what do these markings on my pistons mean?

[dgswinger]

i have a 71 360 torn down to the short block and i look at the pistons and see what i was expecting!! the supposed 71 only 360 flat top pistons!!
as i look closer thru the carbon build up i can see stampings on the tops so i cleaned one off and this is what i see 34189 25 O/S and a large T that is just randomly stamped on the pistons in different locations. i cleaned several of them off and they are all marked with the same numbers. i dont have a bore gauge or i would measure the bore i am thinking that this motor was bored from the factory .025 if anyone has any info on these markings that would be great!!! thanks in advance!! i have a few pics but they arent great from my phone take a look and let me know what you think the 34189 25 is really small so its hard to see on a pic.

 

[805moparkid]

yea over sized .025"

 

[dgswinger]

thanks for confirming my thoughts 805MOPARKID
well thats an odd size i wanted to use these pistons in a std. bore block cause its ready to build... i would have used this shortblock to begin with but the crank is toast and its will need a good cleaning and cam bearings and honed where as the current block im working on that is already done:banghead: now i gotta rethink my build i was trying to get a little more compression using parts that i have in my garage and from what i have read the the 360 stock style dish pistons have around a 9cc dish so thats just lost compression to me!! im dropping my heads off at the machine shop tomorrow to be gone over and milled .020-.030 to hopefully get them into the 67-69cc range i have home cc'd them and came up with 74cc's they are the 73 340/360 "587 J" castings heads im trying to get as close to 9.0-1 compression as possible without having to buy new pistons or have anymore machine work done other than the heads for now. its going in one of my trucks that i will be towing with so i just want a little more out of it than stock....

 

[RustyRatRod]

I see S/C not O/S. Measure them to be sure. The factory used all kinda marks to keep up with stuff.

 

[dgswinger]

I see S/C not O/S. Measure them to be sure. The factory used all kinda marks to keep up with stuff.

it sure looks like O/S in person i know the pics arent the best they all are marked with it. i will measure them to be sure tho! if it were S/C what would that mean?

 

[805moparkid]

it sure looks like O/S in person i know the pics arent the best they all are marked with it. i will measure them to be sure tho! if it were S/C what would that mean?

supercharger pistons for 25psi... just kidding!

 

[dgswinger]

well i bought a cheap inside/outside caliper and seem that the bore is 4" not .025 oversized. look at the pics i measured the bore and piston. guess thats good news i can use them in my build now!! im still curious as to what the stampings mean??

 

[MRL Performance]

You dont measure the piston at the top, you measure it at the bottom of the skirt, about .250" up from the very bottom of the skirt. The top of the piston is smaller in DIA than the bottom. And the tool your using will never give you an accurate reading. You need to find yourself a Micrometer and measure the piston to make sure.

Do it right, Dont assume.

 

[dgswinger]

You dont measure the piston at the top, you measure it at the bottom of the skirt, about .250" up from the very bottom of the skirt. The top of the piston is smaller in DIA than the bottom. And the tool your using will never give you an accurate reading. You need to find yourself a Micrometer and measure the piston to make sure.

Do it right, Dont assume.

thanks Mike for clearing things up for me i now have a digital inside/outside caliper because after looking at the cheap one i had bought i realized a few thousandths can make a difference!! my next purchase will be a micrometer!! i will listen to anything you have to say and have lots of respect for the time and effort MRL has put in to making some very healthy runnings small blocks!! hopefully someday i will be able to call you guys up and have you build me a nice stroker small block!! we dont have a lot of shops around here that really take any interest in working on mopar engines they will do your basic rebuilds and machine work but to actually be interested in what you are building can really make a difference in the end result!! im headed to the garage to take some measurements!!