3.0L, 84mm bore, 89.6mm crank?

[whodwho]

I believe the stock protrusion on an b25i is .7mm

you should be able to compare the stack height and see what you get for a protrusion of yours

1/2 stroke + rod length + piston compression height - block height
37.5 + 135 + 34.2 = 206.7 - 206 = .7

[CarsSuck]

http://www.r3vlimited.com/board/showthread.php?t=243359&highlight=b25+piston

Theres a good picture of a stock b25 piston in the block in there. The top of the crown is more than 1 mm.

[gath]

I believe the stock protrusion on an b25i is .7mm

you should be able to compare the stack height and see what you get for a protrusion of yours

1/2 stroke + rod length + piston compression height - block height
37.5 + 135 + 34.2 = 206.7 - 206 = .7

This is what seems to be the protrusion in my case
44.8mm + 135mm + 28.32mm CH = 208.12 – 206.7 M20B25 BH = 1.42mm

I 'ope the figures are correct because I have two conflicting sources that give the m20b25 block height as 206.2mm and 206.7mm, feel free to give input.

BTW, Stock headgasket is 1.75mm when compressed.

[whodwho]

Those heights are the stack height of a eta and I engine

40.5 + 130 + 35.7 = 206.2 for an eta stack
37.5 + 135 + 34.2 = 206.7 for an i stack


So with the .7 protusion and a 1.75 gasket would leave about 40ish thou squish which sounds about right

[digger]

i measured 2 M20B25 blocks that were 206.2mm, coincidence? i think not

[gath]

This is m54b30 internals in m20b25 block
44.8mm(m54b30) + 135mm(m54b30) + 28.32mm(m54b30) CH = 208.12 – 206.7 M20B25 BH = 1.42mm

[dustyperez]

here is a page with some stock piston compression height numbers. I am doing this build as well and i searched and searched for a suitable stock piston. I didn't wanna machine one down since i will be boosting. And its also possible to get an almost double thickness head gasket. but in the end i chose cp pistons from vac, yes expensive but they know their stuff.
http://cars.forumsee.com/a/m/s/p12-1...ion-ratio.html

[supernaughtIX]

^^^ How is the progress on the build and did you get the #s to machine down the m54b30 pistons?

No progress on my stroker for many months now, I've been working on a major project for work, and spent 3 months in india. Now I've got 600 beehive boxes to make before I can touch the stroker again(workshop space conflict).

All I know for sure is the cutting required is minimal, but valve reliefs will be a must. I have to get my M54 assembly back into a block again to measure then cut on the mill.

[gath]

No progress on my stroker for many months now, I've been working on a major project for work, and spent 3 months in india. Now I've got 600 beehive boxes to make before I can touch the stroker again(workshop space conflict).

All I know for sure is the cutting required is minimal, but valve reliefs will be a must. I have to get my M54 assembly back into a block again to measure then cut on the mill.

Wow! That's gonna be a lot of "Bzz Bzz Bzz" n honey Lol, good luck with both projects.

[gath]

i measured 2 M20B25 blocks that were 206.2mm, coincidence? i think not

Here is some useful info on the deck heights and more

http://www.e30zone.net/e30zonewiki/index.php/M20

http://www.daham.org/16vt/technical/bmwm20/

[digger]

Here is some useful info on the deck heights and more

http://www.e30zone.net/e30zonewiki/index.php/M20

http://www.daham.org/16vt/technical/bmwm20/

useful?

i have seen that data years almost 10 ago and everyone refers to the same original piece of info regardless of whether it is correct or not.......
i just checked again and my M20B25 block code 256k1 was 206.2 so i find it unlikely that exactly 0.5mm has been machined off both my blocks. So the answer is to measure as engines are build to fractions of mm and relying too much on internet data is risky. if you were to rely on a measurement you would use the smaller one as you can always skim off the block but you cant put it back 'properly'

also have plotted both a M20B20, M20B23 and M20B25 cams and dont agree with the Wiki data

[turboplzz]

what ever happened to this m54 rotating assembly measurements? I was super curious reading through the thread waiting to see the results of the piston material that needed to be machined and then it just ended like a bad movie. I'm super curious as to the m54 rotating assembly because I'm trying to get the most displacement without having to buy custom pistons. I would 24v swap it but just love the sound of a worked m20 theres nothing like it and the added weight of the 24 valve swap and already low oil pan I just want to keep it an m20 car. so please someone chime in on what happened here I know its been soooo long since this has been discussed but what happen to it????? I'm super bummed I got to end with no results I'm sitting here with blue balls

[digger]

It’s difficult to machine a set m54 pistons to have the proper squish characteristics and CR.

On a 3L you basically need a 5 to 6cc net dish or so for 10.5:1, thats the same effective dish as the OEM 8.8:1 pistons (hemi dome and offset spherical dish). those ones done by IXER for a 2.8L m20 on e30tech will be too high CR (like 12.5-13:1) for mild 3L street engine on pump fuel.

ive got 3D cad designs including central dish or offset but id just buy a set of 4032 forged slugs rather than stuff around trying to implement something onto a 24V piston unless you own the necessary equipment to machine them and play around/experiment etc

you could do a flat top with central dish but its not gonna work that great, be prone to knock etc

[bmwmaster81]

Im also on the same route. Have got the whole m54b30 internals here since 2 years and would like to build it the cheap way with stock m54 pistons.

The pistons are about 7mm thick. My plan would be to shave them down about 1mm and cut 2mm pockets aditionaly. That would leave a thickness of about 4mm on the thinnest place.

Would this be enough?

I planned to modify the m20b25 chambers to make them hemispherical. Make them round and cut the squish away.

In this way i maybe could get the CR down to about 11.0.

Would this also leave the knock proned characteristic. I mean there are a lot of hemi engines and they run well.


Im not sure how much cc i could gain with this mod.
I think the original shape has about 40cc ? With mod maybe 43 cc?

What do you guys think?

[digger]

you are much better off starting with a 731 nicely ported head and bigger valves, which will work excellent with flat top piston