so, im finally going to get forged pistons and rods. i might be needed to get a .5mm overbore, there fore ill get the 81.5mm pistons for my ls vtec. my question is this. if i have 81.500mm pistons whats the exact bore size i will need so that these pistons wont slap when they are cold. i have heard people following the specs for the pistons and the bore is to large so they knock when they are cold. i would realy like to know how to aviod this. thanks.

 

its not like a knock....more like a slap...it sounds like a noisy valve train...and it doesnt hurt anything...just let the motor warm up a little bit before you open up on it....

 

i just need to know what clearence to use...im thinking like a .030 pistons to wall. im hoping there will be more info with the pistons when they come. btw im using arias

 

Talk to Tom at Arias.You need enough clearance so the pistons don't scuff.

 

well isnt it gong to say something in the box. isnt it going to depend on combusion temps too? there are so many factrs to think about

 

There should be spec sheets that come with the pistons, follow them. Typically if the piston is listed as 81.5mm the cylinder should get bored to exactly that, and the proper clearance will be built into the piston (it will be smaller than 81.5mm)

BUT!!

thats not always the case, pistons have tolerances too and should always be mic'd (measured) for size and bored/honed accordingly.


The thing about forged pistons is they expand more than the stock cast piston since they are of a different alloy of aluminum. Therefore there is more clearance when the motor is cold. This is what creates the slap you are reffering to which is a neccesary evil in order to have the proper clearances at operating temperature. A forged piston motor should always be warmed up to operating temp before driving hard.

 

wouldnt it be alot easier to build the proper clearence while boring it. that makes the most sense to me. does anyone know what arias recommmends?

 

just give the pistons to the machine shop.they may vary a little bit in diameter but he'll bore each cylinder to the spec of that piston so all the clearances are the same even if they did vary a bit

 

the proper clearance will allow for the expansion of the pistons...if you went with a smaller bore to try and stop the slap at cold starts, then when the pistons expanded it will decrease the piston-wall clearance...

 

just listen to that rosko fellow he has the knowledge simplified for you...its the nature of the animal...