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does anybody know what the writing on the top of the pistons mean? i am new to rebuilding engines, i have two 1.9's one of them has an 8, 7, 7, and a 6 stamped on each respective piston. as well as three circles. the other has "sp004" stamped on each piston in the same place, also these pistons have five circles. does anybody know what they mean?
 

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Usually, and I don't mean this is your case, but a stamped piston tells you it's a oversized piston and how much. As for all the stuff you mentioned, there's nothing about it in my Opel book from England. I'd check with a machine shop. Have them measure your pistons and check them against what the stock size is. I would hope some PO didn't put in different size ones in a rebuild, but there are old sorts in the world.
PS: rebuilding isn't really hard as long as you have things checked at a machine shop. Just relax, take your time and pay attention to the little stuff. It's actually kinda fun, as long as you aren't rushed.
 

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Opel engines came with pistons in three (3!) different "standard" bore sizes, to accommodate machining variations of the cylinders when the blocks were bored in the factory. The proper piston was then matched up to the cylinder in the assembly line according to actual bore size. So it is possible to have as many as three different "standard" piston sizes in a given factory engine. Or two, but most engines have a single piston size. The variations are small, but significant enough that you DON'T want to install a larger piston in a smaller bore. And a smaller piston in a bigger bore will "slap".

I don't know the numbering system, but I believe the bigger number indicates a bigger bore (or was it related to the number of circles? Hmm, I used to know but old age is setting in...)

The "standard" piston fits a cylinder bore of 3.661-3.663 inches, while the smaller piston fits a bore of 3.659-3.660 inches, and the larger fits 3.664-3.668 inches. So there can be a difference of as much as 0.007 inch (7 thou), which compares to the specification of 0.0008-0.0012 inch (eight-tenths of a thou to 1.2 thou) clearance. And note that Opel pistons are NOT round! They are "cam ground", such that they are wider at right angles to the pin than across the pin. So make sure that your machine shop measures them in that orientation.

HTH
 

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The engine in my car evidently has a "too small" piston because I get some knocking sound (slap?) for the first minute or two when it starts up cold. Is this going to eventually destroy the engine?
 

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i was dinking around on here looking for various numbers and specs for the 1.9 engines and how to identify various things without having to tear apart the engine, when i came across this thread again...

just an update:

we ended up being able to put together a complete engine for cucumbershampoo's car... he took enough parts from both engines to a local machine shop and had them check out the block and heads, as well as make sure all the internals fit together nicely... engine was painted while it was out of the car, and a few weeks ago it was put in... a few minor problems came up during the break in period, but i do believe everything is working properly now...

i'll maybe try to take some more pictures if i get a chance...
 
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