Hello friends!
I am new on this forum and I apologise for being total newbie in matter of BMW and mechanics (and sorry if I posted this in wrong category).

I got a problem while working on my stroker E36 M50B20 (1992). I am not sure what if I understand correctly calculations I was trying to made, so please, correct me if possible!

First of all, I got stock non-vanos M50B20 - what I am trying to do, is to make 2533cc with changing the crankshaft from M52B28 engine. I would also change intake, injection, air throttle from M50B25 - so no worries. After while I will invest in bigger pistons (86mm) and will make M50B30 - but this is the future.

The problem is - I have completely no clue what should I do about pistons. I mean - I got stock ones with 42.8mm compression height which.. according to my calculations, it's too big.. Let see:
We got Deckheight - 1/2Stroke - RodLength - Compression Height which means:
210mm (I think) - 84/2 - 135 - 42.8 = -9.8mm! So I got nearly 1cm more than 210mm and 9mm compared to stock setting (210.8mm).

Ok, will it fit? Do I need to change my pistons? Or try to get shorter M50B20vanos or M52B20 which are 31.64? and 31.7mm? Then I would lack some height and my compression would be smaller..

About compression, I do not get the calculations. I found some on this forum and would like to use it here:
Swept Volume = Pi/4 * Bore^2 * Stroke
Gasket Volume = Pi/4 * Bore^2 * GasketThickness
Clearance Volume = Pi/4 * Bore^2 * Clearance

I have no idea where to get clearance and gasket thickness.. well.. I know gasket is 1.75 if I am not mistaken.

But I can do it another way, with using this:
Compression Ratio = Swept Volume + CylHeadVolume + GasketVolume + Clearance / CylHeadVolume +
GasketVolume + Clearance

If we take (CylHeadVolume + GasketVolume + Clearance) = R we got simple math:
(SV + R) / R = CR

Which converts to:
SV / (CR-1) = R

Now, using stock values, we can calculate our R for stock M50B20 (will switch to cm for getting values in cc):
Pi/4 * 8^2 * 6.6 = 331.75cc

Our CR is 10.5, so using formula above, we get 331.75 / 9.5 = R so our R is 34.92cc - great, we got something!

Now, with new SVs (from our modified stroker) we can get how much compression rate we can get:
SVs = Pi/4 * 8^2 * 8.4
Svs = 422.23cc

Now using our R to this formula:
(SVs + R) / R = CR
(422.23cc + 34.92cc) / 92cc = CR
13.09 = CR

Well.. its a lot - not too much? Obviously if it can fit! If not, we get new pistons.

New pistons doesn't change SV, only R - so we will just add to R some of new piston difference (assuming Ill get m52b20 one): 42.8mm - 31.7mm = 11.1mm
Pi/4 * 1.11cm * (8cm)^2 = 55.79cc

Now we add this to R and just straight forward to what compression we got:

(422.23cc + 34.92cc + 55.79cc) / (34.92cc + 55.79cc) = CR
CR = 5.654 - well.. Cannot ride that, can I?

Or I don't get something? I could get 140mm con-rods - but I can't find them - what else? Thick head gasket is kind of expensive. Maybe change head? To what? To vanos M52B20 which SHOULD be different, unless this engine block is shorter of 10mm (since complete pistons + rods + stroke/2 is 199,7) despite that M52B20 got higher compression - 11.

I am lost. Please help me chose good pistons.


PS: Swap is no matter for me, I already everything I need except for pistons/piston rings.

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Also, If I am using for example this: http://store.vacmotorsports.com/vac---s50--s52-us-multi-layered-steel-head-gaskets-mls-p1310.aspx

Do I need to calculate thickness with 0, 1 or 2 "normal" head gaskets? Or it fits in there fine enough?

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Another option is getting 130mm rods from M20 (will they fit?) and this thick Cometic gasket 0.14inch which I will get only 211.244mm (0.444mm difference between normal one) considering I need only this gasket to go. What do you think?

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I can also get theese: http://www.cxracing.com/mm5/merchant.mvc?Screen=PROD&Store_Code=CXR&Product_Co de=Rod-BMW-1390x6&Category_Code=BMW3 with M52B20 pistons (31.7mm) and I would get 212.7mm height (only 1.9mm more - will it fit, or not?)