Externally balanced engine troubles - The 1947 - Present Chevrolet & GMC Truck Message Board Network

JMD · 11-08-2016, 01:13 AM

Hi all,
Started my rebuilt 454 (Originally out of a '78 chevy pickup) for the first time today and found that it has a fairly violent shake that increases with engine speed. The only thing I can think of is that the PO put an internally balanced fex plate and harmonic balancer when they should be externally balanced pieces.So my questions are...
1. How can I tell for sure whether or not my flexplate/balancer are externally balanced?
2. Can I just buy an exterally balanced harmonic balancer/flex plate and install them? Or will I need to get the whole rotating assembly balanced as well?
Thanks in advance for amy help
-Jordan

RustyBucket · 11-08-2016, 02:28 AM

The balancer has a weight that is inside the hub and extends nearly halfway around it. I think you can feel it inside the balancer from under the truck. The engine side of the flex plate has a flat plate welded to it you can see with the dust cover off. Google will show you some images on a search. The damper & flex plate aren't balanced to a specific engine so you can put them on your shakin' engine if that's the problem. GOOD LUCK!!

JMD · 11-08-2016, 08:50 PM

Thank you sir! That's exactly what I needed to know. After a quick stop at the parts store I have what I need for under $100. That's MUCH less than I figured this vibration was going to cost me.

un4gvn! · 11-09-2016, 04:05 AM

When we put our 396 back together and started it up, it had a very bad vibration. We thought the machine shop screwed up the engine. He told us to make sure the flex plate was installed right. We thought the flex plate can only go on one way. When you spin the plate with out the bolts on the back of the crank, the bolt holes can only line up one way. We were wrong. The flex plate bolt holes were off 180 degrees. We spun the flex plate 180 degrees (one half turn) and the bolt holes lined up again. We bolted it up and started the engine and it ran smoothly with no vibration.

67 chevelle · 11-09-2016, 12:27 PM

ballencer will have a cavity on the back side of the ring

flexplate will have a weight welded to it

davepl · 11-09-2016, 01:50 PM

If it's an automatic it's easy to put the flexplate on backwards, which would cause it to be wildly out of balance. Just a thought.

11-08-2016, 01:13 AM	#1
JMD Registered User Join Date: Jun 2013 Location: Grand Rapids Michigan Posts: 291	Externally balanced engine troubles Hi all, Started my rebuilt 454 (Originally out of a '78 chevy pickup) for the first time today and found that it has a fairly violent shake that increases with engine speed. The only thing I can think of is that the PO put an internally balanced fex plate and harmonic balancer when they should be externally balanced pieces.So my questions are... 1. How can I tell for sure whether or not my flexplate/balancer are externally balanced? 2. Can I just buy an exterally balanced harmonic balancer/flex plate and install them? Or will I need to get the whole rotating assembly balanced as well? Thanks in advance for amy help -Jordan __________________ My build thread: http://67-72chevytrucks.com/vboard/s...96#post7439296

11-08-2016, 02:28 AM	#2
RustyBucket Registered User Join Date: Jul 2009 Location: Space Coast, Fl. Posts: 1,050	Re: Externally balanced engine troubles The balancer has a weight that is inside the hub and extends nearly halfway around it. I think you can feel it inside the balancer from under the truck. The engine side of the flex plate has a flat plate welded to it you can see with the dust cover off. Google will show you some images on a search. The damper & flex plate aren't balanced to a specific engine so you can put them on your shakin' engine if that's the problem. GOOD LUCK!! __________________ Dad always said, "Son, WISH IN ONE HAND, and ......." -------------------------------------- -------------------------------------- Current toy trucks: '71 GMC 1500 SUPER CUSTOM short/step, orig. dk. blue, 350/700r, ps,pb, A/C '72 Blazer 2wd, ochre & white, 454, tremecTKO 500 5spd. ps, pb, A/C, tilt Last 10yrs of hobby vehicles, had a FEW more in the 50yrs. before these: '66 Plymouth Belvedere City of Miami cop car clone. '70 Nova 406"sb, 13.5-1, solid roller, Brodix, p.glide/t.brake, back halved, 9" Ford, spool 4.88, cage,ladder bar/coilover, 10.5 tire....... SOLD!! '67 C10 short/step side mount spare -- SOLD!! '72 Jimmy 2wd, 350/350, ps, pb, fun driver, lots of bondo & a shiny red paint job..... SOLD!! '69 Nova 350, 4spd, A/C, ps, p.b, ...SOLD!!

11-08-2016, 08:50 PM	#3
JMD Registered User Join Date: Jun 2013 Location: Grand Rapids Michigan Posts: 291	Re: Externally balanced engine troubles Thank you sir! That's exactly what I needed to know. After a quick stop at the parts store I have what I need for under $100. That's MUCH less than I figured this vibration was going to cost me. __________________ My build thread: http://67-72chevytrucks.com/vboard/s...96#post7439296

11-09-2016, 04:05 AM	#4
un4gvn! Registered User Join Date: Mar 2006 Location: Latrobe, Pa. 15650 Posts: 350	Re: Externally balanced engine troubles When we put our 396 back together and started it up, it had a very bad vibration. We thought the machine shop screwed up the engine. He told us to make sure the flex plate was installed right. We thought the flex plate can only go on one way. When you spin the plate with out the bolts on the back of the crank, the bolt holes can only line up one way. We were wrong. The flex plate bolt holes were off 180 degrees. We spun the flex plate 180 degrees (one half turn) and the bolt holes lined up again. We bolted it up and started the engine and it ran smoothly with no vibration.

11-09-2016, 12:27 PM	#5
67 chevelle Registered User Join Date: May 2013 Location: WEST PALM FLORIDA Posts: 1,174	Re: Externally balanced engine troubles ballencer will have a cavity on the back side of the ring flexplate will have a weight welded to it __________________ 68 Long Fleet , ly6 , turbo 350 , 3-5 drop , original paint , front discs 67 Small window , 7 foot bed , tweaked 6.0 LSX 2004R Medium Olive 58 Apache fleet , 235 , offy intake , dual exhaust , 4 on the floor , red/white 69 Long Fleet , Custom , 6.0, 4l60 , AC , Medium Olive

11-09-2016, 01:50 PM	#6
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Externally balanced engine troubles If it's an automatic it's easy to put the flexplate on backwards, which would cause it to be wildly out of balance. Just a thought. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

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