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Rear Eng Mount Replacement - '53 Catalina w/Hydramatic


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Has anyone else replaced the rear engine mounts and had difficulty getting the bolt holes to line up with crossmember? GM mounts are long gone but NOS Doan (after-market service) mounts can still be found. But the location of the mounting holes in the Doan mounts appear to be offset compared to the GM mounts. Unfortunately, the engine/transmission doesn't have enough fore/aft adjustment (limited by the front engine mount) to compensate for that offset. I've been struggling for days and can't get the mounts installed. I can't reuse the old ones because the right side is broken and the left side appears to be bent (probably due to years of struggling to hold the eng/trans in place by itself).

Doan 2064 Mtg Hole Offset.jpg

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Kornkurt, thanks for your feedback and nice find with the revulcanization service. I was aware of Steele Rubber Products but hadn't gotten around to see if these old Pontiac parts qualified. You've given me some options to consider. Before I make a decision which way to go I'm going to spend some more time under the car to see if I can come up with a modification/work-around to make use of these Doan mounts.

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i think you can make a L & R rear mount spacers to go in between the bell housing and the doan made mounts, to get the mounts to line up with the rear mount cross member, likely you will need 4 longer bolts to get the same thread depth into the bell housing. perhaps even use the old mounts flange as the spacers. 

 

Charles L. Coker

1953 & 1954 Technical Advisor

Pontiac Oakland Club International

# 2660

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Charles, I like your suggestion to use spacers because it's precisely the work-around I came up with too. For evaluation purposes I simply used grade 8 washers. Three was just shy of the thickness that's needed. Your idea of cutting off the flanges of the old mounts to use as spacers is brilliant. Thanks for that.

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  • 2 months later...

Gents

Just something to be aware of as a precaution,  if you place the spacers on the rear engine mounts to the supportive crossmember, you will / may well change the angle of the transmission / driveshaft   to the final drive. I would suggest that you measure the current angle of the transmission / driveshaft before you change the mounts and add the spacers. 

Then once you install the new components measure the angles again. On my chieftain the angles are now  -6 deg on the driveshaft unijoint  and  -5.5 deg at the diff pinion flange,  as the operating angles were  un-equal  before hand.  If the mounts are broken , bent or sagged then you may well have to compensate with different thickness  shims to get them somewhere near equal again. Driveline vibration can result  if  the  operating angles are different at either end of the driveshaft. the vibration was substantially reduced when i equalised the operating angles . 

Just a suggestion thats all.

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RussJagoau, That's good feedback. In this unique situation, however, the need for the spacers was to locate the engine/transmission on the crossmember where they are supposed to be. The aftermarket engine mounts had their mounting holes offset from the production locations (don't know why, perhaps a manufacturing error or defect). If I had used the aftermarket engine mounts "as-is" you would be correct, the driveshaft angle could have been affected enough to introduce vibration. I am not happy needing to use the spacers and will continue to search for genuine GM mounts. Thanks again for sharing your experience.

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Hi Bob

So on re-reading your text ( If I have interpreted it correctly )  your plan is to put the spacer/s on the new mounts to abutt to the  the transmission  ( not the cross member ?)  thereby to space the new mounts backwards so that the threaded holes will align to the original bolt holes in the cross member. Given the condition that you found your original mounts in ( broken  -as I did mine ) then most likely the transmission will have "sagged' under its weight as being supported by only  1  sound mount at the rear. Would be interesting exercise to measure your  driveshaft angles before and after the replacement of the old mounts. 

 

I also struggled to find new rear mounts and after a few years of searching was fortunate enough to locate some N.O.S  GM units  on Ebay from  within the States - so i purchased those  - hence the measuring of the driveline angles before and after. Prior  to the changeout the angles as found were significantly different at either end. The new mounts  restored it back somewhat but there was still 2 degrees difference - hence I fabricated some  spacer plates to go under the mounts on the cross member location to raise the rear of the transmission even higher to equalise the angles.  it certainly smoothed  out the dynamic vibration in the shaft even further  as I also had the shaft at balanced at Hardy Spicer as it was also out of balance prior  to the mount changeout.

 

Hope  its a good result for you.

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