RATTLE IN A BIG BLOCK CHEVY

[348tripower]

Ok experts, help me with this one.
 I have a 72 396 chevy. It has a hydraulic cam roller rockers and is a fresh rebuild. Cruising down the road I can push down on the throttle and get a rattle. If I let up it goes away. I can hear it slightly when I raise the rpm while sitting still.  It has not gotten any worse either. It does not sound like spark knock but it is a rattle. It sounds very metallic, but It is eluding me. I have a Mallory mechanical distributor, no vacuum advance,  so maybe that is causing it? I can't here anything with the scope in the fuel pump area. The oil pressure is constant with no fluctuation. I know who did the lower end work to this motor and it should be put together very well.
Suggestions?
Don

[C9]

Any chance your advance curve is coming in to the max too early?

Perhaps you have a drag racing curve?

I had a 400-M Cleveland that the previous owner had recurved the distributor and had it plugged into ported vacuum and part throttle would make the engine rattle at highway speeds.
Didn't sound like the typical spark knock.

Pulling the vacuum line cured the problem during the trip and putting a more sensible curve into the mechanical advance once home took care of it.

Have you done the listen to the engine with a stethoscope - or long screwdriver - bit?

Any chance the rockers are hitting the rocker cover?

[348tripower]

Any chance your advance curve is coming in to the max too early?

Perhaps you have a drag racing curve?

I had a 400-M Cleveland that the previous owner had recurved the distributor and had it plugged into ported vacuum and part throttle would make the engine rattle at highway speeds.
Didn't sound like the typical spark knock.

Pulling the vacuum line cured the problem during the trip and putting a more sensible curve into the mechanical advance once home took care of it.

Have you done the listen to the engine with a stethoscope - or long screwdriver - bit?

Any chance the rockers are hitting the rocker cover?

I scoped it and can't seem to locate the noise. I have a vaccum advance distributor on the way to see if it cures it. Going to pull the covers tonight. And your right about the noise, I have not heard anyhing like it.
Don

[slocrow]

Interesting Don; As you've explained it your either going down the road (so it's loud enough to hear under a load from the drivers seat) or while in a static, free up rev, slightly.
The free rev, are you doing this with your hand on the carb and your head under the hood or is this from the divers seat also? Are the engine mounts solid or is there some flex possible? Is the fan mechanical?
I'm thinking that there might be enough flex to twist the fan to clip maybe like, tranny coolant lines, slightly.
I won't hi-jack your thread but I've got a new chirping i can't figure, coming from mine. Water pump area.......................Frank

[348tripower]

Frank,
Electric fan so that isn't it.  I really think that C9 is on to something. For the record, I can't time this conventionally. The mark won't line up with the timing pointer?  I really think it is the advance built into the distributor.  I don't really know if I am hearing the same noise when I am running the throttle by hand, leaning over the motor. I really don't think so. I have flogged this thing pretty hard and nothing has broken yet. C9 is correct about the noise at cruise. Hammer it or let up and it goes away.
Don

[C9]

but I've got a new chirping i can't figure, coming from mine. Water pump area.......................Frank

Under accelleration or decelleration?

The big Buick in my 32 responds very quickly to throttle.
Most times the V-belt will slip and chirp on both accel and decel.

Keeping it tight helps, but I may not be keeping the belt tight enough.
(Burned up generator bearings when I was a kid by over tightening the belt on a chirping Y-block in a new 56 Ford pickup.  Never forgot it, so I'm a touch leery there.)

I am looking forward to the dual V-belt setup in my 31 roadster and wish there was enough room to do the same in my 32.

[C9]

I scoped it and can't seem to locate the noise. I have a vaccum advance distributor on the way to see if it cures it. Going to pull the covers tonight. And your right about the noise, I have not heard anyhing like it.
Don

If you have another Mallory coming, note that they seem to come through with excessive max timing from the factory and you'll need to back it off a touch.
(The instructions show you how.)

[sirstude]

Frank, What kind of alt/air mounts do you have.  A lot of the high mounts don't get enought wrap on the belt and it squeals on the water pump pully.  Agraveted with running only an alt with no compressor.

Don,  are you loose on the rocker arm adjustments?  Also, what kind of rockers are you running?  I am running Comp roller tip rockers and they are noisy.  

Doug

[348tripower]

Frank, What kind of alt/air mounts do you have.  A lot of the high mounts don't get enought wrap on the belt and it squeals on the water pump pully.  Agraveted with running only an alt with no compressor.

Don,  are you loose on the rocker arm adjustments?  Also, what kind of rockers are you running?  I am running Comp roller tip rockers and they are noisy.  

Doug

Full roller rockers. Gonna check it out the lash tonite.

[C9]

Frank, What kind of alt/air mounts do you have.  A lot of the high mounts don't get enought wrap on the belt and it squeals on the water pump pully.  Agraveted with running only an alt with no compressor.

Doug

You described the alternator mount on my car to a T.

Here's a pic of the same setup with dual belts on my 31.

Upper dual pulley is a Ford model on a Pinto alternator.
The bottom pulleys in the pic are stock Buick, but I have some aftermarket aluminum SBC pulleys adapted to the water pump (a bolt on with 1/8" spacer) and a small aluminum adapter plate to adapt the three bolt aluminum SBC crank pulley to the six bolt Buick dampener.

The 32 is the same except the front sheave on the stock Buick three sheave crank pulley is machined off, water pump pulley is stock Buick and the Pinto alternator runs a single stock Pinto pulley.

Problem with the single belt setup is lack of belt wrap on the water pump pulley.
The dual belt setup on the 31 should overcome the slippage problem.

An idler on the back of the belt would probably help here . . . someday.

[phat46]

I have had what sounds like a PCV valve rattling in my car for years. I swear it sounds like the rattle you get when you shake a PCV valve to make sure it's not gummed up. It has stayed the same for many years and nothing has gone  BLOOEY! ..........yet....

[348tripower]

I made some rocker adjustments to the Stude last night. I stll have the rattle. I comes in after I get some heat in the engine and sounds the same all the time.  
SO, I made a phone call to the owner. (This is a loaner rig)
 I had helped put the top end together in this at least 2 years ago. I was reminded last night that this mother has one piston that differs from the others? So I am assuming that when it gets hot, the odd piston is contacting the head!  :oops:
There has been no change in the sound for 200 plus miles and I have not been able to blow it up. I hope to have time this weekend to pull the intake and heads to see if that is the case, but I am pretty sure at this point.
I didn't pull any plugs yet cause I still need to move it to the right bay to work on it.  
I will post more on this as the story unfolds.

[UGLY OLDS]

Don....My $.02.....
In the original post....

"Cruising down the road I can push down on the throttle and get a rattle. If I let up it goes away. I can hear it slightly when I raise the rpm while sitting still. "

A piston hitting the head is going to hit the head under power, coasting down, cranking, etc...If this was the case you would prolly pounded out the rod bearing in that cylinder buy now......Same thing if it was hitting the head gasket in one cyl.. A piston just touching a valve maybe...Hitting the head, I don't think so..( IMHO)..

"For the record, I can't time this conventionally. The mark won't line up with the timing pointer.."

I think we're getting close.....
Did you verify the balancer at TDC when you built it??   You can still do this now with the "pinkie in the #1 cyl test" at TDC.. (Not hi-tech but it beats "timing by ear" ..)
Just for grins...Try about 5 Gals of Premium fuel & a can of octane booster..LOTS cheeper than pulling the heads....( My kid has to use this "Magic Elixir" in his Dodge...He HAD to have 9.75 to 1 compression when it was built...)

You also could be on the edge of a lean condition...( I'm assuming the "rattle" is only at part throttle..) , Does the noise go away when you accelerate hard enough to get into the "power circuit" of the carb??  
Just some idea's...I hate to see anybody take something apart when it's not neccesary.......
Oops......That's $.07   :oops:  :oops:

[348tripower]

Don....My $.02.....
In the original post....

"Cruising down the road I can push down on the throttle and get a rattle. If I let up it goes away. I can hear it slightly when I raise the rpm while sitting still. "

A piston hitting the head is going to hit the head under power, coasting down, cranking, etc...If this was the case you would prolly pounded out the rod bearing in that cylinder buy now......Same thing if it was hitting the head gasket in one cyl.. A piston just touching a valve maybe...Hitting the head, I don't think so..( IMHO)..

"For the record, I can't time this conventionally. The mark won't line up with the timing pointer.."

I think we're getting close.....
Did you verify the balancer at TDC when you built it??   You can still do this now with the "pinkie in the #1 cyl test" at TDC.. (Not hi-tech but it beats "timing by ear" ..)
Just for grins...Try about 5 Gals of Premium fuel & a can of octane booster..LOTS cheeper than pulling the heads....( My kid has to use this "Magic Elixir" in his Dodge...He HAD to have 9.75 to 1 compression when it was built...)

You also could be on the edge of a lean condition...( I'm assuming the "rattle" is only at part throttle..) , Does the noise go away when you accelerate hard enough to get into the "power circuit" of the carb??  
Just some idea's...I hate to see anybody take something apart when it's not neccesary.......
Oops......That's $.07   :oops:  :oops:

I  gotta bet that it is a piston and head issue. Just cause of the way it sounds.  And the fact that it doesn't do it when it is cold.  Just a couple of thousands in expansion could cause it and not pound anything apart at this point.
It's not detonation cause I can't change it with timing. I should be able to get more or less by changing timing and that ain't happening. The other possibility could be a valve hitting the piston. There again, I didn't do the lower end so I don' know till I pull it apart.  Also, I didn't see the cam and gears in this, so I am thinking, that the cam is advance with the lower crank gear giving me the timing mark issue. This could also lead to a valve contacting the piston if it is oppening too close to it.
I'll keep ya all posted.  Gotta change a torque converter tonight so the stude will have to wait!
Don

[Mikej]

Rocker hitting the valve cover?