About my rebuilt engine ...

[d.hitchcock]

My M10 has piano-top pistons and a rebuilt "e21" cylinder head.

 

Is this enough information to tell roughly what sort of compression ratio we've got?

 

Thanks in advance for any insight. From the noob section ...

[Hans]

you would need to figure out how much has been pruned from head. Is it off the car?

[Mark92131]

There were 3 different OEM "piano top" pistons for the E12 Head.  Each piston's piano was progressively thicker.

 

8.3 to 1 for regular 02s

9.0 to 1 for US spec Tiis

9.5 to 1 for Euros spec Tiis and built motors.

 

Hard to tell what you are running under that E21 head?  Either the pistons were modified (see 4th photo) or the E21 Head was modified for the piano top pistons.  If the head was decked, that will also change the compression ratio.  If the pistons are aftermarket (QSC), their "piano top" pistons are very aggressive (high compression).

 

Mark92131

[Conserv]

My ‘76 (49-state version) retains an E21 head. It has custom aftermarket pistons one might describe as “quasi-piano-top” to achieve a compression ratio of approximately 9.3 to 9.4. My ‘76’s head has not been shaved.

 

Regards,

 

Steve

 

Edited December 12, 2018 by Conserv

[mac]

The answer to your question is, "No." 

 

Mac. 

[williamggruff]

If you have an e21 head and stock piano top pistons from an earlier M10, you aren’t going to have any compression because your pistons are going to interfere with, and bend your valves. 

[d.hitchcock]

The head is off the car at the machine shop, so I'll get some numbers back to indicate whether it's been "pruned" before.

Thanks for all the background on piano-top pistons. The ones in my engine have no markings on the tops, but they look most like "Conserv's" to my untrained eye. They are evidently not piano tops from an early M10 as there is no interference.

Thanks, Steve, for the data points on your engine.

Edited December 12, 2018 by d.hitchcock

[Simeon]

See post from the well informed @LarsAlpina

 

 

[d.hitchcock]

Conserv's piston pictures in that thread look the same as the ones in my engine.

[xr4tic]

2 hours ago, williamggruff said:

If you have an e21 head and stock piano top pistons from an earlier M10, you aren’t going to have any compression because your pistons are going to interfere with, and bend your valves. 

 

Not entirely true.

 

The spare motor I bought had e12 domed pistons and an e21 head.  It had been rebuilt (poorly) at some point in time.

 

It can work, but it's not recommended.

[Simeon]

I am sure that a  BMW dealer actually recommended that I buy and the modify an e21 head to go with e12 pistons. Part of the squish area in each chamber of the head needed grinding away.  Not clear if that was BMW’s advice or just that guy. 

 

Maybe in a post apocalyptic world I would do that so I was mobile enough that I could raid my neighbors for their irradiated water. 

[williamggruff]

6 hours ago, xr4tic said:

 

Not entirely true.

 

The spare motor I bought had e12 domed pistons and an e21 head.  It had been rebuilt (poorly) at some point in time.

 

It can work, but it's not recommended.

 

Your experience was perhaps better than mine. The motor in my first 2002 had been rebuilt by the previous owner immediately before he sold it to me. It had piano tops and an e21 head tha. It ran poorly on the way home. I pulled the head and found the pistons all had valve impact marks, despite having had valve recesses machined into them. 

[d.hitchcock]

We'll be claying the pistons/head. I'll report back.

I'm looking forward to the IE 292 cam and big-valve goodness.

 

[d.hitchcock]

Gosh, I apologize for letting this slide, fellow FAQers.

Reassembly went fine with my engine, and there was no interference in the valve train.

During initial dyno tuning, the motor made 127.5 whp and 125.5 ft-lbs

Edited March 29, 2020 by d.hitchcock

[TobyB]

Did you ever find out where the pistons came from?

 

Those are pretty healthy numbers for a 'street' M10

 

t