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Engine Mounts P15; Harshness Problem Solved


Sam Buchanan

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This is an update to the thread about installation of too-firm transmission mounts on my P15. It seems the issues I experienced were due to the lower mounts I installed. After removing the lower mounts to return to an configuration more like the one shown in the Service Manual illustration the car now runs smoothly as it did prior to installing the new mounts.

 

Here are the new lower mounts that I removed that are currently sold by vendors:

 

transmission-13.jpg.d20d9fb54bc9605789de2c17c3d77e9d.jpg

 

These mounts require longer bolts than the originals due to their thickness.

 

Compare this to the Service Manual:

 

transmission-10.jpg.23756155931576a870416dc489f508b6.jpg

 

It seems the transmission of vibration to the car was due to the mount bolt being captured by the lower mounts. When I removed the lower mounts smoothness returned.  :)

 

It appears the closest arrangement to the original design is to not use the lower mounts that are currently available. The lower mounts may indeed be correct for some applications but my '48 P15 is much happier without lower mounts.

Edited by Sam Buchanan
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Sam,

Were the top mounts you used like the ones from Rock Auto in the picture you posted in the other thread on Thursday, or are those the lower bushings?  (Looks like a completely different type of rubber to me, in contrast to the lowers you took back off.)

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1 hour ago, Eneto-55 said:

Sam,

Were the top mounts you used like the ones from Rock Auto in the picture you posted in the other thread on Thursday, or are those the lower bushings?  (Looks like a completely different type of rubber to me, in contrast to the lowers you took back off.)

 

The mounts shown are the new lowers I removed, the ones sourced from Bernbaum. The Bernbaum uppers are still installed.

Edited by Sam Buchanan
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5 hours ago, Sam Buchanan said:

 

The mounts shown are the new lowers I removed, the ones sourced from Bernbaum. The Bernbaum uppers are still installed.

And the Bernbaum uppers & lowers are made of the same type of rubber?  (I would guess so.)

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16 minutes ago, wldavis3 said:

Sam,

With reference to the service manual sketch, did the top of the new lower motor mount you received fit inside that pocket and touch the

bottom of the top motor mount ?

 

The lowers did fit into the pocket of the frame member but I didn't mic everything so don't know if they were in contact with the upper mounts. Something about removing the lower mounts increased the elasticity of the system and prevents vibes from being transmitted to the car. I suspect the bolts are now more free to vibrate and allow the upper mount to absorb vibrations. But at this point I'm just speculating about why vibration level is now much lower.......but I'm now enjoying a smooth-running drive train.   

Edited by Sam Buchanan
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in my experience, the 40 was the only car to use the lower mounts as pictured.  all the others had a plain flat runner washer.

I felt that the lower mount was a good idea but you must extend the metal tube so the mounts are not compressed at all.

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37 minutes ago, dpollo said:

in my experience, the 40 was the only car to use the lower mounts as pictured.  all the others had a plain flat runner washer.

I felt that the lower mount was a good idea but you must extend the metal tube so the mounts are not compressed at all.

 

The last configuration of the lower mount installation I tried prior to removing the lower donuts was to loosen the bolt to the point that the mounts were not compressed at all by bolt tension, the washers just barely touched the lower mounts. This still resulted in vibration being transmitted to the frame. My engineering degree was awarded under a shade tree but I suspect the vibration was primarily transmitted via the left lower mount as the engine rocked clockwise due to torque impulses. In this scenario the bolt would be pulling the lower washer "up" against the lower mount. Without a lower mount the washer just rides along without contacting anything. It seems the Chrysler engineers had this figured out nicely, we managed to mess it up due to lack of knowledge of how the mounts should be configured. A more detailed discussion of this can be found in this thread.

 

The real problem is retailers not having a clue as to which mounts should be used in which applications. They blindly sell the lowers for a wide range of cars which should not be using them....and those of us with a limited knowledge base don't know any better than to attempt to make them work.

Edited by Sam Buchanan
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Hey Sam - Thanks for solving the vibration transmission problem! I am now going to get new upper mounts for my 48 P15 and get them installed. One question - can the new mounts be installed by jacking up the tranny a bit, removing the old ones and then install the new ones? Is there enough room to remove the mounting bolts? If so, this would be an easy repair. Thanks again for solving the vibration problem!!

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55 minutes ago, Cold Blue said:

Hey Sam - Thanks for solving the vibration transmission problem! I am now going to get new upper mounts for my 48 P15 and get them installed. One question - can the new mounts be installed by jacking up the tranny a bit, removing the old ones and then install the new ones? Is there enough room to remove the mounting bolts? If so, this would be an easy repair. Thanks again for solving the vibration problem!!

 

Don't know if you can install new uppers with the tranny in place, I had the transmission out when installing new mounts, but it would certainly be worth trying. The bolts won't be a problem, you just need enough of a gap to pull the old mounts out and insert the new ones. There was plenty of gap with the transmission slid back free of the bellhousing. I had to use a pry bar to nudge the bellhousing a bit to get the bolts to drop back in place.

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