Seeking clarification on M20 budget stroker (i pistons)

[jc1000]

Hi guys, prepping for my M20 budget stroker and want to iron out a few things I am still unclear on. I am planning on using the ETA bottom end (block, crank, and rods) with late-model/short-skirt i pistons and decking the block ~2mm. I will also be going FI, shooting for 300 hp.

1. Used pistons - Is there anything wrong with sourcing a set of used i pistons (late model) as opposed to buying new OEM replacements? I can find used ones for about $250 and want to go that route if I can.

2. How does the machine shop know how much to deck the block? - I have read that if I take in the block with the crank, rods, and new (used) i pistons, they can assemble one cylinder and then measure how much to deck the block. But what are the specifics on this? Do they measure from the upper-most part of the piston to the deck? Or from the top ring or something? Curious if anyone can explain this to me a bit more.

3. Any suggestions for rings and ring gaps? Are stock replacement rings sufficient? What should I shoot for on gaps? I will be using a 885 i head and running moderate boost, nothing crazy.

[It's Soda Not Pop]

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/64366-1-3-motronic-up-grade-2-7i-how-to

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/378740-forcedfirebird-s-m20-dyno-thread

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/253139-stroking-boring-m20b25-b27-sectioned-blocks-show-critical-dimensions-pics

https://strictlyeta.net/technical/328i/

There is a youtube channel called Denver Beer and Oil (I believe they speak about 2.7i conversions). I have a 2.7, but mine was a seta.

[digger]

if the pistons are not scuffed, worn out or collapsed they should be ok, normally you can inspect them and measure them to see if in spec from a diameter / size persepctive and see how much space between rings and ring groove (machine shop should be able to advise)

you set the piston to head (squish) clearance and adjust to 1mm+/-0.1 or so so mock it up with spare gasket and bare cylinder head and use clay or solder between the angled part of the piston and the angled part on the head. then you machine off the head to bring the measurements inline with the goal of 1mm. you can also use a MLS where you can vary thickness to fine tune things but they are a bit more finicky and expensive

ring end gaps on looser side of stock tolerance will be ok so around 0.020" 1st and 2nd. this compares to what JE recommend (for different material) 84/25.4*0.0050,0.0053 = 0.017"/0.018"
https://www.jepistons.com/ring-insta...-instructions/. People make more power than what you are proposing without modifying anything as far as ring gap so bear this in mind its unlikey to be an issue

[jc1000]

if the pistons are not scuffed, worn out or collapsed they should be ok, normally you can inspect them and measure them to see if in spec from a diameter / size persepctive and see how much space between rings and ring groove (machine shop should be able to advise)

you set the piston to head (squish) clearance and adjust to 1mm+/-0.1 or so so mock it up with spare gasket and bare cylinder head and use clay or solder between the angled part of the piston and the angled part on the head. then you machine off the head to bring the measurements inline with the goal of 1mm. you can also use a MLS where you can vary thickness to fine tune things but they are a bit more finicky and expensive

ring end gaps on looser side of stock tolerance will be ok so around 0.020" 1st and 2nd. this compares to what JE recommend (for different material) 84/25.4*0.0050,0.0053 = 0.017"/0.018"
https://www.jepistons.com/ring-insta...-instructions/. People make more power than what you are proposing without modifying anything as far as ring gap so bear this in mind its unlikey to be an issue

Thanks, digger.

When you say "machine off the head", are you talking about the cylinder head or the surface of the block? Isn't it the block I am supposed to be shaving down? Also, I had the head machined already and .008" was removed.

[digger]

Thanks, digger.

When you say "machine off the head", are you talking about the cylinder head or the surface of the block? Isn't it the block I am supposed to be shaving down? Also, I had the head machined already and .008" was removed.

Sorry it should be machine off the block not the head.

[jc1000]

Sorry it should be machine off the block not the head.

That makes sense.

So if I take the machine shop everything - the head, new pistons, block/crank/rods, etc. - and say I am looking to have a piston to head (squish) clearance of 1mm (+/- .1), is that enough info for them to take it from there?

[digger]

May need a spare head gasket but they could use some shims to account for thickness if needed but otherwise that’s it. They will mock it up measure the squish and deduct the delta.

The squish band is at a slight angle so they will need to adjust the numbers slightly. if the squish band was flat like most domestic engines you could have measure the stick out of the pistons without the head