block so have nothing to compare it to. If I were to see someone else's block without mine right beside it I probably wouldn't notice. Maybe the front block pad stamping could help with the intended use?
 

MRHP

 
Supporting Member 1
Change between 348 and 409. Could also be changes in heads required different notches. I have noticed the difference in size as well and this is what I have concluded. Your mileage may vary.
 

1964SuperStocker

Well Known Member
Supporting Member 3
Answeres, kind of. Big notch is 409 truck block and small notch is 348 car block. I believe it would be irrelevant which head or application the block would be used for. Large pistols would need larger valve reliefs in order to ensure a desired value to bring compression down. 1 point in compression for a 409 isn't the same as for a 348. My best guess.
 
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