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Username Post: SBC Water Bypass Hole In Block        (Topic#187700)
Posts: 88
Loc: Upper Left Coast
Reg: 06-12-06
04-19-08 10:42 PM - Post#1415334    

I purchased a Cloyes aluminum timing chain cover ( p/n 9-221). It comes with a gasket that does the cover and the water pump all in one piece. Pic below:

As you can see, it is a one piece gasket that services the timing cover and the water pump. The instructions state.." Make sure you place the gasket so that the water bypass hole on the passenger side of the block is covered". The hole in the gasket for the bypass is not there.

Question #1. I purchased a timing chain cover, not a water pump so why are they telling me to block the water bypass hole?

Question #2. What would be a logical reason to block this hole? GM had a good reason to put it there. Water circulation and warm water to the heater core when the thermostat is closed.


Dedicated Enthusiast
Posts: 5189

Loc: south western pa.
Reg: 01-25-00
04-20-08 04:52 AM - Post#1415406    
    In response to buns

we always blocked them in race engines because it over cools that front cylinder with extra water. a street engine with a thermostat i would punch a hole in the gasket

retired race engine builder,former NASCAR tech inspector. new corvettes owned 1959,1962,1963,1964,1965, 1966,1997,1999,2002,2005, 2008 plus 30+ other chevy cars and trucks along the way. 2008 corvette sold and waiting on a C-7.

Edited by motorman on 04-20-08 04:54 AM. Reason for edit: No reason given.

Posts: 88
Loc: Upper Left Coast
Reg: 06-12-06
04-28-08 09:19 PM - Post#1422553    
    In response to motorman

Thanks for the reply. Just today I found this little tidbit of information in a book called Small Block Chevy Performance by Dave Emanuel.

Quote ..... "Thermostat Bypass System.— Reworking a small block's bypass system can pay off with improved cooling. Under normal circumstances, when an engine is cold, a bypass in the water pump housing allows coolant to circulate enough to avoid a pressure build up when the thermostat is closed, and to allow the thermostat to open properly. Some engine builders have found that improved cooling efficiency results when this bypass hole is plugged (with either silicone or a standard plug) and four holes are drilled in the thermostat. Typically, four 1/8" diameter holes are adequate. However, Myron Cottrell of TPI Specialties recommends four .200" holes in a 160 degree thermostat for summer operation and four .080" holes in a 180 degree thermostat for winter operation. This may seem like splitting hairs, but an engine that's overcooled produces less horsepower.The thermostat modification also improves block filling since air in the waterjackets has an escape path.

Even if everything in the cooling system is operating properly, drilling holes in the thermostat and plugging the internal bypass usually results in a measurable drop in cooling temperature. And it may cure an overheating problem if the "wrong" parts have been installed. Some race waterpumps and aftermarket cylinder heads ( a passage in the bypass circuit leads from the block to the heads) do not have provisions for the internal bypass.If the circuit is already blocked, the thermostat modification will cool things down."

Source: Dave Emanuel

p.s. It's a good book. Well worth owning, in my opinion.


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