piston confusion again lol

[erickmarciano]

hy guys looking to rebuilt my engine next winter and was starting my research .
my car is a late 71 euro. when I look on SLS and other sites it shown euro with a steep and flat for USA , should it be the other way around?
should I get flat USA?
thanks
Erick

[Benz Dr.]

Go with the flat top piston if you can get it. The stepped piston will give you about 20 - 15 PSI less compression per cylinder, or about 150 PSI on most engines. I have a set of 87.5 mm (max or 2nd over size ) in stock should you need that size.

[erickmarciano]

My euro car has original pistons and are flat .The new USA pistons are flat and euro stepped ?
Thanks
I will only know what size I need once I take the engine apart

[Benz Dr.]

Some pistons are stepped but they were normally flat when new. Over sized pistons are generally 3 mm shorter to compensate for increased compression due to over boring so stepped pistons only add to compression loss. I'm not a big fan of stepped pistons and will only use them if nothing else is available. In some cases I will deck the block to return proper piston height. Compression is HP on our engines and knowing how far to take machining is key to a strong engine.

I've seen engines where the pistons were .020" under deck height. I've also seen engines where the pistons were above the block parting surface .020" which is allowed. Ideally, you should aim for deck height if possible.

[MikeSimon]

My opinion - may be wrong - is, that the stepped pistons originally were intended to compensate for the increase in compression when going oversize due to a bore. i have 87.00 (86.97-98) pistons in my car and they are stepped

[Benz Dr.]

My opinion - may be wrong - is, that the stepped pistons originally were intended to compensate for the increase in compression when going oversize due to a bore. i have 87.00 (86.97-98) pistons in my car and they are stepped

New pistons are shorter to compensate for any compression ratio increases so stepping them is overkill.

[MikeSimon]

New pistons are shorter to compensate for any compression ratio increases so stepping them is overkill.

If the pistons were shorter, at TDC they would sit below the upper edge of the block deck. They do not. And if they are shorter, they may also increase the displacement, thus negating the increase in combustion volume to keep compression somewhat