I have a freshly rebuilt LT1 bored .030 over with 6" Forged Eagle Rods and SRP forged pistons. The engine has only about 30 miles on it and already I'm getting horrid piston slap when the car is warming up. It takes 2 or 3 minutes before the slap disappears. IS this normal for forged bottom ends to be like this? My stock engine would slap a bit when cold but dissappeared much faster.

I forgot to add that the internals are balanced and the block is line honed and decked and bored with torque plates

 

The material of a forged piston is denser than stock pistons. It takes longer for it to heat up and expand.

 

What was your piston to wall clearence done at? I also agree with anaik's statement.

 

The clearance figure I have to get monday from the builder but from what others have told me the slap noise is normal as a forged piston is slightly oval when cold and expands more when hot. I'm also told the slapping will not hurt the cylinder as long as I take it easy and wait til the motor reaches operating temperature before mashing the accelerator

 

Yep, you are correct in that. Depending on what you asked for, some machine shops will give u slightly abit more piston to wall clearence to factor in for n2o or boost applications in the future if u ask them too, not sure if they did the standard .005" (mite be off on that) or .006-.007" for lets say nitrous like I did. I did a .0065 cus i will be using nitrous and SC in the future, so i decided to plan ahead. JE told me to do this, and the only down fall would be a slight piston slap when the motor is cold until the piston heats up and expands.

 

As long as the slapping is not hurting the cylinders I can live with it.

 

I had this same problem. I only went with .0045 and my my car would clack for about 5 mins before it would warm up enough to completely go away. This was with JE/SRP forged flattops. Now that I spun some bearings and have rebuilt it, we will see what Wiseco's sound like with .004 I just hope it dont sound like the last time.

I dont see how anyone can go with .0065! That must sound horrible.

 

.0060-.0070 is almost required if your running n2o and a boost application. The stock NA piston to wall clearence was stated by JE is .005. Now if you add nitrous that is gonna make the piston itself expand more because of the added heat, now the extra expansion + not enough clearence = BIG problems. I told JE my engine shop put the stock .005" piston to wall clearence in my 383 and they told me to call the engine shop right away and tell them to change it. I said will .005" work with nitrous, they said barley, and its not worth the risk. Now add N2o and boost and he said he knows for a fact the piston will pretty much seize in the cylinder hole cus it doesn't have enough room to expand. I think alittle piston slap during warm up is worth it for peace of mind when it comes to piston expansion and clearence :-)

 

If the SRP pistons uses the 4032 alloy then the clearance on their sheet is 0.0025" for NA application. This alloy does not have a high expansion factor as the 2618 thus requirin more clearance.

 

Im sorry, i neglected to say my pistons are custom JE 2618 alloy, so yea mine will require more then the normal forged pistons.

 

This is great information thanks.
Will the slapping during the cold operating temperature affect the cylinder walls at all? Is it advisable to let the warm up for 5 minutes or can I drive it lightly until it reaches operating temp?

 

Well its my opinion that you should always kinda let a motor warm up after you start it, not just start it and tear *** haha, even in the summer i do it. You shouldn't have a problem, unless its clearenced to much and its slappin even after its warmed up. Find those clearences out for us...Piston to wall that is.

 

.004" is the clearance. I never take the revs over 2000 when its cold. The builder says thats ok

 

JE manufactures two different pistons. Their high end 2618 aluminum JE, and their SRP line made from 4032 high silicon content aluminum. The 4032 is more suited to street driven engines as it has a lower expansion rate. With a lower expansion rate, less assembly clearance is called out by mfr., thus making for quieter cold engine startup. This contrasts with the 2618, which may experience piston slap at cold startup. Unless you are comtemplating FI or nitrous, there should be no reason to have piston slap at cold engine startup, with the 4032 aluminum SRP piston. If there is, the incorrect assembly clearance was used, regardless of what your builder told you.

 

I think I'm gonna email JE on this one