Can I install my pistons backwards? Hear me out...

[JamesRamsay]

I'm putting a 1.8 head onto my 2.0 E12 block, reusing my 2.0 pistons, and I'm thinking of doing something silly. The 1.8 head has a totally different combustion chamber design, so in order to use it with my 2.0 pistons I understand I'm going to need to remove material from the combustion chamber or the piston crowns. However, looking at the parts in front of my, another plan comes to mind:

 

 

1.8 pistons on the left, 2.0 pistons on the right. Annoyingly, the raised areas of the pistons are almost 180° opposed to eachother. You can see that where the 1.8 pistons are raised, the 2.0 pistons are flat, and vice versa. I made an overlay image to see where I'll have to remove material and it looks like a pretty arduous job that'll leave me with a pretty low-compression engine:

 

 

Alternatively, what if I just put these pistons in backwards? Suddenly I've got no more interference, no grinding/filing necessary, maybe not an ideal compression but at least a working engine:

 

 

So my question is, obviously, is this possible? I'm no mechanic, just a bozo cobbling an engine together, so I don't know if I'm making the rookie-est of rookie errors here. If I'm being dumb, please feel free to tell me 😂. Are pistons symmetrical, in terms of where the wrist pin connects? Am I going to screw anything up by flipping them around? And, assuming it's possible, can I install the whole piston/rod assembly backwards or should I keep the rods at their original orientation and only install the pistons backwards? I'd really appreciate some advice on this from you guys who know what you're doing.

[uai]

1 hour ago, JamesRamsay said:

Are pistons symmetrical, in terms of where the wrist pin connects? Am I going to screw anything up by flipping them around? 

No and Yes

Edited May 5 by uai

[Tommy]

I like the simplicity of the answer above 😄

There's reasons why usually the first advice is to be sure to match the pistons with the right kind of head.

[TobyB]

uai's answer is what everyone says.

 

MOST pistons are asymmetrical.  Thus, they have an arrow on them, pointing forward.

 

The E46 pistons I'm working with right now are obviously cast asymmetrically, 

with significant differences on the skirts for thrust and non- thrust side.

Put them in backwards, and they won't last long.

 

YOUR pistons are asymmetrical- I mean, jeeze, just look at the crowns of them.

 

BUT now look at the undersides.  Measure the location of the wrist pin, the thicknesses of the skirts,

see if you can figure out where (if) there are strengtheners cast into the form.

If that all measures identically, you can certainly try it.  The worst that will happen is that they will

wear rapidly and develop piston slap.

but I bet you discover that the wrist pin's not exactly centered in the piston....

....and putting that in backwards will cause excess friction, wear, and a short, possibly galled life.

 

t

it's good to question things like this, and measure for yourself.  After all, the absolute worst that'll

happen is that it'll die quickly and take the block with it.

 

[conkitchen]

FWIW: I put an e21 head on using e12pistons (piano tops) by reshaping the chamber. I made a molded plug e12 chamber impression out of an oven hardening clay. Then I started grinding away with a hand grinder ever so gently. Slowly checking by test fitting the "plug" into the chamber being reshaped.  Worked great and on a turbo boosted motor. I would say why not just reshape the head to work with your pistons. Or as the wise old one Toby asks, what's the worst that'll happen? 

 

But I'm crazy like that-I put a double row timing master link on a single row chain. They all said it would blow up. 

Edited May 5 by conkitchen

[tech71]

7 hours ago, JamesRamsay said:

However, looking at the parts in front of my, another plan comes to mind:

That's a bad plan.

Better plan would be to install 1.8L crank and pistons into your 2L block. Yeah, you lose .2L displacement but it might not blow up quite as fast🙄

Edited May 5 by tech71

[JamesRamsay]

11 hours ago, uai said:

No and Yes

 

Fair enough, can't argue with that 😄

[TobyB]

6 hours ago, tech71 said:

That's a bad plan.

Better plan would be to install 1.8L crank and pistons into your 2L block. Yeah, you lose .2L displacement but it might not blow up quite as fast🙄

Wait a minnit- 1.8 pistons, 2L  crank, and then REALLY mill things down until it all fits.

 

11:1, 12:1, 14:1?  All possible!

 

t

would measure to see about the backwards thing, and then when it doesn't work, do the 1.8 crank.

The better fit and squich might actually make more powah than pistons that really don't want to fit.

[tech71]

26 minutes ago, TobyB said:

Wait a minnit- 1.8 pistons, 2L  crank, and then REALLY mill things down until it all fits.

Thats not what I said dude....

"Better plan would be to install 1.8L crank and pistons into your 2L block".

Edited May 6 by tech71

[JamesRamsay]

To be honest I think I might just build a 1.8L engine after all. I have this beautifully clean E30 1.8 sitting on a stand and here I am coming up with all kinds of goofy schemes to frankenstein it up to two litres. Just making life hard for myself; my car is a 1602 anyway!

 

Yeah, forget this. I'm going to throw some seals and gaskets at this 1.8, swap over anything I need from the old engine (sump, engine mounts, etc) and call it a day. Throw my 2.0L crank and pistons up for sale and get the money moving in the right direction. Thanks for bringing me to my senses, fellas!

[Tommy]

30 minutes ago, JamesRamsay said:

 Throw my 2.0L crank and pistons up for sale and get the money moving in the right direction. Thanks for bringing me to my senses, fellas!

Or you can save them and maybe build a 2l later when you stumble on some more parts that fit the picture. 

[TobyB]

11 hours ago, tech71 said:

Thats not what I said dude....

"Better plan would be to install 1.8L crank and pistons into your 2L block".

Oh, of course not- but you got me thinking, and it was gooooodddd.

 

James, you are on the right track to having a reliable driver sooner than later.  

 

t

 

[conkitchen]

13 hours ago, JamesRamsay said:

. Thanks for bringing me to my senses, fellas!

Well that is what this forum is good for. All sorts of good bad information.