Hello I have a ZZ383 that came with 7 +.005 pistons and one STD piston. The previous owner was tired of messing with this vette so I picked it up. I am down to replacing the undersized piston with a +.005 like the rest to fix the knock. I need the piston to bore specs and the top/btm ring gaps. I have tried Scoggin but I was put on hold after having to alomst beg for help(I wont be calling back). Any help will be great.


Oh yeah this engine only has one dyno run on it due to the mystery knock(small piston).

Thanks

 

out of curiosity, how was the small piston discovered?

 

I pulled the head off and the piston tops all but one have the same number stamped on them. They even have .005 on top of 7 of them then a STD on the small one. I ordered the correct size piston/rings from SDPC and the tech guy said it is a common problem with the ZZ383 and ZZ4s since the Hyper pistons get a tighter bore clearance. Now I remembered that I need some specs so I called back and I get the BS treatment.

here is a quote from a GM TSB:GM Piston Slap Fiasco. A loud embarrassing and annoying internal engine knock. Vertical piston and cylinder wall scuffing. extent of the "piston slap" engine defect. GM TSB #'s 01-06-01-022, 01-06-01-028 condition should be considered "NORMAL

I guess its normal to have different size pistons?

 

Are you removing the motor to perform this work?
I would find out who makes the hyperutectic piston for Chevy and contact them for piston to wall specs.
I would make sure you carefully check the bore diameter and properly hone and fit the new piston. The rings should have a spec sheet included for ring end gap.
If this was my car, I would also remove the other 7 pistons and I would check piston to wall clearances on them as well and strongly consider new rings and honing for all cylinders. If they screwed up one bore, they probably did not do very well on the rest.
Best of luck.

 

Engine is still in the car and I have pulled the other piston/rod assemblys on the even side of the motor now. They all 4.0075 bore with 4.005 pistons. I have been trying to see if .0025 is the correct clearance or not. It also seems that a .030 top compression ring gap and a .033 bottom is normal for this application and it fits the KB specs. This motor has run a total of 1 hour max. before being shut down due to the piston slap. All cross hatching is still present in the bores and all bores are even with zero taper. I guess Ill press the new piston on and see what she will do. It made almost 400 rwhp from 2500RPM up last time and 500 rwhp at 4500RPM. Thats when they shut it off deciding that the knock wasnt going away as it heated up.

I still cant believe that 3 GM dealers couldnt tell me the specs on the parts they sell?

 

I thought the ZZ383 had full float pins?

 

ah ha... good job. as long as you have the heads off I see opportunities for other lil mods.. are you considering replacing all 8 pistons as long as you have the heads off? rings? any head work?

 

I can't find the specs for your particular engine, but I did find some specs for late model GM small block, I work at a dealer and these are legit. The max it's giving cly bore to piston diff is .0026 by my calculations your at .0025 (except for the odd ball piston obviously) the outer edge of spec, but still in spec.
The ring gap specs I found are as follows top compression ring end gap is .010 - .015 second comp ring end gap is .009 - .019. As I said I don't know if these specs apply but maybe they'll help.

 

KB would have all the specs, obviously. I know for fact the ring end gap for the top ring is .055 per inch of bore. The top ring gap is critical on the hypereutectics. Too much is WAY better than too little. The 2nd ring is much more forgiving. If you're reringing it, I would tighten up on the 2nd ring. I believe the spec for that one is .045 per inch of bore. For a total of .018. Not sure on the piston to cyl clearance. I want to say .004, but I could be off.

 

OK, you may not want to hear this and I hope it's not the case with your engine. I have a friend that use to work at GM's Tonawanda Engine Plant. He said when they ran marine engines they would use an oversized piston and a standard bore. The marine engine runs cooler and can benifit from the tighter clearances. But, a street engine would eventually seize from the the extra heat expansion. It sounds like you may have 7 marine pistons and one standard(street) piston.

-Mike

 

Thanks for all the help guys! I got ahold of a friend that runs a high end race shop here and he contacted one of the cheif eng. in the zz383 build project. It turns out that the problem with this zz383(early builds like this one) was that they use a standard 350 block bored to accept cast or forged pistons. The cast and forged pistons will expand much more than the hypereutectics used in the zz3s,zz4s and the early zz383s. This leaves the bore to piston clearance too big which results in piston slap. To correct the problem GM had some custom +.005 pistons made to use the larger bore blocks and reduce to clearance to .0015-.0025. According to Keith Black tech dept. I can run the piston to bore as high as .0045....really not too sure of this. For the ring gaps I found that between .026 and .038 is good for a top ring gap since the Hyp. pistons run very hot on the top ring and end butting has been a problem. The bottom ring will vary depending on the use of the engine. I paln to be turbo'd so ill leave the gap just like it is.

I really hope that no one else runs into this, but if you do PM me and Ill try to get you some contact #s most dont have access to.

thanks again!

Chris

Oh yeah I am running a set of Edelbrock e-tec 200s, a modded superram, lt4 hot cam and once this is all worked out a pair of stock Grand National T3 turbos with 8-10 psi and meth injection for cooling. Thats as many mods as I think I want to run for now since it is just a street car. I would have liked to install some nice I beam rods and forged pistons but I have this as a budget project right now so those will have to wait.

 

I just can't see the LT4 hot cam being the best choice for a turboed engine.

 

I was about to ask how you got 500 RWHP out of a ZZ383, but that answered my question!! Be careful, that's a lot of power to be pushing though that rotating assembly! What car is this going into?

 

It really isnt the best choice for a turbo motor but thats what is in it and I am on a budget. I have a bit of fudge factor with cyclinder pressure since Im at 6800 ft elevation. For strength, I feel the crank is up to it(4340 forged and nitrided with a main girdle) as are the rods but the pistons are by far the weak point. As long as I keep the fueling and timing under wraps it will be ok.

 

If you're running a 4340 crank and hypereutectic pistons you need to have your head examined. i would much rather run a cast crank and forged pistons than vice-versa. the hypers are a short wick. You'll never know, and then boom! They give no fair warning. Its not as if you're going to hear anything. its not like "as long as it doesnt ping". That sort of stuff. if you plan on forced induction and running hypers, its not a question of IF but WHEN. As long as you think it will be OK if you "keep the fuel and timing under wraps" you're fooling yourself.
PS If you're on a budget, cheap out somewhere else. The INEXPENSIVE way is to do it right the first time.

 

I understand your opinion is your opinion. With that said I am by no meens a newbie at this, I have been running turbos, blowers and N02 for over 20 years. I know the limitations of hyper pistons and of the other internals so I guess Ill get my "head examined" at my next regular check up.

Thanks for the concern of my wellbeing.

 

I thought I had heard it all. All this stuff you want to cram in this thing and you summerize with "I'm on a budget". If you dont start with a solid foundation, what do you have? We build all our "street" motors with the same rotating assy as FI motors:4340 cranks, H beam rods and forged pistons with file fit rings. All for DEPENDABILITY. Ditch the turbos and build the bottom end to last, and spray the snot out of it. It will cost less and last longer. If you're on a budget, whos gonna tune this thing with twin tubos and hyper pistons? nobody in their right mind, thats who!
PS the price differance in hypers to forged pistons aint but about 200$. How cheap are you?

 

here is a link to the GM ZZ383 specs.

http://www.monstermarketplace.com/Au...415a73193.html

If you read them you will see that the forged crank and hyper pistons were coupled by GM. Its not like I went out and picked this list of parts. You ask "who will tune it" your answer is ME. If you really want to get into specifics just ask. This thread is titled "ZZ383 help". The question I asked is for clearance specs...if you dont have them then keep your trap shut and SCREW up some one elses post.

It never fails, someone asks for a bit of tech and some know it all starts to spout off. I really enjoy ALL OF THE HELP but this has gotten so far off topic it is pointless.

From here on out I am only repling to clearance and ZZ383 tech in this thread.

Chris

 

Lighten up when folks are trying to help, even if you don't like their answer. Now the thing I'm unclear on is this. Are these GM pistons or KB's? I ran KB hypers in my last engine and at standard bore, it was recommended a .026" top and .018" second ring gap. It worked well for over 20K miles until I installed the new engine. But turboed engines will have a different spec recommendation for wall clearance at least, I'm not sure about the gap. KB's website has all of this info but if they're GM hypers, then I can't help you.

 

Whatcha wanna say now? I'd bet its the most accurate post in your thread. still doesnt change the fact that twin turbo and budget dont belong in the same sentence. But what do I know?