# Mystery Hole

**URL:** <https://community.cartalk.com/t/mystery-hole/33128>\
**Category:** General Discussion\
**Tags:** chevrolet\
**Created:** [July 9, 2009, 5:13pm UTC](https://community.cartalk.com/t/mystery-hole/33128 "2009-07-09T17:13:27Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![rustychevy](https://avatars.discourse-cdn.com/v4/letter/r/ebca7d/32.png) [@rustychevy](https://community.cartalk.com/u/rustychevy)\
**Post date:** [July 9, 2009, 5:13pm UTC](https://community.cartalk.com/t/mystery-hole/33128/1 "2009-07-09T17:13:27Z")

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I am putting in a 6v inline engine in my 65 chevy and thought it was all hooked up but when I put the coolant it came out a hole in the engine block. The hole is half inch across within the exhaust manifold openings. I am having trouble figuring out if there needs to be anything important screwed in there or if I can just plug it up as my dad suggests.

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**Author:** ![Whitey](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/whitey/32/24680_2.png) [@Whitey](https://community.cartalk.com/u/Whitey)\
**Post date:** [July 9, 2009, 6:03pm UTC](https://community.cartalk.com/t/mystery-hole/33128/2 "2009-07-09T18:03:58Z")

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It is probably the previous home of an expansion cap (that probably isn’t the correct term–can’t remember it). These caps are made so that if someone forgets to put antifreeze in the engine, and the coolant freezes, these will pop out and might save the engine block. I would try to replace it rather than just plug it up.

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**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://community.cartalk.com/u/GreasyJack)\
**Post date:** [July 9, 2009, 6:56pm UTC](https://community.cartalk.com/t/mystery-hole/33128/3 "2009-07-09T18:56:00Z")

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Does the mystery hole have threads? It could be the engine block coolant drain.

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**Author:** ![TwinTurbo](https://avatars.discourse-cdn.com/v4/letter/t/8baadc/32.png) [@TwinTurbo](https://community.cartalk.com/u/TwinTurbo)\
**Post date:** [July 9, 2009, 7:05pm UTC](https://community.cartalk.com/t/mystery-hole/33128/4 "2009-07-09T19:05:05Z")

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Freeze plug. They’re usually larger than 1/2". If it’s low on the block, it’s likely a black drain plug. If it’s high on the block, it sounds more like a coolant temp sender to me. Never heard of a 6v inline engine though…

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**Author:** ![meaneyedcatz](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/meaneyedcatz/32/692_2.png) [@meaneyedcatz](https://community.cartalk.com/u/meaneyedcatz)\
**Post date:** [July 10, 2009, 2:06am UTC](https://community.cartalk.com/t/mystery-hole/33128/5 "2009-07-10T02:06:11Z")

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For what it’s worth…the freeze/expansion plug may do what you say it does but the real reason it is there it totally different.

When casting the block from molten metal sand is used to provide openings for items like the water jackets. After the block is cast the holes provide a way to let the sand out. I hope this makes sense, I do not fell I am articulating my words correctly.