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Arias Forged Pistons 10:1, 90mm


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Price: $100
Location: Denver, CO


Description:

Hi! I've got an old set of Arias high compression (10:1) forged pistons out of an M10 with an e12 head, for a 90mm bore. They're in remarkably good shape, they were bought in 1998 to overhaul the engine by the previous owner. However, it looks like they bored the cylinders too big back then and the pistons were very slightly smaller than spec'd. So it burned tons of oil and my 5 speed swap "pulled the thread on the sweater" so to speak. 

 

The end result is I dropped a bunch of money on new pistons and boring to 90.5mm, and you get a really solid set of pistons for the cost of a single new piston, without a 6 week lead time. 

 

 

Per one of the commenters below, he believes they look like 121 head pistons. I can't guarantee they fit those, but they probably should. The shape of the dome does not determine the compression, just the volume and deck clearance. The volume and height above deck is greater than standard pistons. 

 

Comp height: 1.704" or 43.28mm

Dome (including negative volume of valve cutouts): 11.2 cc

Deck clearance on a standard engine (217.5 deck height): -0.78mm (above deck) (218.28mm above crank centerline). 

 

Calculated CR is 10.3 with a 217.5 deck height and e12 heads. A 217.3 deck height and 121 heads is calculated to 11.2. So pay close attention if you are going to use these on a 121 head, and consider a thick head gasket based on measurements of your engine. 

 

The piston's diameter at the specified measuring point is 3.5358-3.5363 (89.81- 89.82mm) at 68F. They're likely 2618 alloy, so they expand a good deal, and have a specified piston/wall clearance of 0.006. 

 

Rings are hastings 652 (for a pack of 8 ) 😎🫢 or 582 (for a pack of 6), can do plasma moly (2m652) or chrome (2c652), or any other company's equivalent rings. 

 

Asking $100 OBO shipped in the continental US. Located in Denver/Aurora for local pickup. 

 

I'll also be posting a working 4 speed, a dual dcoe manifold, and a thermal reactor if any of that is interesting to people. 

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Edited by wmax351
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25 minutes ago, mac said:

Those look like pistons for a 121 head. Not an E12.  

How 'bout double checking?........

 

Mac. 

They were running fine on an e12 head. Motor is a late 1974 production/ 1975 model year. PXL_20240201_012024226.thumb.jpg.c21a2bdecce9808d246907917948f352.jpg

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7 minutes ago, Furry Camel said:

They will physically fit an e12 head but that’s not what they were made for. The 10.0:1 compression stated is for a 121 head as mentioned. Quite a bit less in a larger e12 chamber. 

 

They were ordered specifically for this engine. I can measure them to check what the actual CCs are. 

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22 hours ago, wmax351 said:

 

They were ordered specifically for this engine. I can measure them to check what the actual CCs are. 

My best advice is to stop asking questions of people that have many eons of experience with everything BMW, specifically M10. You clearly don't think we are qualified to know the difference between an E12, E21, and 121, and what goes in them. Maybe try Cat Fanciers.

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I'd like to confirm fitment for anyone who wants them. "Double checking" as mentioned above. You don't have to be a dick.

 

They were custom pistons ordered by the previous owner 25 years ago, so I can't just look at part numbers, etc. They're still in good shape and could be good for someone, otherwise I'd just toss them in the recycling. 

 

 

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1 minute ago, wmax351 said:

I'd like to confirm fitment for anyone who wants them. "Double checking" as mentioned above. You don't have to be a dick.

 

They were custom pistons ordered by the previous owner 25 years ago, so I can't just look at part numbers, etc. They're still in good shape and could be good for someone, otherwise I'd just toss them in the recycling. 

 

 

You asked a question and was given an answer twice, but you don't want to accept the fact that we are giving you accurate information, and your info isn't.

E12 on the left, 121 on the right, for comparison. It's a simple google search, buddy.

IMG_0009.JPG

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8 hours ago, Myinka said:

What do they weigh each.

Confirmed 443g each. 

 

I also measured the volume of the dome,  including the valve pockets, which is 10-15 cc within the limits of my quick measurement (tape to make a clear cylinder above the piston, add known volume of water, measure height). It protrudes 0.300" from the flat surface. Compression height is 1.704." So 218.3mm compression height relative to the crank centerline. So 10:1 seems reasonable on any of the heads. 

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17 minutes ago, wmax351 said:

 So 10:1 seems reasonable on any of the heads. 

A bit of engine builder knowledge to be shared here: Stated compression is in a stock head before any valve pockets are cut. Valve pockets deduct from that, in the COMBUSTION CHAMBER MEANT FOR IT, and even less in a larger combustion chamber such as E12.

Buyer beware. Disregard any reference to an E12 head in regards to these pistons, despite any claims made by this seller.

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1 minute ago, Furry Camel said:

A bit of engine builder knowledge to be shared here: Stated compression is in a stock head before any valve pockets are cut. Valve pockets deduct from that, in the COMBUSTION CHAMBER MEANT FOR IT, and even less in a larger combustion chamber such as E12.

Buyer beware. Disregard any reference to an E12 head in regards to these pistons, despite any claims made by this seller.

Agree. They came out of an e12 and ran fine without valve interference issues. Per your comment, they should work fine (maybe better) in a 121 head, which is nice to know. I can only comment that they physically fit an e12 head, and I can't say the same about the others and don't take responsibility for them not fitting. They're a set of forged pistons for the cost of a cast piston. 

 

Based on my measurement with water (including the valve pockets), they have a 15cc dome with the pockets cut. (50cc of water over the piston gets a depth of 0.41". Margin of error includes surface tension) 

 

If someone wants to use them and cares about the absolute exact compression, they're welcome to make an offer and measure. 

 

If they care more than that, they can spent 1000 bucks and wait a couple months for new pistons. 

 

 

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I measured the actual dome and valve cutouts and put it into a parametric CAD system. Actual volume of the dome, after subtracting the volume of the cutouts, is 11.2cc. So if you want, you can calculate the exact compression for your specific engine. 

 

In the end, they're cheap, well made pistons that are higher compression than flat tops. Put them in your lemons car or budget rebuild and enjoy them. 

 

 

Edited by wmax351
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