harmonic balancers? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Custom 68 · 10-21-2002, 12:08 PM

I am rebuilding my 327 putting TPI on so it wont be a real high RPM screamer. I have the original Balancer on it now should it be replaced or how do you tell. What would you recommend? I will be taking the engine and parts to the shop soon to have it all checked and balanced and such. The guys there will probably be able to tell me more just wanted your thoughts.
Thanks
Dave

guitarman · 10-21-2002, 12:13 PM

Your stock balancer should be fine. I believe it's good for 8,000 plus rpm's. Tpi motor's don't turn any where near that.

mikep · 10-21-2002, 12:13 PM

If its 30+ years old replace it. Rubber doesnt last forever and even if you cant see damage it will fail at some point no matter what it looks like so perform a little proactive maintenance and change it. You can get a new 8 inch elastomeric balancer for $70 that will do a better job of soaking up torsional vibes . Its cheap insurance.

toms68cst · 10-21-2002, 12:49 PM

If you don't mind me butting into you post custom 68, How does a harmonic balancer work? What's inside that thing?

I've always wondered.

bigjimzlll · 10-21-2002, 02:59 PM

there is a hub then a layer of rubber then a ring....when the motor is turning the outer ring , do to centrifical force, is lightend...Oh hell I dont know how it works....lol

Michael A. K. G · 10-21-2002, 04:45 PM

On a working crankshaft, most of the motion (of course) is rotation about the centerline. But what else is going on? When engineering a moving or load-bearing part, you have to consider how the steel behaves under stress.

There are several obvious vibrational modes that get superimposed with rotation, on a heavy rotating shaft. One is "flexure," where a slightly unbalanced crankshaft will wobble about its centerline. What you do about that is make several main bearings, rather than just two. And balance the crankshaft by drilling holes in the counterweights.

The next is the "torsional" mode, where the front and back ends twist in opposite directions. There is a characteristic frequency for this that depends on the length of the crankshaft and its stiffness. The harmonic balancer is designed to have just the right properties so it matches this frequency and twists in the opposite direction as the front of the crankshaft. Thus, it cancels out the motion.

The frequency matching means you can't interchange harmonic balancers without also interchanging the crankshaft, unless the two crankshafts are identical.

Custom 68 · 10-23-2002, 10:45 AM

MikeP suggested an 8 inch one. I measured mine and it is 6.25 inches across. again this is a 327 forged crank. I forget the year but it is the pre 68 with the small journal.
After I looked at it i agree with the replacement the rubber looks pretty bad.
Thanks

mikep · 10-23-2002, 11:05 AM

The 8 inch damper will do a much better job of dampening at higher RPM's even on a 327. The 8 inch chevy balancer as found in later model heavy duty 350's will work fine on a 327 as it is neutral balanced . Make sure to have the entire assy balanced before assy though. The nice thing about modern balancing is that everything gets its own dynamic balancing job on neutral balance engines. That means the crank gets done by itself as do the balancer and flywheel so if you ever need to change them your balance will remain even as long as the part changed is correctly balanced . Even rods and pistons can be changed as long as the weight remains the same as the part that is being rplaced.

Ackattack · 10-23-2002, 11:15 AM

do the 327 balancers have bolts that go into the crank? In my service book it doesn't list a torque specification and there isn't a bolt in mine; is there supposed to be, or is it just a press fit?

Custom 68 · 10-23-2002, 11:18 AM

Thanks Mike so basically if it will fit I can run it? and yes Ackattack mine does have a bolt in the front of the balancer. it is a press fit that may be the way they are supposed to be but I did bolt mine. Seemed safer

crazy longhorn · 10-23-2002, 11:22 AM

some of the sj engines didnt have the bolt,or the threaded hole in the in the crank. i ran into this on a 283, that I had drilled & tapped at the time of overhaul. This engine didnt have a damper, just a steel hub that was press fit......crazy AL

**ScottH** · 10-23-2002, 12:01 PM

So if you get your engine balanced, should you have the balancer also checked? what about aftermarket balancers, should they be checked as well?

crazy longhorn · 10-23-2002, 12:24 PM

Theory is that you should be able to swap balancers on the sb.....except the 400 sb, as they are nuetral balance(400 is external balance,& has to be on the crank to balance,as does the flexplate). Practice doesnt always follow theory , due to manufacturing tolerances! I have seen many 350 street eng's balanced without doing the balancer & flex plate, & no problems. Both of these pcs can be balanced off the crank, so swapping doesnt cause a problem(265-350ci). crazy AL

10-21-2002, 12:08 PM	#1
Custom 68 Registered User Join Date: Nov 2001 Location: Springfield, MO Posts: 2,932	harmonic balancers? I am rebuilding my 327 putting TPI on so it wont be a real high RPM screamer. I have the original Balancer on it now should it be replaced or how do you tell. What would you recommend? I will be taking the engine and parts to the shop soon to have it all checked and balanced and such. The guys there will probably be able to tell me more just wanted your thoughts. Thanks Dave __________________ Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!"

10-21-2002, 12:13 PM	#2
guitarman Do it well or don't do it Join Date: Apr 2002 Location: Georgia Posts: 457	Your stock balancer should be fine. I believe it's good for 8,000 plus rpm's. Tpi motor's don't turn any where near that. __________________ Try to be the person your dog thinks you are. Excellent paint and body site: http://www.autobody101.com/ Visit my Web Site http://home.attbi.com/~agfosteriii

10-21-2002, 12:13 PM	#3
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	If its 30+ years old replace it. Rubber doesnt last forever and even if you cant see damage it will fail at some point no matter what it looks like so perform a little proactive maintenance and change it. You can get a new 8 inch elastomeric balancer for $70 that will do a better job of soaking up torsional vibes . Its cheap insurance. __________________ No truck :-(

10-21-2002, 12:49 PM	#4
toms68cst Registered User Join Date: Jun 2000 Location: Bismarck, ND Posts: 4,285	If you don't mind me butting into you post custom 68, How does a harmonic balancer work? What's inside that thing? I've always wondered. __________________ Unrestored 68 C-10 CST. Original 327. 4-Speed CH465. 50k or so miles. TREASURER, Drum Brake Club.

10-21-2002, 02:59 PM	#5
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	there is a hub then a layer of rubber then a ring....when the motor is turning the outer ring , do to centrifical force, is lightend...Oh hell I dont know how it works....lol __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

10-21-2002, 04:45 PM	#6
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	On a working crankshaft, most of the motion (of course) is rotation about the centerline. But what else is going on? When engineering a moving or load-bearing part, you have to consider how the steel behaves under stress. There are several obvious vibrational modes that get superimposed with rotation, on a heavy rotating shaft. One is "flexure," where a slightly unbalanced crankshaft will wobble about its centerline. What you do about that is make several main bearings, rather than just two. And balance the crankshaft by drilling holes in the counterweights. The next is the "torsional" mode, where the front and back ends twist in opposite directions. There is a characteristic frequency for this that depends on the length of the crankshaft and its stiffness. The harmonic balancer is designed to have just the right properties so it matches this frequency and twists in the opposite direction as the front of the crankshaft. Thus, it cancels out the motion. The frequency matching means you can't interchange harmonic balancers without also interchanging the crankshaft, unless the two crankshafts are identical.

10-23-2002, 10:45 AM	#7
Custom 68 Registered User Join Date: Nov 2001 Location: Springfield, MO Posts: 2,932	What do I need to replace it with MikeP suggested an 8 inch one. I measured mine and it is 6.25 inches across. again this is a 327 forged crank. I forget the year but it is the pre 68 with the small journal. After I looked at it i agree with the replacement the rubber looks pretty bad. Thanks __________________ Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!"

10-23-2002, 11:05 AM	#8
mikep Used to have a truck Join Date: May 2002 Location: port orchard WA Posts: 1,552	The 8 inch damper will do a much better job of dampening at higher RPM's even on a 327. The 8 inch chevy balancer as found in later model heavy duty 350's will work fine on a 327 as it is neutral balanced . Make sure to have the entire assy balanced before assy though. The nice thing about modern balancing is that everything gets its own dynamic balancing job on neutral balance engines. That means the crank gets done by itself as do the balancer and flywheel so if you ever need to change them your balance will remain even as long as the part changed is correctly balanced . Even rods and pistons can be changed as long as the weight remains the same as the part that is being rplaced. __________________ No truck :-(

10-23-2002, 11:15 AM	#9
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	do the 327 balancers have bolts that go into the crank? In my service book it doesn't list a torque specification and there isn't a bolt in mine; is there supposed to be, or is it just a press fit? __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index

10-23-2002, 11:18 AM	#10
Custom 68 Registered User Join Date: Nov 2001 Location: Springfield, MO Posts: 2,932	Thanks Mike so basically if it will fit I can run it? and yes Ackattack mine does have a bolt in the front of the balancer. it is a press fit that may be the way they are supposed to be but I did bolt mine. Seemed safer __________________ Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!"

10-23-2002, 11:22 AM	#11
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	some of the sj engines didnt have the bolt,or the threaded hole in the in the crank. i ran into this on a 283, that I had drilled & tapped at the time of overhaul. This engine didnt have a damper, just a steel hub that was press fit......crazy AL

10-23-2002, 12:01 PM	#12
ScottH Senior Member Join Date: Sep 2002 Location: Hampstead, NC Posts: 8,187	So if you get your engine balanced, should you have the balancer also checked? what about aftermarket balancers, should they be checked as well?

10-23-2002, 12:24 PM	#13
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	Theory is that you should be able to swap balancers on the sb.....except the 400 sb, as they are nuetral balance(400 is external balance,& has to be on the crank to balance,as does the flexplate). Practice doesnt always follow theory , due to manufacturing tolerances! I have seen many 350 street eng's balanced without doing the balancer & flex plate, & no problems. Both of these pcs can be balanced off the crank, so swapping doesnt cause a problem(265-350ci). crazy AL



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