Street Hemi rattles when running

[skyman51]

I have been told many have asked about this. When running the street hemi rattles. It is not a valve tappet noise but a true rattle. I have been told this is normal. Anyone else care to chime in?

 

[RustyRatRod]

Does it sound like all eight doing it? It's probably floating wrist pins. I have a friend in Warner Robins, Georgia with an original 66 Hemi Charger and his does the same thing. Kinda sounds like a diesel, but quieter.

 

[Hemirunner]

I have been told many have asked about this. When running the street hemi rattles. It is not a valve tappet noise but a true rattle. I have been told this is normal. Anyone else care to chime in?

Are you asking about a particular street hemi or saying that you've heard that they all do it? Very confused. Hemi's in general produce alot of top end rocker noise but a true rattle would be detonation and you'd eventually beat the bearings out of it.

 

[rev.ronnie]

Hemis with the stock type TRW forged piston, have .005" piston to wall clearance. This will cause them to have a "piston rock" or rattle when cold. It doesn't help that the piston pin height is really high on the Hemis. The piston is top heavy above the pin....a 7" connecting rod would really help matters, but would not be stock, of course. I use up to 7.1" rods even with 4.5" stroker engines. They are real stable.


When I have custom pistons made for them, I get more offset in the wrist pin built into the piston. Adding a moly graphite coating allows tighter piston to wall clearances as well. The KB pistons run at half the OE clearance, so they run quiet as well.

The wrist pin clearance in both the rod and piston is less than .0015". If they are making noise, you have serious problems. I would doubt this is the case.

Now, remember, they did make this noise when they were new, so if you want it correct...you have to deal with it. Thicker oil Wil help it a little and when the engine is up to temp, it should be almost nonexistent. If they rattle hot, the piston to wall clearance it too loose, or the skirts have collapsed.

 

[skyman51]

I just bought a restored 1967 Hemi GTX. The engine rebuilt by Larry Shepard, so I don't think anything was not perfect. I am going to talk with him. I do know the engine was rebuilt stock but was bored .020 over and has new Ross pistons. I have several friends that are longtime 66 and 67 Hemi owners (solid lifters) and they both said that rattling sound is normal, that theirs have been doing it for years.

 

[RustyRatRod]

When I said floating wrist pins, I did not mean the wrist pins themselves made the noise. Full floating pins allow the piston some movement that actually frees up some horse power by reducing friction. They allow the piston to rock in the bore ever so minutely relieving stress. THAT'S where the noise is likely coming from. Couple that with forged pistons and you can have a rather noisey, yet perfectly healthy engine. I never have liked the idea of piston pins being off center. This is done for only one reason. To make them quieter. It does absolutely zero to aid in performance and in fact, causes friction. That's what keeps the pistons quiet. Constantly being under load from an offset pin. This is why old schoolers rebuilt engines and turned the OEM pistons around backwards on the rods and also why all performance pistons are made with zero pin offset. Reversing the pistons on the rods put the offset going the other way to reduce friction. While it certainly makes for a louder operating engine, it frees up some horse power. It's dyno proven. I took for granted everybody knew what I meant. I guess not.

 

[rev.ronnie]

If a press fit wrist pin is binding enough to restrict piston movement...it will gall. If the clearance is correct, the movement will not be restricted. The performance difference between floaters and press fit is probably not measurable. In fact, Smokey Yunick did testing on this very thing many years ago, finding no measurable difference between the two. His conclusion was that floaters were easier to service and that was about it.

The purpose for the piston pin offset is indeed to quiet the pistons operation. With most restoration stuff I deal with, overall drivability and reliability is the paramount concern, not every last HP.

If HP is the primary concern, noise is the least of the worries.

 

[skyman51]

Does it sound like all eight doing it? It's probably floating wrist pins. I have a friend in Warner Robins, Georgia with an original 66 Hemi Charger and his does the same thing. Kinda sounds like a diesel, but quieter.

That sounds like the noise I am describing. The wrist pins are no t offset. I have been told that in 68 when the Hemi went to the hydraulic cam, they were much quieter. I am learning, I am a Chevy guy.

 

[rev.ronnie]

It's funny that you mention installing the pistons backwards. I had a customer that used to install them that way in big Chevys he would build for people, 20 some years ago. He would have me cut his valve reliefs so he could do this with the cheap cast pistons he used.. He probably paid for my piston vice

 

[b-star69]

dang picky hemi owners! lol hey triple r where in georgia do you live i'm probly 50 miles from wr, are you going yearone on sept 8 th

 

[RustyRatRod]

If a press fit wrist pin is binding enough to restrict piston movement...it will gall. If the clearance is correct, the movement will not be restricted. The performance difference between floaters and press fit is probably not measurable. In fact, Smokey Yunick did testing on this very thing many years ago, finding no measurable difference between the two. His conclusion was that floaters were easier to service and that was about it.

The purpose for the piston pin offset is indeed to quiet the pistons operation. With most restoration stuff I deal with, overall drivability and reliability is the paramount concern, not every last HP.

If HP is the primary concern, noise is the least of the worries.

Yeah, I shook Smokey's hand in 1986 in Atlanta. Got to shoot the breeze with him for an hour or so at the old Speed Distributors vendor's showcase, along with Vic Edelbrock, and even Richard Petty. Smokey was a cool old dude, no doubt about it.

Different folks have different opinions and results on it. But, the fact remains that floating pins DO allow some movement. That cannot be debated. I have personally spent some time in the dyno room at Mercer University in Macon, Georgia and had the pleasure of watching James Lovett (RIP) making several runs on a couple of different engines. He was our auto instructor in high school and a retired Benz mechanic from the 40s to the 70s. He always contended the same thing. That there was some friction loss to be had with full floating pins. Friction loss=horse power.

I look at it like this. All of the heavy hitters had floating pins. Heck, I even had a friend whose dad had an Olds Delta 88 with a 425 "Ultra High Compression" as it said on the air cleaner. That was one of our auto shop projects in high school. Rebuilding the engine. Guess what? Yeah. Big, gnarley 4 door luxo barge.....with full floating pins. lol

I ain't arguin just for the sake of it. Being a machinist and mechanic a LONG time, I have torn down a LOT of engines. One thing I an say was a pattern I saw. Low compression=pressed fit. High compression=floating pins. Not everytime.....but dang near it. Them factory engineers knew something. I just don't buy it that ease of assembly was the only reason.

 

[rev.ronnie]

Most pressed pistons were way too tight from the factory, I'll give you that, for sure. Even now, when you buy a new Fed Mog forged piston, the clearance is like .0003". I hone them out to .001"-.0015" depending on the application. Tight and seized wrist pins were the cause for most of the collapsed skirts I saw with these pistons over the years.

You were lucky to have met Smokey, he was a legend! I've owned my machine shop for close to 25 years now and I still enjoy reading his old books, out dated or not. He was such a candid guy. His autobiography is stellar.

As far as the OP and his noise, I am still going with piston to wall. It's just a nuance of its design.

 

[RustyRatRod]

Most pressed pistons were way too tight from the factory, I'll give you that, for sure. Even now, when you buy a new Fed Mog forged piston, the clearance is like .0003". I hone them out to .001"-.0015" depending on the application. Tight and seized wrist pins were the cause for most of the collapsed skirts I saw with these pistons over the years.

You were lucky to have met Smokey, he was a legend! I've owned my machine shop for close to 25 years now and I still enjoy reading his old books, out dated or not. He was such a candid guy. His autobiography is stellar.

As far as the OP and his noise, I am still going with piston to wall. It's just a nuance of its design.

For sure! I am thankful I had that opportunity. Speed Distributors was much like Summit back in those days, except they were wholesale only. Since I worked for a small mom and pop auto parts and machine shop running the machine shop, I had the opportunity to attend. I did every year and met some cool people. 86 was by far the best year. Or was for me anyway. They had said that it was even better in the late 70s, but I would have been a tad young then. I also agree with your honing method.

You know Don Garlits always said "run clearances loose and run lots of oil pressure". I am a firm believe in that. Loose makes power. I even will run low tension rings in a street motor. I mean....either you're serious about makin power or not, right? Most any serious hot engine will smoke a little anyway. All the old timers I used to hang with always said those were the ones to watch out for. lol

 

[rev.ronnie]

Yeah, I run low tension rings in everything except some supercharged engines. I've done it for years too, way before it was "cool" Now look, GM uses 10-12# oil rings in pretty much everything.

I don't like bearings to run too loose though.. Windage eats power. We had one on the dyno a few months back that had one qt. too much oil in it...it lost 40 HP!


Remember the old saying; if it too loose, only you will know, if it's too tight, EVRYBODY will know!

 

[Hemi Rebel]

That sounds like the noise I am describing. The wrist pins are no t offset. I have been told that in 68 when the Hemi went to the hydraulic cam, they were much quieter. I am learning, I am a Chevy guy.

HEMI didn't get hydraulic cam till 1970

 

[Meep-Meep]

My hemi is reasonably quiet aside from some valve train noise (solid cam). Best I can describe piston slap is the engine sounds like a diesel when cold. And yes, a 2000 series alloy will be set at looser clearances and will produce noise - kind of a muffled thunk sound.

A rattling sound to me could mean a tinging metallic sound and no engine should do that. Do you have an automatic or 4 spd? If auto maybe check the flex plate? How about the heat riser in the exhaust manifold? Windage tray interference (assuming you have one). Just trying to cover all the bases. One thing I have done to isolate a potential rod or piston knock is to adjust the RPM until you get the steady noise, then pull plug wires out of the cap one by one until the sound changes. If you can turn the sound on and off by pulling and replacing the wire you most likely found the affected cylinder.

 

[skyman51]

HEMI didn't get hydraulic cam till 1970

You are absolutely correct.:grin:

 

[skyman51]

I spoke with Larry Shepard this morning and he told me what I am hearing is the rocker assembly and the forged aluminum pistons until they expand. He also said that every Hemi he builds, the only oil he puts in and recommends to the customer is Pennzoil 25W50 racing oil. He says it has 2000 ppm zinc. He added he goes further and puts only 5 1/2 qts. in with the filter and adds a bottle of either Comp Cam or Lucas ZDDP additive. He said you can start the motor even in very cold weather with no problems with this oil. I have a heated garage so it never crossed my mind.

 

[RustyRatRod]

I spoke with Larry Shepard........

Pretty much says it all right there.

 

[rev.ronnie]

I spoke with Larry Shepard this morning and he told me what I am hearing is the rocker assembly and the forged aluminum pistons until they expand. He also said that every Hemi he builds, the only oil he puts in and recommends to the customer is Pennzoil 25W50 racing oil.

Hmmm, cold piston slap and thicker oil...sounds familiar..did someone mention that?