M42 build for E36 Compact

[Warsteiner]

I think that all measuring should be done again from the beginning.

You can raise compression by simply having less of a dish (less cc's) in the piston. It only takes a couple of cc's in the dish to make a significant difference in compression ratio without changing anything else.

I have to agree with Nick that this really does come down to measuring things. I would measure the piston CH first and see if you got the right piston. The dish looks ok from the pic. Then measure the block deck height. Should be 212mm.

That would be where I would start.

Cheers,
~Ralph

[wazzu70]

Yes, I have seen people sent pistons with the wrong specs before. Unfortunately its not all that unusual

[MrPhatBob]

I'll be over there tomorrow.
The rods were 140mm when I checked them, it has to come down to the compression height.
Lots of good info on the crank - I shall have a good old check.
The block has yet to have been decked - we'll see how much I have to take off it to get it ground flat.
But all things will be measured.

Taking ~1.6mm off the top of the pistons will not be a problem - I have compression to spare, according to the compression calculation program taking that amount off the pistons will put me up at around 11.6:1, plus I have loads of valve depth with those cut outs as they are... And it will make the pistons lighter.

[Warsteiner]

Ok..........wait a sec!

If you have not decked the block and you know for sure that the rods are 140mm.......then why would a stock S52B32US piston make you stick that far out of the bore with a stock 81mm crank? You should be -.65mm inside the bore.

It doesn't make sense mathematically. Unless you have the wrong pistons. And if you did buy these pistons new then why couldn't you have made them to your spec to fit the motor you're building?

I would now check the CH and cc the dish to see what you actually have.

Cheers,
~Ralph

[MrPhatBob]

Okay so I've had all the measuring tools out this morning.
Stroke confirmed at 81mm
Rods length confirmed at 140mm
Compression height  32.8mm (32.78mm)

(Stroke/2) + rod length + compression height = 40.5 + 140.0 + 32.8 = 213.3
213.3 - 212.15 = 1.15mm above the block.

So I am going to deck the block to get it flat and then cut the pistons down about 1.3mm, crown to ring height is 6.65mm which would reduce to 5.35mm
The pockets are cut into the crown leaving a crown to ring of 4.78mm so my modifications wouldn't go below that.

As for getting them to spec, it was a bloody nightmare getting any pistons at all, none of the UK companies were quoting anything close to sensible prices, Wossner weren't interested in playing 4 cylinders and the JE piston order was, in short, rushed.

[Warsteiner]

Ok that sounds more like it. So now you should measure the piston itself and find that you are probably at 86mm which is a 3.0L piston which is correct for the 32.8mm CH. Glad you measured again to get it straight so you have peace of mind.

Just another thing to think about......This list is not only for sharing information with each other but it's also about helping each other out. If you needed a specific part and couldn't get it by you, any one of many people would have helped you get what you needed here in the US I'm sure. I will say that I would have helped you get whatever parts you needed and shipped them. That's what a community is about no matter what the distance or how many ponds we have to cross  

Let us know....It looks like its gonna be great!!

Cheers,
~Ralph

[MLM]

On a Topic which has cofused me over the years, and this may be an appropriate time to ask given the discussion on piston height and your acess to some excelent engine build knowlege. What squish clearance are you targeting and what are your thoughts on this?

[MrPhatBob]

Well I asked, and the answer was - "It depends on the engine"... From what I can understand its best to try and replicate what came from the factory, the pistons came to the top of the block leaving the gasket thickness as squish clearance. There's going to be more squish area as the bigger bore pistons will see to that.

So... I put the block on the grinder

To get the whole thing flat - took 0.004" off the deck in total
Then the pistons and rods went back in

and the crown to deck heights measured - indicated by the numbers on the crowns.
These were then machined down on the lathe - a touch concerning because I was plunging a cutter into my new pistons.
While the pistons and rods were out, I checked the crank endfloat - 0.12mm which is pretty much spot on.

[Jaker]

Nice clutch/flywheel!!!

[MrPhatBob]

That's one from one of the race engines on the bench I was using, mine is a 5kg TTV steel flywheel with standard M44 clutch.

[wazzu70]

Its really hard to get a proper answer on squish clearance! I think too many factors are involved to have good data to follow as a general guideline.

Following what the factory did is pretty wise. There is no way any one of us can replicate the analysis and testing performed by the OEM!

[MrPhatBob]

New oil squirting jet arrived so...
There they are, the last time (for a while) that I'll see them - see if you can spot the three old ones

Rings were gapped according to JE's spec

Pistons fitted using 86.5mm sleeve

Then finally the core plugs


Unfortunately the people at BMW do not have any 92-95 M42 crankcase gasket sets, so now the fun starts trying to find them.

[jrw21]

Looking real clean! You just reminded me of something I forgot to check when I left the machine shop the other week...to make sure all the oil squirters were still there!

So, your final piston compression height is 31.5mm after the shave?

[MrPhatBob]

Its close on 31.5mm, we took a bit off the block to get it straight so its probably closer to 31.4mm now, it became more important to get the height correct with respect to the deck.

[colin86325]

If it were me, I'd ditch the stock gaskets and use Yamabond (or similar) instead.  I don't like having to fix leaky seals twice!