Bought a built jdm b20b vtec stamped p3f (with knock sensor) and it had blown head gasket. So went to swap it and noticed cracked cylinder walls. So planned on dropping it out and putting in my sleeved b18.

The pistons weren't stock b20 pistons, domed about as much as a type r piston but no markings on piston exept the direction arrow and the number of the piston order (1-4 just a single digit stamped on the center of the piston. There is also a very faint etched marking reading 050. But that's it. No serial number nothing on the skirt nor the underside. It does however have a pattern from the casting process(just vertical and over lapping horizontal lines. Any have a clue how to determin what these are?!?!?!?!?!?!?!

 

050 means its .5mm overbore, so 84.5mm

 

Take a pic.

If it's a .50 oversize piston, it's not factory. OE pistons only came in standard and .25 over. You most likely have RS Machine/Nippon pistons in there.

 

Yea I did some research and got that part. Does going .05 over just compensate for wear and honing? Also, Any way to tell what TYPE of pistons they are? Compression and all that? So I have to put it all back together and do compression test?

 

And sorry no pics... I'm using my phone, looks just like rs machine, except with a silver skirt. Thanks guys!!

 

They aren't .05 OS, they are .50 OS... as in mm. .5mm = ~.020".

Generally you bore the cylinders out when they are worn or tapered or out of round beyond the allowable specs. Some people do it for a little more displacement.

Generally, I advise people to wait on ordering pistons until after the motor is torn down and measured so that they can decide if the cylinders need to be bored or not. If the purchase absolutely has to be made with the motor still together (daily driver or whatever), just go ahead and plan on boring it and buy the OS pistons.

The factory oversize pistons are only .25mm (.010") OS. You can easily hone that out on a machine, I wouldn't recommend trying to do it with a ball hone or by hand (). Just about everything aftermarket comes in .50mm increments though.

As far as telling what type they are, there are only a few options out there for the 84.5mm cast pistons. The P73 clones are the most common though.

 

.05 was a typographical error on my part **0.50. But thank you much. I was thinking they were p73 knock offs bit couldn't be sure.

So another question same topic... The cylinder walls are cracked. If I get it sleeved, would I get it at 84.5? Running into all these issues has drained my wallet so hoping to salvage at least the pistons and spend less. They seem to have to have some carbon built on the sides of them between the rings, can I just replace the rings and clean the pistons and check the roundness of them?