9 replies to this topic

[rowanxx]

Will probably be looking to increase the compression ratio of my mini due to a new head.

Is it preferable to skim the head or to deck the block bringing the pistons in line to the top (or very slightly below)?
Difficult to fine info on the internet as searches aren't helping.

[Spider]

Well, I tend to skim the block and bring the pistons flush (or in some builds, have the pistons just poking out    ), then any further adjustment take off the head.

[nicklouse]

It depends.

Are you having any head work done?

Often it ends up with a bit of both.

Your engine builder will be the best to advise.

[rowanxx]

I will be building the engine myself, only taking to a machine shop to hone, skim and press in some bearings. Thinking of decking block, as new 998 blocks are cheaper than nice heads.

[dklawson]

Have the machinist check the block.  If it is flat and true, leave it alone.  It is much easier to skim a head at any point than to mess with a block.  However, to each his own.

[Rob Gavin]

Do some calculations and see what you actually need to remove before you make a decision.

[carbon]

As Moke Spider says, the main reason for decking a block during rebuild are to get optimum piston crown heights. Other reason is to get rid of any warping of the deck.

 

Before decking a block you really need to do a dry build of the engine, as the compression heights of pistons can vary quite a bit (possibly more so for 1275 than 998 motors). No point in taking more off the block if the pistons are already sitting flush.

 

As Doug says, for raising compression ratio you would be best to skim the head.

[Cooperman]

Well, I tend to skim the block and bring the pistons flush (or in some builds, have the pistons just poking out    ), then any further adjustment take off the head.

 

That is what I always do. Bringing the pistons flush, or within a few thou of the block deck at TDC is best. Then tir CR is finally set by skimming the head to get the combustion chamber volume correct.

With a 998 engine with a fully gas-flowed head it can be necessary for the pistons to 'stick out' above the deck level slightly to get the CR high enough.

[Spider]

Just to perhaps de-mistify why I suggested skimming the block and bring the pistons flush. This is to maximise the squish between the piston and the combustion chamber. By maximising the squish, we get one final 'go' at atomising the fuel, before ignition.

[Cooperman]

When I do the 'trial build', if the pistons are more than 0.010" down the block at TDC I always skim the block to bring the highest piston to between 0.002" and 0.004" from the block deck level. As above, it optimises the 'squish' for best performance and economy.