Engine - Over-boring blocks for 73.0, 73.5 or 74mm diameter pistons

Issued 10 Sept 2008 Amended 3 Oct 2008

I frequently hear the line “big-bore engines tend to be unreliable long-term because of head gasket problems”. This is complete rubbish, even when non-Payen head gaskets are used. These big-bore engines can be supremely reliable - providing they are machined and built correctly in the first place. Something that continually astonishes me is that this (correct machining and build) doesn't happen even when engines are built by some of the self-proclaimed A-series 'experts'.

The main problem stems from the fact that when the head is torqued down and compresses the gasket’s fire ring around the top of the bore, it reduces the internal diameter of the gasket to around 72.5mm across the centreline of the bore - i.e. between the cylinders. This is not much of an issue where 73mm pistons are used since all pistons are a smaller diameter at the crown, and even smaller across the wrist pin axis as they are also slightly ovalised. However, where the larger 73.5mm and 74mm diameter pistons are used, unless steps are taken, the piston crown will clip/hit the gasket where it over-hangs the bore. The obvious and unavoidable conclusion is the head gasket will fail - either quickly or after some time. But it will fail.

There are a couple of ways to prevent this from happening, rather than hoping to get lucky. First is to make sure the block is bored using the head gasket to centre the re-bore rather than the original bore centres in the block. These original block bore centres can be anywhere between spot-on and central to the gasket or absolutely miles out. Not much of an issue on the usual over-bore sizes of +0.020", 0.040" or 0.060" - hence the factory's lack of attention to detail at getting the bores located more accurately. So when getting your block bored to take the much larger piston diameters, make sure you supply the company with a head gasket with instructions to centre the re-boring to the gasket, across the centreline/wrist pin axis plane. If they are familiar with or are generally seen as A-series specialists, just ask the question and make sure they are doing this.

Where a block has already been straight bored out using the original bore centres, position a gasket on the block using several head studs, but especially picking up on the gasket locating stud holes. These locating stud holes are the short stud between cylinders 1 and 2 and long stud behind cylinder 4. These head stud holes in the gasket are 3/8" diameter whereas all the others are 7/16". With the gasket located in this way you can now assess how much and where the gasket fire ring over-hangs the bores, bearing in mind it will over-hang a little more once clamped down by the head. If you are lucky and there is barely any over-hang and it is distributed evenly all round the bore, you will get away with it dependant on piston position at top dead centre. If it over-hangs more on one side than another, front, rear or side to side, then steps need to be taken to ensure no gasket failures occur because of the piston hitting the fire ring.

One option here is to make sure the pistons are located a short way down the bore from the block face when they are at top dead centre - 0.010" to 0.015" below would be plenty. Do a trial build to establish piston to block face distances. If the pistons are less than 0.010" down the bore, then the piston crowns will need machining down to make sure they clear the gasket fire rings. The main problem here is that you have to have the pistons fitted to the rods to be able to make the measurement in the trial build. Not everybody has build pins - wrist pins with a section ground down to be a slide fit in the con rod - that can be used as 'floating' pins in the rods/pistons during the trial build. The vast majority of folk will be using the original type interference-fit wrist pin to rod set-up. Also - once the pistons are properly fitted to the rods, not all machine shops are capable of machining the piston crowns with the rods attached to the other end. The answer here is to get the piston crowns machined before they are fitted to the rods. The best way of doing this is to machine either a reasonable chamfer or a small 'step' in to the piston crown edge to allow for the gasket over-hang.