Can anyone tell me what bore means? and the difference between the higher and lower messurements?

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92 Camaro 305 TBI Automatic. Everything stock so far except a shift kit.

Future mods. Spark plug wires and plugs, tint, new 3inch pipes. Maybe putting 2 glass paks for effect. any other advise on getting to 300+ HP would be appreciated. Thanks

 

I'm not trying to talk down to you, so don't take this the wrong way. Simply put, a V8 has eight cylinders; each containing its own piston. The diameter of each cylinder is called the "bore". The distance that the piston travels down inside the cylinder is called the "stroke"(this has more to do with the crankshaft). There's more to the subject, but those are the basics.

As far as your question about the higher and lower measurements, I'm not quite sure what you're describing. Are you talking about compression, or bore/piston oversize, or something else?

 

The higher and lower sizes may be the bore size and the piston size. The piston is actually a few thousandths smaller in diameter than the actual bore. Otherwise the piston wouldn't slide very easy and you'd have a hell of a time getting it in. Everything has tolerances.

Jason

 

most piston makers show where all the differant measurements are taken on the piston. might be a good idea for you to get one and look at it.

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ICON Motorsports
1st & 3rd
MM Black Diamond 538 F&AM

 

OK, I was looking in a Summit catalog and was looking at the pistons. Describing the piston it said...

Bore 4.030"
comp height1.555
stroke 3.48"
rod5.7"
comp ratio 10:1
pin Dia .927" and the price... I just wasn't sure what bore ment...

 

The bore is the measurement of the diameter of the cylinder. The 350 (except the LS1) have a 4" bore. When a machinist inspects the cylinder walls for wear, they may be so worn that he has to grind off .030" of material to clean the walls, take out any tapper, and then he hones the metal properly. The common overbore size are also .040" & .060". That's how the 4.030" bore is had. I dunno what the comp height is. Sorry. Maybe I'll learn on here. Anyway, the stroke is the amount of up and down movement the piston does. Again a 350 (expt. the LS1) has a 3.48" stroke. The only way to change this is to change the crankshaft. They give those specs so that you don't put those pistons on a 400 cid crank which has a 3.75" stroke. Otherwise it'll hit the heads. The comp. ratio is usually set by using a 64cc chamber head. If your heads are another size chamber, then you have to recalculate. And of course the pin diameter is just that. The size of the pin in the piston to hold it to the connecting rod.

As said before, don't take this as "duh, you should know that." Like I said, I dunno what the comp. height means. I've just never been informed. You'll never know until someone tells/shows ya.

Edit: Sorry, the 5.7" rods are also on the 350 (expt. the LS1) You can buy aftermarket 6" rods, but then the pin in the piston has to be raised, again so the piston doesn't hit the head.
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92 RS w/t-tops 305 TBI Auto.
170K miles and don't burn a drop o'oil
-K&N Truck filter #1500 w, open ele.
air cleaner
-Dynomax 2 1/2" cat-back
-B&M TransPak
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-JVC CD--Alpine speakers & 4ch. amp
Rockford 2ch. to 2 MTX 12" subs

[This message has been edited by AJ_92RS (edited June 16, 2001).]

 

Compression height is the distance from the center of the wrist pin to the top of the piston, which is most commonly considered the largest flat area on the top of the piston that would be level with the deck of the block rather than the absolute highest spot like a domed piston would have.

 

That was my guess. Thanks. I just didn't wanna tell someone something I guessed at. I just never realized that there was that much space there. Never measured so I never knew.

Thanks again though. Like I've always been told, the dumbest question is the one never asked!!!!!

Oh yea, does that hold true for a dished piston too? Do they measure from the "rim" of the piston, outside of the dished area? I assume they do, but it's safer to ask. And don't worry, soon I'll be askin' all kinda questions on here. I'm just about ready to put together a 350 for my car. First timer here.

 

Yes. The measurement is generally taken to the "quench" area of the piston face, which is the large flat spot of the piston outside of the head chamber area. The purpose of the quench area is to create a volume which, when the piston approaches TDC, is so close to the face of the head that all that fuel mixture in that space gets rapidly squeezed out into the open chamber area and resulting in very high amounts of turbulence. The quench space on most engines is only .030 to .050 inches. Racers try to adjust this quench volume as small as possible to improve this turbulent squishing action, but if they get too close they risk having the piston smack the heads at high rpm. I have an aquaintance who got his quench space so small on one motor that he ended up transferring some serial numbers from the piston face onto the metal of the head, sort of like a 6,000 rpm punch press. That was too close for comfort.

 

Thank you and im not taking it like im dumb... if that was the case everyone would be.. We have to learn some time and thats what I want to do.. Thank you explaining it to me.. I could I know if people dont tell me.. Thats why I asked you! Thanks again!!

 

do you knwo what this measurement would be on a stock 305?

 

Most stock Chevy small blocks come out around .030" -.035" that the piston is down in the bore at TDC. The stock deck height is 9.025", measured from the crank centerline to the deck surface; so in the case of a 350, take ½ of the stroke (½ of 3.48", or 1.74"); plus the rod length of 5.700"; plus the compression height of 1.555"; you come up with 8.995". That's .030" less than the block's deck height. That .030" is the deck clearance. All combinations of stroke, rod length, and piston pin height must equal no more than 9", and preferably as close to that as possible, in order to work properly.