Background

As was mentioned in the Fox Tuning Guide, Volkswagens came with two types of 8v cylinder heads (excluding the cross-flow head, which I won't discuss, because the work involved to install one in a Fox is too extensive).
• "Small valve" cylinder heads have the following valve sizes:
• Intake valve 38 mm
• Exhaust valve 33 mm
• "Big valve" cylinder heads have the following valve sizes:
• Intake valve 40 mm
• Exhaust valve 33 mm
Due to the design of cylinder heads, oversized valves can also be installed.  I currently have 40.5 mm / 33.5 mm valves installed.  I have heard that it is possible to install 42 mm / 35 mm valves into a stock 8v cylinder head, but I do not know how this is physically possible, due to the available space between the valves.  Have a look at the pictures below.  I have purposefully left them in a high resolution so the details are clearly visible.
 

	Small Valves: 38mm 33mm
	Big Valves: 40 mm 33 mm
	Oversized Valves 40.5 mm 33.5 mm

	Here's a comparison of my stock Fox cylinder head ([left] 38mm / 33mm valves) and the one I upgraded ([right] 40.5 mm / 33.5 mm valves).  Click for a big picture.

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