Vent Some Steam

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POSTED ON October 30, 2017

Hot spots in an engine can lead to warping of deck surfaces, leaks and even detonation. Keeping temperatures even around the bores and in the cylinder heads can not only increase engine life – it can also make power. Perth Street Car journeyed to A1 Hi Performance, in Myaree to prise a few trade secrets from proprietor and engine builder, Leon Withnell.
When overhead valve smallblock engines started becoming popular for racing in the early Sixties, no-one could have predicted just how far their development would progress. Through the countless generations of Chevs, Clevelands, Windsors and now the LS1/2/7 family, more is being demanded of an engine than was thought possible in those halcyon “few cents a gallon” days. Modern engines are now capable of producing well over 400hp from the factory, with compression ratios around 11:1. Not only do they manage this on pump unleaded fuel but they also return incredible economy figures and will idle in the heat all day with the air conditioning blasting.
In recent times, engineers have realised the importance of even cylinder head and block cooling. This is evident in the SVO blocks made by Ford. They feature additional vent holes around the bores which allow hot coolant to flow into the main stream heading for the radiator. While these holes are not large enough to upset coolant circulation, they do serve a critical function by eliminating hot spots.

If you are a Ford fan then there is good news when it comes to both sealing and cooling your Windsor or Cleveland more efficiently. SVO produce a head gasket which provides extra holes to match their SVO blocks. As these blocks are a combination of Windsor and Cleveland engines when it comes to coolant and oil flow, this means that the gasket can be used on both types of engine. Now a standard Cleveland block does not have these extra vent holes but they can be carefully drilled through the block and cylinder head decks using the SVO gasket as a template. We suggest you take your parts and have this professionally drilled, in order to avoid a costly mistake but there is no reason why a competent DIY engine builder could not perform the operation.
Clevelands have a particularly nasty hot spot at the upper front section of the deck face. There is a very large space between the water passages at this point which may become so hot that the iron changes colour in some cases. This overheating of the block and cylinder head material naturally causes it to expand, crushing the head gasket in the process. When this superheated area is allowed to cool it resumes its previous dimensions in most cases but the head gasket does not. Once compressed past its desired thickness, the gasket will remain compressed allowing either coolant and/or gases to leak. Eventually the head gasket will fail completely at this point. Drilling two extra holes inside this trouble spot, with corresponding holes in the cylinder head can eliminate this problem entirely. How many Clevos actually run cool on the street in summer? Not many.

This SVO gasket is used on an SVO block. Note that the bottom cooling holes are aligned. This late model 302HO head gasket sits on an SVO block. Note cooling holes are misaligned.


AFR Head with cooling slots added to align them with the cooling holes in the block and gasket. This rectifies the cooling port misalignment. Extra steam holes have been added around the bores to eliminate hot spots. Note the head face has been machined with receiver grooves.

The SVO gasket is good news on other fronts as well. It also allows head and block swaps to be made far more easily. For example, fitting Windsor heads to an SVO block or even a Cleveland. Bolting your favourite AFR or TFS Windsor heads onto an SVO block will require slotting of their steam vents in order for them to line up with the holes in the block. Fitting Cleveland heads to a Windsor also requires drilling a large hole in the front of the intake manifold face to allow water to flow through a Windsor intake manifold.
A further benefit of the SVO gasket is its lock wire fire rings. These function in the same way as the o-rings used with copper head gaskets by fitting into a receiver groove milled into the cylinder head. Typically, this groove is only 10-11 thousandths of an inch deep and it must be precisely machined by a competent machine shop. This gasket was developed specifically for Ford engines fitted with supercharger, turbos or nitrous oxide as the lock wire rings are far stronger and are able to contain much higher cylinder head pressures without failure. So, the gasket offers the combustion sealing benefits of copper and o-rings without the need to use sealants or goo on the deck surfaces and risk blocking those vital steam vent holes. You see, in this modern world, you can have your cake and eat it (or should that be steamed pudding?).


The standard cylinder head prior to machining and drilling.


The arrows indicate the location of two vital extra cooling holes drilled into this Cleveland block.


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