Hi everyone, just after some help. I've cooked my 3K and most likely cracked the head. I have a spare 4K sitting around and the bottom end of the 3k was recently rebuilt so i don't want to get rid of that. My plan was to get the 4K head ported/ polished and decked and just bang it on once its ready. Seems like the only drawback ill have is a slight drop in compression? will there be any fitment issues? thanks in advance. ( Can't find any threads on this, only 3K on 4k blocks )

If they both have a CR of 9:1, (Wiki figures) then the compression drop will be the percentage difference in capacity.

 

1166cc to 1290cc is about 10%, so your compression with the 4K head will be 8 to one plus a bit, pretty useless.

 

The volume in my 4K head was about 31ml, and the combustion chamber area 30sqcm. You will have to reduce the chamber volume 3ml at least, so with the area of 30sqcm that means a skim of 3cubic cm/30sq cm or 1/10cm, or 1mm. So skimming the head by 50thou will take you back to stock compression. Then you'd think, "well, an extra pass with the mill will give better compression, so take off 75thou...." That would give you around 9.3-9.5:1

 

..and you would have to look at your rocker ratio because now the pushrods are effectively 1.5mm longer and the rocker ends might run off the valve stem.

 

So- measure the volumes of your 3K and your 4K heads and work out exactly what compression ratios you have now and will have with the 4K, then how much to skim off. If you've never done it before, research it until you understand what's going on or PM me.

 

Good luck!

why not just drop the 4k in? if you've "cooked" the 3k you might want to compression test it to make sure the rings aren't "jellied" before you go swapping heads.