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Tii pistons vs stock '02 pistons

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Hi folks-so my local engine block shop is going over the lower block from a stock 2002 for me. They want to replace the pistons due to wear, but they are finding pistons that are rated for a Tii motor(they can't be returned apparently). Before ordering, he wants to try to verify that they are identical. My understanding is that they are, but I wanted to ask here, as you folks would know better than I do. Can anyone confirm? Thanks.

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Diameters are identical between 2002 and 2002tii cylinders and pistons.  I suspect by “tii” they are referring to compression ratios: U.S.-spec tii’s came originally with compression ratios of 9.0 to 9.5 while U.S.-spec 2002’s originally came with compression ratios of 8.0 to 8.7.  Lots of 2002’s, by now, use higher compression ratio pistons for added power. My ‘76, for instance, came with 8.3 compression ratio pistons, but now has a 9.3 compression ratio.  Of course, it now runs on premium fuel, a necessary component of the “upgrade”.

 

Your machine shop should also know that, except for flat-top pistons (generally 8.0 to 8.3 compression ratios), the pistons need to be matched to the head.  For example, your car’s head is likely a 121, an E12, or an E21. Unless the pistons have flat (flush with the block’s deck) tops, they must be shaped for the appropriate head’s combustion chambers.

 

My summary is perhaps an oversimplification, but I’m hoping it, at least, points you in the right direction!

 

Regards,

 

Steve

 

Edited by Conserv
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To elaborate on conserv’s post.

 

step 1) identify which version of m10 cylinder head your motor is running.  This mark is cast on the intake side of the head (e21, 121, etc.)

 

step 2) cylinder head type coordinates to the shape of the piston (pop-up).

 

step 3)  The height of the “pop-up” relative to the face of the top of the piston coordinates to possible compression ratio (the taller the pop up, the higher the compression ratio).

 

***it is best to do away with the notion of “tii pistons” as these days that means little, given the number of m10s that have been modified/changed.

 

If you need some visual references, see this thread (

)

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Ok, so I have verified a refurbished E12 head will be going on the block. Now how do I suggest a piston to my machine shop accordingly?

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Just to add.....what is the thickness of the "refurbished" head? That will determine either the thickness of the head gasket and /or popup.

 

You won't need high Octane gas if you have a knock sensor ignition such as a J&S, but you will prefer the way it drives.

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As others have said, you need to know what cylinder head you have before you order pistons.

 

Source for the below info. 

http://www.bimmers.com/02/

 

The following piston options are available for the various 2002 cylinder heads:

121 Cylinder head:

8.5:1 CR U.S. "Spec" piston on 1968-1972 2002's
         42.3 mm pin-to-deck, flat top

9.0:1 CR U.S. "Spec" piston used on '72 US-model 2002tii's

9.3:1 CR European 2002TI (twin-carb model)
         42.1 pin-to-deck, 5.2mm dome

10:1 CR Early euro-spec 2002tii
        42.5 mm pin-to-deck, 4.4 mm dome

E12 Head:

6.9:1 CR Factory "Turbo" pistons (used on 1974 2002 Turbo)
         42.0 mm pin-to-deck, 3.7mm dish

8.1:1 CR flat-topped piston (much like the 8.5:1 CR used on the
         121 heads)  used on '75 US-model 2002's, also on most
         US-model 3-liter cars of the era. (keep in mind that
         the 2-liter 4-cylinder with the E12 head and 3-liter
         6-cylinder cars can interchange pistons)

8.3:1 CR U.S. "Spec" piston on mid 1972-1974 cars

9.0:1 CR U.S. tii pistons

9.5:1 CR European tii pistons

E21 2.0 Head:

8.3:1 CR flat-topped pistons (used on 49-state '76 2002's,
         US 2-liter 320i's)

9.3:1 CR domed pistons (used on european 4-cylinder 320i's)

Note: compression ratios given are with a stock head, and zero deck height.

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