canbaufo

Fellow cz28.com member is helping me with the new motor. He is in another state so I haven't seen it run yet personally. He says he can hear piston slap and believes it is the cause of false knock showing up on the scan. The machine shop said they clearanced the pistons to .005 We had asked for .004 because of reverse flow cylinder head cooling. It was never actually checked by the mechanic who assembled the motor.

The fellow who's helping me says if it's the cause of the false knock I will probably just have to live with it or disable the knock system. I am definitely not comfortable disabling it. He said we may try loosening the sensor in the block a little (yes, it's an LT4 module on the ECM). I am afraid if we do that it may not be able to hear real knock either. Any way to get the sensor or module/ecm to differentiate between knock and slap?

How many of you who've rebuilt went with .005 piston clearance? Do you have the same problem?

jonaddis84

I think you are a little tight, are you sure .005" is the number you are thinking?

Ive always been told, and recently by a nascar/top fuel builder that .080 is the ABSOLUTE MINIMUM, and .100 is a safer number to shoot for. .005" is like inkjet paper i just measured in at .004" so I dont think thats what it is set at, or if it is its definitely a problem.

biggtime

You didn't state who's pistons you are using so i do not know if it is to loose. I have my SRP forged pistons with .0045 clearance with no noise at all. .005 is a little loose but then we need to know what pistons they are (forged, cast, hypertuetic). Some run tight at .0025 to .003. Unfortunately the person who assembled the engine did not check so this is all of a guess. I have seen some right ups that people have changed the threshold of the sensors sensitivity with different resistors but have not done this myself. I personally run a DFI without a knock sensor so this is no real crises to me.

SStrokerAce

All depends on the brand of pistons, piston alloy, where that brand measures for P to W clearance.....

Depending on the piston .005 could be loose.

Bret

Denny McLain

Usually when you buy pistons the specifications are included and everyone except backyard mechanics checks the clearances. Also is it doing it just while it's cold or all the time. It's not uncommon for LT stuff to rattle some when cold, but goes away once it's warm.

You can program your knock sensors to not pull timing out if you sure the noise you hear is not detonation. A bnch of people do that even though I don't think it's the best of ideas.

marshall93z

you might be thinking about P to V clearance!!

jonaddis84

Good call

Didnt even notice it. Disregard my post.

OneFlyn95z28

Well i have a guy here who has the same problem. and we now know why. As you pointed out the mechanic did not check to clearance. Wrong answer for him.

THis customer did check his clearance and thought it was good...then found out he measured it the wrong way.

Now add to this the fact the pistons he bought were clearanced for a 4.0300 bore and the Machinest cut them to 4.0350 and you quickly find a .0100 clearance.

As many have pointed out with out all the clues like piston producer and what the final bore was hard to tell

Good luck man!

1racerdude

If anyone can hear piston slap from .004 to .005 They are damn good. .001 is not going to sound like a diesel either.Ya got to get way more than that to get a good set of pistons to knock.
IF,and I say IF you have slap@.005 you will only hear it when it is cold,it will go away when the engine get's to NOT.
Don't believe it's a problem but if it is get you the correct resistor from Radio Shack and put it inline to your knock sender and be done with it.

canbaufo

Sorry I forgot to mention piston type. They are Speed Pro forged pistons. My mechanic checked everything else like bearing clearances and deck height .....beats me why he didn't check the piston to wall clearance. Anyway, the machine shop claimed they did it at .005 which is probably what the pistons call for on the box they come in.

Can someone explain this to me anyway ...I don't really understand what the clearancing is. I thought if you were bored .030 over you were bored .030 over ...how/when does additional clearancing come into play?

Also, yes .....it is happening after the car is warmed up too. As you rev the motor the module pulls more timing as the RPM goes up.

Mindgame

The clearance is the difference in the bore diameter and the piston diameter at it's checking area. Let's say you've bought a set of .030-over pistons and the manufacturer recommends .003 clearance for your type of build. You mic the pistons (where the manufacturer recommends you mic) and they measure 4.028. So, your boring operation is shooting for 4.031 to establish the correct clearance. Just an example but that's why the pistons need to be in the builders shop before any boring is done.

HTH.

-Mindgame

canbaufo

Well, the pistons were at the shop when they did the work ...so hopefully they did it right. Thanks for the explanation ...makes sense. I think this is what has happened: The machine shop is a little "old school" and pretty much treat the LT1 Block just like any other SBC. The piston Manufacturer (Speed Pro) recommended .005 .....however, my engine designer recommended more lik e.0035 to .004 due to reverse flow cylinder head cooling (less heat - less metal expansion). The machine shop liked to do things "their way" and just did it at .005 like you would for a SBC. I think this is what has happened. Since you hear it even when warmed up .....it's clearanced a little too much. Darn it I hope it isn't really the cause of the false knock.

Oh yeah ....by the way, if they overclearanced the block wasn't I pretty much screwed with those pistons anyway? Once you've overclearanced it's too late and a different set of pistons would be required.

1racerdude

The noise is probably something other than slap.
There is all different kinds of false knock in a built engine. Do a search and get the resistor that goes inline with your knock sender and get rid of any false knock.Get a good dyno tune also to make sure your timing and fuel are correct and you should be good to go.

canbaufo

Are you saying you can get a resistor that will filter out the false knock frequency but allow the frequency from real knock to go through? I would think the frequency emitted from real knock would change in different situations. Where should I do a search? Here? Only reason I ask is my connection is horrible and it takes forever to do anything on the internet.

jonaddis84

I dont know the expansion properties of the pistons you are running, or even pretend to. But if you cant get rid of the knock, try running the engine hotter, like at factory temp, assuming you probably have a 160stat in it.