Piston size problem

[89custom]

so the idiots ive been geting sh*t from gave me 2 wrong size pistons, im sure some of u heard me bitching somewhere on here. but im stuck on pistons. they only make wisecos in like .25 .50. 75. 1 blah blah. my cylinders 67.54 and i need to know if .04 mm is enough clearence because i dont want to bore, it will be like 100 bucks or some gay sh*t. .04 is really small isnt it? is it guna be to tight? i through the old piston in there, and we measured the old one for 67.34 but it fits really nice in there. where the hell did this size come up? its like in between the 2 sizes closest.

for now, im running the old piston, some how it has more compression after we rebuilt, dont ask how but it does for sure, same rings, piston and cylinder was honed. also i have a knocking noise, is that just cuz im running 87 octane? i know i should be running more but thats all i had... ive heard ocatane changes the pinging/knocking sound from detonation, what else could be making the knocking?

 

[01blasterdude]

The pistons sizes are general, when running strict tolerances like that, you need to get a piston and take it to the machine shop, he will measure it with quality calipers, and then will bore the cylinder to match. Also over time your cylinder can gain some size (not visible, but measurable with good tools) it is from the wear on the cylinder. .04 is not small considering how pistons are supposed to run fairly tight to the cylinder, if they didn't, they would slap and wobble around.

You can cause serious engine damage by running too low of octane gas, pinging or knocking, is being caused by the gas igniting from the compression, before the spark plug sparks. When this happens, it is combusting while your piston is still trying to come up stroke, and it is very hard on your engine, can ruin your piston pretty easily also.

 

[89custom]

OH sh*t. that sounds liek what might have blown my crank bearing. does that make sence? the gas is firing before the piston is all the way up its stroke, pusshing down on the crank compressing the bearing. is this what might have done it? and what octane should i go with?

 

[wrench man]

.04mm = .002", thats plenty of clearance, the machine shop will measure the piston with a micrometer and then set the clearance in the bore by honing the bore and they will measure the bore with a dial indicator set up to measure ID.
The knocking sound is probably your old piston slapping in the bore?, is it constant or some what intermittent?