E30 Zone

Bit of a random question, but does anybody know if the M20B20 engine block is shorter from the centreline of the crank to the deck than the M20B25 block?

Basically I'm pretty sure my pistons are right, but they stick up above the top of the deck and I'm pretty sure they shouldn't!



I'm using an M52B28 crank and 135mm rods, and when I measured the pistons, the gudgeon pin was 4.5mm closer to the top of the piston than standard which would allow for the 9mm increase in stroke.....which all leads me to think that maybe the block is too short

'Elp!

Cheers!
Alex

Dont you use the 130mm rods from a 323/320 or ETA for a 2.8
The blocks are the same apart from the 320 block dont have the steam holes.

as above only the 130mm rods wil work for both 2.7 and 2.8 build. 135 rods make it look like yours!! and all 130 rods are the same irregardless where they come from i.e. from 2.0, 2.3 and ETA

second the steam holes (2.3 does not have them either - they are not necessary but better to have) and of course the 2.0 is 80mm vursus 84mm bores, they can be bored out but cheaper to use the 84mm 2.5 block with the steam holes.

as i am sure you know the BMW stretch bolts are only good once.

mrlee

Are these custom pistons or not?

Turbo-Brown wrote:


I'm using an M52B28 crank and 135mm rods, and when I measured the pistons, the gudgeon pin was 4.5mm closer to the top of the piston than standard which would allow for the 9mm increase in stroke.....which all leads me to think that maybe the block is too short

'Elp!

Cheers!
Alex

Yes they're custom piston's.

The only thing I can think of is that your block has been decked at some point.... Or the pin is not quite 4.5mm higher up the piston.

Looking at my B25 pistons, yours are a bit flatter on top, as if they've had some shaved off the raised bits.

In the wiki im sure it says use the 130mm rods


To keep the con rod angle correct, BMW made the 325i conrods 5mm longer. So if you fitted a 2.7 81mm crank into a 325i block (basically the same as the Eta block), the pistons would stick out of the top of the bore by 3mm. To get around this, you need to use 130mm Eta/2 litre rods. Then the pistons don't come to the top of the bore - about 2mm short in fact. It is no good guessing how much to take off the block face because early Eta blocks were very slightly taller (we are talking half a mm maybe) so you need to fit a 325i piston and 130mm rod to the Eta crank and block, take it to a machine shop and get it measured up properly to get an exact figure. It will not cost any more, and it will be bang on.

Hope this helps man...

Mike

Sorry guys, I wasn't very clear in my first post what I've been working with.

I've got the M52B28 crank, 135mm rods and I had some pistons custom made to suit so that I could keep the 135mm rods without having the pistons poke up above the deck.

In theory, the custom pistons should have had their gudgeon pins 4.5mm further up than standard, and I am 99% sure that they have (they are flatter on top than standard as they're 8:1 compression)

What's really puzzling me is why they do poke out, especially as I can't imagine the 320i which the block came out of had had its block decked or the standard pistons would've done the same

Get the feeling I need to strip this all down again and measure up don't I?

206mm CL of mains to deck is the given OE spec Alex(B20B23B25, B27E are 206.5mm)

Dont forget that the pistons do pass right through the headgasket fire ring when @ TDC, so you'll need to factor in the HG thickness as well dude.

TBH they look OK in the pic

Hey Ant, how ya doing mate? Had hoped you might spot this thread

Maybe I'm crapping my pants over nothing then.

Guess the next thing to do is to put some plasticine on the piston and drop the head on with gasket in place.

I make the buildup of a standard B25 set of pistons this:

Compression height: 34.2mm
Rod length: 135mm
75mm stroke/2: 37.5mm

Giving a total from crank centreline to piston shoulder of 206.7mm....so if I've got about a 0.7mm protrusion, I'm back in business!

Do a dry run Alex, you could always go with an oversize headgasket if your still not happy?

To really realize your current compression ratio you´ll have to measure the heads cc´s.
as well as how much the piston is poking out of the deck height in cc´s

factor in the headgasket thickness + stroke and you´ve got it exact.
I suggest you try and find this out,

You can use clay or similar molding to create a piston depression over the deck to then measure against if you don´t want to take them out.

Certainly a dry run to check for piston to head clearance is on the cards Simon! Just hope there's 1mm or more clearance!

Will have a measure of the chamber and piston volumes too, just to be on the safe side.

The pistons do look much flatter than stock ones though.

I'll keep praying that everything's as it should be!

Cheers for the input guys, much appreciated as always!