1098cc Head & Block-Gold Seal Engine

[Stubmiester]

I am having the head & block checked by a local engineering firm, because the head gasket keeps failing. Does anyone know what is the maximum you can skim off the head and block? The engineer posed the question.

Thanks

Stuart

[philthehill]

The maximum you can skim off the top of the block is the remaining height between the top of the piston when fitted and at Top Dead Centre (TDC) and the top of the block. Normally called decking the pistons.
But in your case skim only as much as is required to give a fully machined top surface to the block and no more.
If your block has not been skimmed before that is easily achievable without worrying about breaking into a water channel. It may take more than one pass of the milling machine to achieve a good surface.
Put a straight edge on the top of the block and inserting a feeler gauge between the straight edge and the block should identify the low spots on the top of the block and allow you to determine the maximum amount to be removed i.e. the max thickness feeler gauge plus say 0.010".
The same applies to the head only take off what is required. I would not go above a 0.060" skim as there is the possibility of breaking into the horizontal oil way which feeds the rocker assy. The oil way feed can be identified by the plug on the N/S front of the head. Leave enough metal between the oil cross feed and the gasket face of the head.
If the engineer is asking you - the top of the block and gasket face of the head must be in very poor condition otherwise I would have expected the engineer to be able to determine what is required.

[Stubmiester]

On the ball or what Phil! Thanks you. The engineer asked the question when I dropped off the block and head (I am also having the crank checked as well) so I think the question was in anticipation of the worst case? never the less I will pass on the info and thanks again.

[bmcecosse]

Yes - I would wonder about the 'engineer'... If a LOT has to be taken off the block - then the pistons can of course be turned down on the surface so on assembly they don't stick up above the block surface. If you were 'blueprinting' the engine, you would carefully assemble and measure each one - determine if all the crank throws are identical, check if all the con-rods are the same length, and if the pistons are all the same height from gudgeon pin to top surface. Then these are corrected where found wanting (sometimes just be juggling the parts into different bores - although this is not 'ideal' ) and then the pistons would be skimmed to get them all to the same deck height. Finally of course - they would be balanced for weight, as would be the conrods... A lengthy process ! Do ask the engineer to tell you how much he removes from both block and head...if it is up to 40 thou total then that will be a useful increase in the compression ratio. Any more than that then you will need to ease some metal out of the combustion chambers (around the inlet valves) to keep the CR within reason.