Stroke on a 1.8

[Beantown]

Hello all. 

I am building a low budget 1.8 for my 1973 BMW.  I know, I know,  it should only have a 2.0 in it. Well, its what I have so that's what its getting for now...

 

I have 2 blocks that I am using to build one and have run into a problem with the length of the stroke on both blocks.  I am using the block from one(block #1) and the pistons from another(block #2).  After installing the #1 piston and connecting rod from block #1,  the piston clears the top of block #2 by about 5 mm.  So it seems that I have to either use the crankshaft from block #1 or the connecting rods from block #2.  So my question is,  would it be better to use the block #2 crankshaft to gain a shorter stroke or use the shorter connection rods from block #1 to gain a shorter stroke?  The blocks are both the same height.

 

The reason I'm not using block #1,  is because it was locked and has rust on the cylinder walls

The reason I'm not using the piston from block #2 is that they are a little beat up and are flat top pistons(Block #2 has domed pistons)

 

Thanks a bunch for all your help.  I just hope I can keep focused on this project and actually finish it...

 

Beantown

[PatAllen]

M10 block heigh are the same on e21/318 1.8 and 2002 2.0

only the crank stroke and piston pin location is different

same rods

 

hth

[PatAllen]

 

[Preyupy]

Pat is correct. The blocks are the same, the difference is the stroke of the crankshaft  1.8 is 71mm stroke and 2.0 is 80mm stroke.  The connecting rods are the same, the difference is the position of the wrist pin in the piston it is 4.5mm lower on the 1.8 piston.  That is why the piston is sticking 4.5mm out of the top of the block if you are using a 2.0L crankshaft. 

[Beantown]

Ahhh.  Great post,  thanks Pat! 

 

After inspection,  it seems that the block I am using has a 2.0 crank installed.  So now the question is, should I put the 1.8 crank in that I have or look for pistons from a 2.0?  Anyone have any pistons from a 2.0 they want to sell?  89.97mm.   I hear that the 2.0 crank is stronger,  but would I even notice the difference if I put the 1.8 crank back in the block?

 

Thanks all.

[PatAllen]

if it where me id go with 2.0L all the way thru.

its a 2002 after all.

you need to know what head is planed to be installed to get proper piston and expected CR

[Beantown]

The head is a 1.8 from a 1981 320i.

[PatAllen]

humm, with the CIS injector holes....

i dont know what 2.0 piston would fit the 1.8 head other than flat tops, which will give you around 8:1 CR

otherwise you will need to check piston clearance if you choose other.

oem used high CR pistons for the 1.8 head is quite hard to source in america.

better get another head, or another complete motor

someone else will chime in

 

hth

[2002iii]

The block is exactly the same but the head is different. I would suggest getting a 2.0 head and pistons. I would put an add in the parts wanted section or just keep an eye on the parts for sale. 320 heads are only good for the internals, the bare head is just a big chunk of scrap aluminum.

Edited August 28, 2019 by 2002iii

[Beantown]

Thanks so much for the info.  So why are the 1.8 heads so bad?  I understand that they have the injector holes that need to be filled,  but what else makes them so bad?  I thought that the 1.8 heads were decent?

[PatAllen]

they are not "that bad" they are just not really meant for the 2002. they flow well, the combustion chamber is the most evolved of all the heads variations.

what makes them bad:

it has the injector holes (bad #1) and the piston choice is limited (bad #2) and the hole for the fuel pump rod is not drilled (bad #3) AND the distributor gear on the cam is meant to work backward compared to 2002 cam

 

hth

[xr4tic]

2.0 is the way to go.  You would lose HP because of compression ratio with the flat tops, but you would lose more HP by going to a smaller motor.

 

Plus 8:1 is a good turbo compression ratio.

 

Where are you located?  I have a used set of flat tops from my motor, no idea of condition, you can have for the cost of shipping.  I also have some used piano tops, but it would require boring the block and a matching head.

 

 

[PatAllen]

please dont make a korean drama with hp losses vs cr and lets be realistic

 

keep in mind that most us spec delivered 02 had very low/moderate CR to accomodate emissions, they where all rated at 100hp square

differences in HP is very marginal at thoses magnitude of CR on a stock motor

 

even if you choose the 1.8 head, flat tops and 2.0 crank you will still have a very potent usable and fun 02 to drive

 

you dont loose nothing if you dont have anything to start with. my 02 cent.

[kbmb02]

2 hours ago, PatAllen said:

please dont make a korean drama with hp losses vs cr and lets be realistic

 

I'm glad you're back, Pat. -KB

[Beantown]

Wow!  You guys are great. Thanks for the info and thanks for dealing with my ignorance on the subject.  Even though I have owned 2 of these in the past,  I never had to get into the nuts and bolt of the engines.  So are you saying that if I run the crank from the 2.0 in my 1.8 block, It brings it up to a 2.0?

 

XR4,  thank you so much for the offer.  I live in Asheville NC,  Do you think the flat top pistons are good enough to use?  and are they 88.97 mm? 

 

Pat,  thanks for the honest response.  This is what I needed to hear.  I could take the first block and bore it and build a better engine once my 02 gets on the road and funds are more available. 

 

Thanks again!