Recommended Piston Pop-Up Height for a Decked Block?

[Anthony]

I've just had one of my 2.5 blocks decked to remove the recess.

After test fitting the pistons they pop-up out of the block by 25 thou.

I understand the Payen head gasket usually has a compressed thickness of 34 thou.

 

Given my current measurements I'd have a squish of only 9 thou, which would most likely be asking for trouble.

I have seen a few people recommend 5-8 thou pop-up for the pistons, which would give 29 -26 thou squish.

 

My question is:

Is there a recommended squish for these engines?

Unless anyone has any other recommendations I'm planning on machining the pistons to give a 5 thou pop-up.

 

Thanks,

Anthony.

 

[TimW]

I've no idea what the correct squish should be but when I decked the 2.5 pi saloon block the engine builder didn't machine anything off the pistons. I was running a standard 2.5pi head at the time but I've since fitted an early TR6 head thars been ported, chambered and heavily skimmed. I don't have any issues and achieved 176.6bhp on the dyno a couple of weeks back. 

[Nick Jones]

25 thou is too much IMO. You need to allow for some thermal expansion and some rod stretch. 

I went with 5 on my 2L.  This was after skimming the block to remove the recesses and then skimming the pistons back..  I don't remember what pop-up I had initially, before skimming the pistons, but I don't think it was as much as 25.  I do remember that it varied somewhat between cylinders and I spent some swapping pistons and rods around to get the most even results and each piston was marked accordingly and machined to give the 5 thou on the nose.

I'm surprised you have as much as 25 as the 2.5 pistons are usually around 20 thou down from deck to begin with and the recesses around 40 thou at most.  Are you measuring in line with the gudgeon pins ?  This pistons don't sit quite flat in the bores when fitted without rings so you can get some odd measurements if measuring away from the pin line.

So I wouldn't argue with your 5 thou.  It's working just fine for me.

 

Nick