Hot Rods ZZ 350 with top end knock

I am helping a friend with a ZZ mtr in an early Ford. They are not for me but---
When cold it has a definite knock of some sort. When warm it goes away. It runs very strong with great oil pressure and runs cool as well. I've never fooled with these ZZ roller mtrs--suspect piston slap or ? Some say this is not uncommon with these mtrs but it is irritating--were it mine I would replace it with a small journal 327 but that's just me. Ideas???
Thank you

 

does it do it with the belts off?

 

If it's got forged pistons, it's piston slap. Once it comes up to temp, the rings expand enough to fit properly.

 

sounds like only one piston probably #4 or #6 but I know sounds travels Yes noise it there with no belts and it is more top end than bottom end it sounds like--gonna runs it on stands with wife in car and load it a bit when cold. Had a 2003 GMC company truck with similar sound when cold--drove it 183K-sounded the same as when it was new.

 

The first versions of of the ZZ 350's definitely had piston slap. The problem was the silicone content in the pistons. They changed that and the problem went away. Maybe you have the earliest version engine.

 

Chevy website says high silicon pistons. That means they're most likely hypereutectics, and with the tight piston to bore clearance used on hyper pistons its unlikely to be piston slap unless something's bad wrong.

Companies like GM don't throw money at stuff unnecessarily, so while the pistons could be forged 4032 alloy and still fall in the category of a high silicon content, its highly unlikely as the hyper pistons are plenty strong for the power levels that engine is built to produce.

 

I had a ZZ3 sounded like the pistons were swaping holes when cold. ran very strong I pulled it out and built a small journal 327

 

A lot of the mid 90's 350's did that. My 1994 Buick Roadmaster knocks for the first 15 seconds or so of running, then quiets down. 124K miles on it, and it still sounds the same as it did 8 years ago when I got it.

 

Pull the plug wires one at a time piston slap will change on a dead cylinder.

 

I got a friend with a ZZ and it has piston slap 'til it warms up. He bought one of the first ones to go in his street rod.

What is he using for exhaust. I chased a tappet ticking for a month once that turned out to be an exhaust leak on the number 8. Found it by accident. LOL if he is running manifolds and not headers it may warm up and expand enough to seal up. Just thought that I would throw that out there.

I don't know about now but those engines used to come with a build sheet if your friend got one and didn't toss it, it would help to take a look and see what you're working with.

 

This things not running mechanical fuel pump, is it?

Actually it's the piston skirts that expand to reduce clearance with the cylinder walls.

 

Are you serious????? and who told you that shit?

 

Well he got the expansion part right just got the wrong part expanding. in the end something swells up and it gets tight.

 

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Beaner,I guess he got his tongue caught in his eye tooth and couldnt see what he was saying

 

You know, a tech teacher told me that a long time ago. I guess it made sense at the time, and I never really thought about it afterwards. I personally have never had that problem.

 

All the early ones had piston slap because the wrist pins were centered. At some point GM went to offset pins to cure that problem.

 

I have a CRATE ZZ3 and now have 2,000 miles on it.
Mine also demonstrates noticeable piston slap noise when cold and to a lesser extent some mild piston noise when fully warmed up.
Uses no oil and runs like jack the bear.
Characteristic forged piston noise of the early ZZ engines.
The later ZZ4 engines had the wrist pin location changed to minimize the piston noise but it's not affecting any durability issue.
Personally, I've always liked the noise of forged pistons and solid lifter noise.
Takes me back to the fun and raucous 60's.

 

Wasn't trying to stir things up, just going for a little clarification. Your tech instructor may have been explaining about filing piston ring end gaps, so that at operating temperatures the ring ends don't butt together and try to seize the ring in the bore.

 

That could very well be it. I do remember someone was asking about piston slap and the teacher started talking about the the rings expanding. Note: It was a night class, and I was working a full time job and went to school 3 nights a week. I could have very well misunderstood a thing or two.

 

Wear ear muffs until it warms up!