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Domed or flat


Beantown

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Hi All,

 

The motor that I will be using for my 2002 is a 1982 1.8.  The original block had some rust, on it so I picked up another block from a member,  and the blocks have different pistons in them.  I was wondering which pistons I should use for different applications.  I plan on using a 32/36 carb and intake or the FI from the 320i.  But didn't know if the domed pistons or flat would be better.  I think that the domed would make for more compression?  Does that equal more HP?  Also,  would the flat pistons work better for a turbo'd motor?

 

Thanks,  see pictures.

IMG_0151.jpg

IMG_0152.jpg

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Flat pistons would theoretically be better for Turbo, yes. But flat forged pistons would actually be better for forced induction. 

 

What'll be more important is what clears your combustion chambers of your specific head, and if your cylinder walls will need to be over-bored, or just honed. After that, is if those pistons will fit the bores you want to put em' in well enough to rationalize re-using anyway, if they're in good enough condition to re-use at all. 

 

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The domed pistons are in the block that the head came off of,  so the clearance shouldn't be a problem.  But I will check to make sure it wasn't decked.  The cylinder walls look very good and I think should only need to be honed.  I had no plans on boring.  So which pistons would be better for a naturally aspirated(32/36 carb and manifold) motor? 

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For NA, compression is king, so i'd go with the domed pistons.

 

Somebody with more knowledge than I will chime in eventually, but I'm gunna bet the compression ratio difference between the two is pretty negligible. 8:1 vs. 8.5:1? I think there was an 8.3:1 piston at some point too... 

 

But every little bit helps!! 

Edited by 2002Scoob
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Here's another opinion-

 

don't use the 1.8 crank.  It's usually not forged,

and the pistons for it have the pins very low in the skirt.

It's not a good design to start with, and since you're

going to be spending real money no matter how you cut it,

start with a 2l crank and pistons.

Ireland used to make (and may still) pistons for a 

1.8i head with a 2l crank.

 

If you're doing a $2.50 motor, use the domed pistons

for either normally aspirated or turbo- they only give you

a low (8.x) ratio anyway, and they're not detonation- prone.

 

t

doesn't like the 1.8i crank

 

 

Edited by TobyB
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"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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The 1.8L motor with the 89mm bore (like the 2L motor) and short 71mm stroke (like the 1600 motor) configuration was first developed in 1968. Prior to that the 1.8L BMW motors had a smaller 84mm bore (like the 1600) and the longer 80mm stroke (like the 2 liter motors). BMW switched to the the bigger bore ad shorter stroke for the 1.8L because the short stroke motors are smooth motors. By about 1980, BMW stopped using forged cranks in most of their production cars. If you want to use a forged crank for your 1.8L, you can hunt down a 1600 or 1802 crank from 1970 and up. Prior to that most 1600 cranks were of the 6 bolt vs. 8 bolt design. But, the cast BMW cranks are good as well for street motors. In my opinion, the 1.8L motor from 1968 and up is a good design as it is very smooth and economical. The motor is more comfortable with high rpms due to its short stroke. It also feels great with the 245 OD 5 speed transmission and 3.90 diff.

 

Slavs

Edited by Slavs
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The early 1600 and 1800 evolved into the 2 liter.

 

the late 1.8 liter is just a destroked 2 liter, with the same 135mm rods

and a low pin to get the crown to the top of the stroke.   Half as many

counterweights, and no forging hits.  Maybe if you ran a longer rod, some

of the torque might come back- but then, rods cost more than the 2L cranks, so...

 

 

For the $2.50 motor, tho, (and I've done a couple of those, too) go domed or go home!
 

heh

 

t

 

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Why does it always have to be a 2 liter ?

The 1.8L is a very good motor with a big bore (89mm) and short stroke (71mm) combination. This particular 1.8L was developed in 1968, well after the 2 liter. It's a very smooth motor. Due to the short stroke, it was not necessary to fully counterbalance the crank. The stock cast pistons are very high quality and strong. And, the piston pin location is mounted lower, but I don't see why this is relevant. With the exception of the bore, BMW uses the same block in all of its M10 variants. The main bearing to block deck distance is the same for all blocks. BMW uses the same connecting rod length for all its M10 variants. Different stroke is achieved through the different crankshafts and piston pin location. The short stroke motors are purpose built for smooth operation and high rpm's. Engineers often de-stroke motors while increasing the bore to maintain displacement when building high performance racing motors. BMW, Porsche and Alfa have all done this. The Alfa Giulia GTA 1600 is one such example while the BMW Formula 1 motor from the early eighties is another. I'm aware most of you guys here are 2 liter centric, but smaller displacement M10 motors were also produced, and they are not inferior to the 2 liter. They have their purpose. The 1802 is a very nice car with a silk smooth motor.

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Great information!  I love knowing the small differences for the blocks and knowing what I have to move forward with.  I will be adding the domed pistons to my good block(If I can get the piston arm caps off,  they seem stuck?).  That being said... I have read that I have to replace the head bolts and piston arm bolts because they stretch.  Being that this is a $2.50 rebuild,  do I really need to replace these bolts?  I will be adding new rings to the pistons and other various parts,  but they bolts look good and I don't want to ruin my budget. 

 

Thanks all! 

 

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13 minutes ago, Beantown said:

 

...piston arm...

 

 

Let’s call the piston arm, which connects the piston to the crankshaft, a “connecting rod”....??

 

Regards,

 

Steve

 

  • Like 1
  • Haha 1

1976 2002 Polaris, 2742541 (original owner)

1973 2002tii Inka, 2762757 (not-the-original owner)

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Most shops always replace the head bolts during rebuilds. But, I've never done it on my everyday street motors, as I was on a budget. But, I always check head bolt torque and valve clearance after the initial 500 miles. I've never had a problem. But, if your wallet permits, I suggest you use new head bolts. If I was building a motor for someone else, I would use new head bolts.

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Done literally hundreds of head jobs in the last 45 years, most of it at dealerships and we never replaced the headbolts or rod bolts, reused them more than just twice on some cars     Only ever did rod bolts when we built very high performance engines   but as Slav s said, retorque and valve adjustment after 500 miles but I also do this after the first 30 min run in as well, gets rid of all the assembly crap    oil and filters are cheap

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