2.7 build using a m20b25 as basis?

[Sjoerd320i]

Hello lads,

It's been a while since I've been on here.

I now own a 1990 320i auto and i got me a complete 2.5 engine of ebay.
planing on a complete rebuild of the engine and maybe a 2.7.

I looked around on here but almost all the topics how to build a 2.7 are about using the eta bottom end.

I was wondering what I need to build a good spec. 2.7 using the b25 bottom end and a diesel crank.

I hope you guys can help the foreign guy out

[reggid]

http://www.e30zone.net/modules.php?name ... ic&t=89665

[Sjoerd320i]

Yes I had a look at that topick but all 3 use the eta block if I read it correctly

[daimlerman]

Eta block and 325 block are the same casting.I would try your diesel crank with a 320 rod and a 325 piston,and see how far up the bore the piston comes...we do not see the diesel crank here so you will need to experiment and let us know how you get along.

[march109]

The bore on a M20B27 and M20B25 is the same =the displacement difference comes from the stroke difference.

I'm not sure but I think the blocks are nearly identical anyway, so I would just rebuild the 2.5 bottom end with the 2.7 crank, reusing the 2.5 pistons but I expect you neet to use the eta/320i or custom 130mm pistons.

It is definatly possible to do, the only thing I think you need to change (in terms of the long engine) are the rods and crank.

[Sjoerd320i]

I'm using a late 2.5 engine so if it all is the same as an eta engine I should just be able to use one of the options from the sticky 2.7 topic?

The difference between an eta and diesel crank is the material. all the sizes are the samen stroke etc.

[reggid]

late M20B25 pistons, 130mm rods, 324TD crank, 325i head, approx 2mm off block, adjustable cam gear

[daimlerman]

late M20B25 pistons, 130mm rods, 324TD crank, 325i head, approx 2mm off block, adjustable cam gear

do take note of the 'approx' part....this will give you a very sound base that will rev. better than the eta based engines will.Good luck with the build,and keep us posted.

[Ant]

and eta blocks are taller.........

using a B25 block is bad practice IMHO, the increased stroke meaning the pistons/rings will go lower than before in the block to BDC and will pass from the "worn" used part of the bore to fresh un worn sections below.

given that oil control is the lowest ring, I'd opt for a build using a mint eta bock or a rebored
+4mm B20 block to ensure the bored are 100% round, 100% parallel and 100% unworn

BMW bores dont tend to wear to the naked eye, but there will be some wear on the thrust side of the bores and thats what would worry me.

You can do this on the cheap, but you will do it more than once..... do it right 1st ime and she'll do 150K with ease.

[Sjoerd320i]

Is it an option to hone the the 2.5 bottom end and use oversized piston rings?

[tomson]

Is it an option to hone the the 2.5 bottom end and use oversized piston rings?

Probably but as Ant said rebore a 2.0 block to 84mm, you have one already!

[Sjoerd320i]

Is it an option to hone the the 2.5 bottom end and use oversized piston rings?

Probably but as Ant said rebore a 2.0 block to 84mm, you have one already!

Yeh that's true, but I also have a 2.5 engine that's not mounted in a car.

Not sure yet what I going to do...