Thread: bore & piston size

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i just dialed in my block and piston and came up with 4.025 on the piston and came up with 4.046 on the bore this is pretty new to me and i dont have a choice all the machine shops here shut down and the closest one is 5hr away.any help with these numbers would really be appreciated.thanks

Hmmm, doesn't seem quite right there Len. Ya either got the wrong pistons, wrong block, or wrong measurements. Are you by chance measuring the top of the piston? If so, not a good place to measure. If you have skirt protection on the pistons (like Sealed Power), then be sure to measure the bottom skirt below that coating (don't measure the coating). Otherwise, measure the middle of the piston.

Just curious, are you using a caliper, micrometer, bore guage?

All piston measurements are made on the lower portion of the piston, perpendicular to the pin. A piston is ground in an egg shape and you should come up with two different measurements, approximately .008 difference between the two. The top, ring land area, should be .020 smaller than the rest, and round. The general rule of thumb in piston grinding is for every inch of bore you allow .001 for expansion. According to the numbers you supplied, it appears there is way too much clearance, unless you are measuring the top, and in that case it will be too tight. This all may be different depending on what material the pistons are made of. I hope this helps. You could always check a Motors manual, they give piston clearance specs, at least they used to.

what type of motor is it?

Specifications

General
Cyclender #'s (front-rear)

V8
Left (driver's Side) 1-3-5-7
Right 2-4-6-8

Firing Order
V8 1-8-4-3-6-5-7-2

Bore & Strokes
V8 Engines
283 3.875 X 3.00in
305 3.736 X 3.48in
307 3.875 X 3.25in
350 4.000 X 3.48in
396/402 4.125 X 3.76in
454 4.250 X 4.00in
400 4.125 X 3.75in

Cam Shafts
Small Block V8
400 1.9482 to 1.9492in
All others 1.8682 to 1.8692in
BIG block V8's 1.9482 to 1.9492in


I hope this helps any i just took it out of Haynes GM AutoMotive Manual.

Sorry i forgot the bore diameter's

283 & 307 - 3.8745 to 3.8775in
305 - 3.7350 to 3.7385in
350 - 3.9995 to 4.0025in
396, 400 & 402 - 4.1246 to 4.1274in
454 - 4.2495 to 4.2525in (4.2500 to 4.2507 for 1991 models)

if ya need anything else let me know and ill look it up

the motor is a 350 chevy 010 casting.i measured it from the the bottom of the skirt this time came up with 4.055 its a digital caliper so it kept going from 4.055 to 4.056 on the piston skirt.thanks all for the help.

Originally posted by len
the motor is a 350 chevy 010 casting.i measured it from the the bottom of the skirt this time came up with 4.055 its a digital caliper so it kept going from 4.055 to 4.056 on the piston skirt.thanks all for the help.

Something's not measuring up right. According to your original post the bore measures 4.046 and the measurement you are coming up with for the piston is 4.055? Sound's like it's time to measure the bore once again. The best way is to use a bore gauge, if you have one. That way you can measure the entire bore length. Stay away from that top ridge as it will measure smaller.

Originally posted by Streets
PS. I still don't understand what "len" was askin' about, did I miss somethin somewhere along the line?? [/B]

I gettin a bit confused now too. I'm sure he'll straighten us out.

i took my pistons down to the machine shop here and thay said that the piston are 0.60 over but he checked to many times so i want to be sure that when i order the new ones that i get the right size for my engine.i used mm to measure the bore and it came up 103.08 mm and the piston came up 103.03 mm.

Len, It looks like you do have .060 over pistons and the block is also bored .060 over. If you just leave the cylinders alone and don't hone them you will end up with roughly .003 clearance. For a street motor that will not see any hard times, that's fine. But, if you are going to run this harder than normal, the clearance is a tad too tight. For the bore you have, you should have .005 clearance. You can get it by honing the block. If you're gonna order a set of pistons, then obviously you want .060 over.

thank for all the help from everybody.