The Standard Motor Products V-237 valve used in the last 387 orders I have sent out of these PCV systems uses 2 inches of mercury out of an available 18-20. Meaning, the amount of vacuum is so small that it is not possible no matter how worn the engine is for there to be any oil or 'oil mist' entering into the intake.To prevent this problem further, I use a 3/8" diameter tube because the bigger the diameter, (panic is correct), the less possibility of liquids getting involved. Also, placing the valve further away from the bottom of the engine (perhaps further up the tube) wouldn't make the suction any less. Lots of testing and research went into this project (now years ago) to make sure just the right valve was chosen for these particular engines. No fouling of spark plugs, no oil usage, no problems.
To answer your original question, the new style V-237 has two ports. The 3/8" one is used and the smaller one is capped off. The smaller one can be used for testing the vacuum if you so choose. I keep in close contact with most of my customers and there have been no negative comments from those who have actually purchased it. I give full refunds and no takers so far. I am glad you are enjoying the book! PCV, stock or my system is a good idea to reduce crankcase gasses. Its a good conversation to have.The road draft tube works on the Venturi Principle after the vehicle reaches about 30 mph. The rest of the time, crankcase gasses are forming and wreaking havoc. The 235/261 engines were the last to NOT have PCV, My name was in the topic, so I felt the need to set the record straight. Hope this helps!
Link on Amazon for the engine book:
https://www.amazon.com/Chevrolet-Inline-6-Engine-1929-1962-Rebuild/dp/1613254881/ref=sr_1_1?Link on Amazon for pre-orders of the full restoration book:
https://www.amazon.com/How-Restore-Your-Chevy-Truck/dp/1613255020/ref=sr_1_6?(I hope all my old friends here on the premier forum for all things Advance Design are doing well! :))
