ok, did an autopsy on my 305 in my 86 t/a. it showed 232k miles on the clock. when i took the heads off i was surprised to see they were in decent cond. i was also surprised to find a cross hatch hone on the cylinders.. i measured the cylinders and it is indeed a 305. the pistons have 4 valve reliefs in them. i'm wondering if anyone can tell me if the above piston came stock in these motors? i'm trying to figure out if it's been rebuilt and/or what's been done.

 

Should be a dished piston with 4 valve reliefs and a "notch" on the face pointing towards the front of the engine. Anything beyond that would require tearing the pistons out and trying to find something to cross-reference.

Chances are, if the bore is still stock (3.736") you have an original engine. Anything that's been over-bored (+030, +040, +060) has clearly been rebuilt sometime in it's life.

Please note that this just means the engine is STOCK, not ORIGINAL to the car.

 

thanks. guess it solves that question. they are stock pistons. when i get the bottom end apart i'll see if perhaps some one did a re-ring job ect. i can't believe i can see hone marks at that kind of millage. the cam bearings are showing tipical wear for it's age too. i'm really not going to use any of this stuff anyway. just currious. i didn't measure the bore exactly as i was just concerned it if was a 305 or if someone had replaced it with a 350 thanks again.

 

An '86 would have flat-top pistons with 4 valve reliefs.

If they are oversize, there should be a stamped number on the crown.

If the number on the pad in front of the passenger side cylinders includes numbers from the VIN, it's the original engine.

 

Five-seven: Now that I think about it, I tend to agree with you. 85-up 305 in a 3rd gen SHOULD be flattops to get you the higher factory compression rating of 9.3:1.

However.... I have the original 305 short block out of my wife's 92 RS (TBI engine, still rated at 9.3:1) but the pistons are definitely dished with 4 valve reliefs. They're also definitely original. Not big dishes, but still dished. I'm not disagreeing with you but I wonder whether GM was "cheating" the compression ratio in some of the lower output 305s, like my wife's TBI engine.

 

Flat tops, 4 valve reliefs

 

Through '86, flat-tops with 4 valve reliefs, 9.5:1 static CR.

'87-up, almost flat-tops, slight dish, 4 valve reliefs, 9.3:1 static CR.

 

Sometimes, to get a useful answer, you have to ask the right question. So, before getting into a stupid argument about a bunch of nothing, let me ask one question about something useful, that will perhaps shed some light on whatever it is you really want to know:

You've got a dead motor laying there in pieces. What diference does it make if it's stock or not? That is, what would you do differently, based on that info alone?

Measure the bore. If it's 3.736", then it's probably stock, and you should throw it away because it's a 305. If it's larger than 3.736" but less than 4.000", then it's not stock but rather has been rebuilt, and you should throw it away because it's not only a 305, but also it's a completely used up 305.

No matter whether it's "stock" or not, the correct response to the situation, is to throw it away.

Did that clear anything up?

 

That's too funny! I'm gonna put that in my sig...