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Old 01-21-2019, 06:56 PM   #1
Russ/40
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Default Body Shudder - Rear Motor Mounts

My '29 RPU has an annoying shudder throughout the sheet metal when down shifting down, or on deceleration. I'm not speaking about clutch chatter, but a vibration throughout the entire chassis. In my search for a resolution I backed off the three engine mount nuts on each side. It made a big difference in the amount of vibration in the chassis. Nowhere have I found any particular instructions on how tight, or loose, these 6 bolts should be to allow the rubber pads to absorb vibrations, but not so loose as to cause any negative consequences. I'm looking for feedback of your experiences on how you have dealt with this issue. Also any other ideas on how the quiet body vibrations would be welcome.

All parts are new in the mounts. I had the 6 bolts snugged down initially. Probably to the point where the tubular collars were not allowing for any movement.
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Old 01-21-2019, 07:58 PM   #2
Gary WA
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Default Re: Body Shudder - Rear Motor Mounts

What type front motor mount? and is it worn out ,springs etc?Flywheel housing alignment? Is this a new engine (miles), just start doing or been going on for a while? Many variables?
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Old 01-22-2019, 09:33 AM   #3
DJ S
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Default Re: Body Shudder - Rear Motor Mounts

Universal joint or pinion bearings?
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Old 01-22-2019, 09:41 AM   #4
BRENT in 10-uh-C
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Default Re: Body Shudder - Rear Motor Mounts

Quote:
Originally Posted by Russ/40 View Post
My '29 RPU has an annoying shudder throughout the sheet metal when down shifting down, or on deceleration. I'm not speaking about clutch chatter, but a vibration throughout the entire chassis. In my search for a resolution I backed off the three engine mount nuts on each side. It made a big difference in the amount of vibration in the chassis. Nowhere have I found any particular instructions on how tight, or loose, these 6 bolts should be to allow the rubber pads to absorb vibrations, but not so loose as to cause any negative consequences. I'm looking for feedback of your experiences on how you have dealt with this issue. Also any other ideas on how the quiet body vibrations would be welcome.

All parts are new in the mounts. I had the 6 bolts snugged down initially. Probably to the point where the tubular collars were not allowing for any movement.



There should be some metal spacers (A-5093) that control the proper spacing for you. Just tighten the nuts and key them and you will be set.
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Old 01-22-2019, 09:44 AM   #5
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Default Re: Body Shudder - Rear Motor Mounts

I just rebuilt the rear motor mounts in my 29 AA, to me the 3 tubes prevent anyone from over tightening and squashing the rubber, so I snugged the bolts down then turned them to the next slot for the cotter pin.
The shutter problem occurred before and after the MM rebuild, could it be that the 3 main bearing have too much clearance.
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Old 01-22-2019, 10:51 AM   #6
Farrell In Vancouver
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Default Re: Body Shudder - Rear Motor Mounts

Checked Muffler rear hanger?
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Old 01-22-2019, 11:02 AM   #7
Ernie Vitucci
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Default Re: Body Shudder - Rear Motor Mounts

Good Morning...You might want to take a close look at your frame. You might have a frame crack that is allowing the vibration to occur. Clean off the frame for a foot in front and about three feet behind the motor mount. This area is where the major stress is on the frame. Cracks do sometimes appear in this area....Ernie in Arizona
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Old 01-22-2019, 03:48 PM   #8
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Default Re: Body Shudder - Rear Motor Mounts

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Universal joint or pinion bearings?
That was my first thought too.
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Old 01-23-2019, 06:30 PM   #9
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Default Re: Body Shudder - Rear Motor Mounts

Russ , while reading your post #1 you mentioned about bolts tightened and not allowing collars to move? I have all sorts of vibrations in a 29 and trying to eliminate one by one, are you saying that this mount moves a little on these collars ? thanks for any advice
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Old 01-23-2019, 07:00 PM   #10
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Default Re: Body Shudder - Rear Motor Mounts

Quote:
Originally Posted by Russ/40 View Post
My '29 RPU has an annoying shudder throughout the sheet metal when down shifting down, or on deceleration. I'm not speaking about clutch chatter, but a vibration throughout the entire chassis. In my search for a resolution I backed off the three engine mount nuts on each side. It made a big difference in the amount of vibration in the chassis. Nowhere have I found any particular instructions on how tight, or loose, these 6 bolts should be to allow the rubber pads to absorb vibrations, but not so loose as to cause any negative consequences. I'm looking for feedback of your experiences on how you have dealt with this issue. Also any other ideas on how the quiet body vibrations would be welcome.

All parts are new in the mounts. I had the 6 bolts snugged down initially. Probably to the point where the tubular collars were not allowing for any movement.
It is NOT bolts, motor mounts, chassis etc that is CAUSING the vibration.
IT IS THE ENGINE.
Whoever assembled and balanced the engine, screwed up.
You did have it balanced didn't you?
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Old 01-23-2019, 09:55 PM   #11
larrys40
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Default Re: Body Shudder - Rear Motor Mounts

While I do agree that some vibrations can be caused from internal engine issues and poorcrwbyilds just because he has a vibration doesn’t mean it’s the engine itself .
I have solves vibrations from correcting improper front mounts and adjustment of them, poor installation and rear mounts, driveshaft issues and yes, torque tubes even.

By design the 4 cylinder model A engine is inherently imbalanced.
I suggest making sure your engine mounts are installed properly in the rear as Brent indicates and your front mount and springs are correct as well. Properly aligned you should be able to get a crank in easily. Some of th stiffer and replacement beehive champ style mounts allow the front to sit too high and cause stress at the rear mounts.

Larry Shepard
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Old 01-24-2019, 12:39 AM   #12
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Default Re: Body Shudder - Rear Motor Mounts

ALL engine vibrations are caused by MOVING PARTS.
If the engine is not running, there is no vibration. Changing engine mounts, muffler clamps,
brake rods, drive shafts and anything else can and does sometimes minimize or mask the vibration by changing the resonant frequency of the vibration, BUT it does not address the root problem. BAD BALANCE JOB.
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Old 01-24-2019, 11:42 AM   #13
Russ/40
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Default Re: Body Shudder - Rear Motor Mounts

Quote:
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It is NOT bolts, motor mounts, chassis etc that is CAUSING the vibration.
IT IS THE ENGINE.
Whoever assembled and balanced the engine, screwed up.
You did have it balanced didn't you?
Pete, I suspect your correct. The engine was a rebuilt short block I received when I purchased the truck as a basket case some 30 years ago. I disassembled it and checked tolerances, but did not get it balanced. The flywheel and pressure plate were balanced. I'm running a modified flywheel for the V8 pressure plate and a 39 tranny. When the engine is pulling, there is no vibration. When idling no perceptible vibration. It seems to be worst during deceleration. I tightened up the rear mounts as Brent suggested, and the shudder is much worse. The front mount is correct with perfect alignment of the front crank bearing.
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