Piston Slap and now what???

[Vette66AirCoupe]

Chapter 1
I had some work done on my '66 recently and decided to add the chore of removing the exhaust manifold gaskets that aren't supposed to be there to the short list of stuff that needed to be done. I was warned that the gaskets might be there for a reason but they would try removing them and if there was an exhaust leak I could always put them back on. I also asked for some advice on some engine noise that I can't idnetify.

Chapter 2
The result was that the exhaust was fine without the gaskets and the engine noise was diagnosed as piston slap with the disclaimer that it's really hard to diagnose on a side pipe car. It was always a little louder when cold. Of course this is annoying when you crank it up after a cruise and people get that WTF is that noise look on their faces. I decided to live with it as the fix would lead me down the path of a complete engine rebuild. The repair facility's opinion was that it was not a dangerous situation.

Chapter 3
I backed the '66 out of the garage last Sunday so I could get under the '60 to finish up the clutch replacement job. It was colder than normal, around 16 degrees outside, 45 in the garage as I hadn't fired up the heater yet. The car sat outside for about 4 hours. When I got back into it and started it up there was LOTS of engine noise. I decided to take it for a ride and it became obvious that I had an exhaust leak as well as the engine noise diagnosed as piston slap. Almost as soon as the engine temp got to normal the exhaust leak went away. The piston slap, if that's what it is, remained, though much diminished since the motor was warm. This is a numbers matching coupe, 427/390 with A/C and P/S. The oil level is full and the pressure is fine.

Chapter 4
Lying in bed, daydreaming while watching the Olympics, whatever it is I'm tossing these problems around in my head. Could I have the exhause manifolds decked and solve the leak problem? Might I find out it's not the manifold surface but the heads that are not true? Will the piston slap lead to a major problem that might destroy my numbers matching block and wipe out some real value? Should I just bite the bullet and go for the rebuild, a task I do not want it undertake on my own? What would it cost to do it right? Will I end up with damage to the body?

Chapter 5
Here I am asking a million questions hoping that the sheer volume of this thread doesn't deter you guys from commenting. I'm in the fog and would really appreciate your input.:confused

 

[BarryK]

Terry
i can't help on the piston slap except tell you that if your pistons are slapping you, than hit them suckers right back!

on the exhaust manifolds, I'd say the easiest fix would be to just reinstall new gaskets as you had in there before. Have you decided whether or not you plan on taking the car thru NCRS judging or not. I know you were talking about it last year. If you are, you will get some deduction points for having the gaskets there but if your not worried about judging than I'd just put the gaskets back on and be done with the problem. It's an easy enough job to put the gaskets on.

 

[JohnZ]

The exhaust leak issue usually results from slight warpage of the exhaust manifolds (which isn't unusual); a machine shop can true them up with a light cut, or you can use the gaskets.

The piston slap is quite common with high-mileage forged pistons, but they were only used in the solid-lifter engines; the 390hp engine used cast pistons with much tighter clearances (forged - .004"-.006", cast - .0009"-.0015"). I'd be concerned if the piston slap noise is loud and has just started, vs. coming on gradually over a long period of time. It may be time for a rebuild.

 

[redvett]

I like piston slappers they were part of the older corvettes with forged pistons and knock isnt a bad thing. My 2001 A4 Corvette has the cold knock but boy will she run. 12:98 @ 109 mph in the 1/4 mi bone stock. )

 

[Vette66AirCoupe]

John,

The slap did come on gradually over a period of time. I have no way of telling whether this engine has been rebuilt but would suspect so based on it's age. 59K on the odometer but who knows what reality is. I like the idea of the cut on the manifolds.

Red,

Mine's no race car being a stock 390 hp engine but it runs real strong, idles nice and doesn't smoke! RE your ride, nice ET for a stock vette, yeeehaw!

 

[Kid_Again]

rebuild that puppy...first, you have JohnZ's advice...second, why would you feel comfortable driving a car that makes a LOT of noise?...doesn't matter whether it's cold or warm, it ain't right

only a matter of time

BTDT

 

[IH2LOSE]

I like piston slappers they were part of the older corvettes with forged pistons and knock isnt a bad thing. My 2001 A4 Corvette has the cold knock but boy will she run. 12:98 @ 109 mph in the 1/4 mi bone stock. )

Nice times

I have an 02 with an A4 also

Funny but after awhile you get used to the car and think its actually slow for a vette until you have something pull beside you and you leave them in the dust and actually wonder if they let off or something.

If you want to hear a piston slap,so you absolutely know what it sounds like find some one with a 01 to 03 C5 and listen to them start it when its cold. You can hear it clear as a bell with the hood closed

What worries me on this is that you say people hear it at a cruise,A slap is almost like a whisper down deep in the motor but you can hear.

If it that loud that folks are hearing it loud it may be the time to pull the 390 and bag it in the garage and throw a crate motor in it,

I purchased a 454 HO 425 hp for under 5000.00 it still has the boss for a clutch cross shaft and a mecanical fuel pump . Its pretty much a bolt in replacement, Runs on pump gas with 36 degrees of total timing.

Good luck, Would be a terrible think if its actually a rod promlem and it lets go and does some major block damage.

Sorry and best of luck

 

[Vette66AirCoupe]

Does anybody have a ballpark estimate on a complete rebuild?

 

[JL66REDCPE]

Terry -- from your message it did not sound like the mechanic that diagnosed the sound was 100% confident that it was a piston slap (based on the sidepipe comment). Without hearing the engine is it possible that it is a lifter problem. The 390 has hydraulic lifters and maybe one or more of them is leaking down and not pumping up properly. I would suggest a second opinion on the noise from another shop before I went looking for a rebuild.

 

[BarryK]

Terry
My buddy was going to have his L79 SB from his '65 completely rebuilt about a year or two ago and he said it was between $4K-$5K.

There is a place locally called Engines Plus that is suppose to have a decent reputation, you may want to check with them and see how much they estimate for a rebuild. Bill Skinner may even has a suggestion for an enginer rebuilder that is good.
Whatever you do, be sure to remember to tell them not to deck the block!! :eek

 

[gmjunkie]

Just a thought!! I have had BB Chevy Valve seats Disappear running low lead or no lead gas!! Newer engines have harder seats!!! Pre 72 if stock have softer seats and Low lead or no lead will let the valves Beat the seats right out of it!! And they Can run hard and sound like piston slap!!!!! My LS6 in my haul'n Truck Lost 5 exhaust seats and still Ran Like a "Raped Ape" but was Nosier than Hell!!! When I removed the Heads and Disassembled them, there was 5 exhaust seats Completely GONE!!! The valves were sealing on the Head Castings!! Anyway $500. later Manley swirl polished stainless valves and ALL new hard seats!! Quiet as a church mouse now,and still runs Like a "Raped Ape" on 87 oct.!!

 

[Kid_Again]

I have direct experience on your question about rebuilding a 427/390hp IF you don't need any replacement parts. I'll correct that by saying I switched to '781 heads with new valves cut in and minor stuff like that. It's $5K on a good day. IF you need 427-specific pieces, you are going to blow through some serious money.

I rebuilt my 427 and regretted it. I now have a zz454. Save $5K now and go to a crate motor. Or, if you're into # match, expect to pay up to $7K if you live near a major metro area and things just don't go well for you.

PS, If you want decent heads for your 427 rebuild, give me a call. Frankly, I'd rather see you buy a crate motor but business is business.

good luck

 

[Vette66AirCoupe]

Junk,

That's an interesting thought. I remember when I bought it the seller said something to the effect that surely the valve seats must have been replaced by now but there was no way he could know for sure. If I'm real lucky, it's a lifter. If I'm moderately lucky, getting the heads done right will be the end of it. If I'm unlucky, I'll need the rebuild. Thanks for posting!

Kid,

I hear you, and I appreciate the advice. But I think I'd probably rebuild it rather than have a spare engine laying around in the way. Besides, I have a pipedream of getting it judged some day. If I have to pull the engine out, given that I have NO history on the car, I'd probably do the following:

First and foremost, a complete disassembly and a thorough check of the block before anything else.
Then I'd have the crank checked and turned as necessary, or replaced if need be.
I'd probably be buying new pistons and rings as I'd bet I'd end up .030 over on a rebore. (It could already be there for all I know)
Complete head work, new valves, lifters, rockers, springs, retainers and push rods.
New cam bearings and check the camshaft.
New rods, rod bearings and crank bearings.
New oil pump, timing gears and chain.
New water pump and fuel pump.
Starter Rebuild.
exhaust manifolds planed.
Possible clutch job.
At least the carb and distributor are in good shape. :eyerole
Throw in the gaskets set, a set of wires and some other incidentals and $7K starts sounding like a bargain.
I'm sure somebody will remind me of something I've missed.
I have to go now.

 

[Kid_Again]

Yup, I hear 'ya. Good luck. All I know is I got a #match 427....sitting in the basement :eyerole

 

[ChuckG]

If you want to hear a piston slap,so you absolutely know what it sounds like find some one with a 01 to 03 C5 and listen to them start it when its cold. You can hear it clear as a bell with the hood closed

My 2000 has piston slap, especially when cold. Goes away when the engine heats up. Just about every C5 I've listened to has piston slap on start-up.

My 00 is nothing special. Base/unmodified engine, automatic, performance axle.

Runs 13.50's at the strip consistently, AND you don't even feel like you're going fast. Chuck

 

[GTR1999]

On esuggestion if you rebuild. Know who you're going to use. I have an original L-82 4 spd 75 that I broke down to the bare block and measured before bringing to a local "specialist"
When I got the block back he must have never thought I'd measure it again. It was bored over by .005" completely useless at .030" know I have to go to .040 and buy all new parts. The genius,when confronted, tried to say corvette run hotter then normal cars and he builds race cars and knew more then Federal-Mogul engineers. Yet he couldn't explain why the ring gaps were .034" when installed. We're going to dicuss it further in court soon.
Point is, make sure of the job before you assemble it. BTW, others have used this guy to machine and assemble the engine and it failed a few weeks later.
Good luck with your car,
Gary