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318 Poly/ big block motor mounts interchange?
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58Jackie
Posted 2020-01-12 8:37 PM (#592841)
Subject: 318 Poly/ big block motor mounts interchange?



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Posts: 123
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Location: Southern Idaho, USA
Hey guys,
I'm asking if the 58 318 Poly motor mounts will work on my 440?

So i just was under my car (58 Belvedere, 318 Poly) measuring the motor mount tabs/ears on the side of the block that the cup style mounts attach to. Measuring from the outside diameter of the mounting tabs, its 4". On the inside of the mounting tabs, diameter is 2 1/2".

On my 440, 1970 RB motorhome motor, it has the exact same 3-bolt mounting tabs on the block. Measured this one too, and the diameter of the mounting tabs is exactly the same as the 318.

Please let me know if i can use these 318 motor mounts on my 440. I hope so, as the dimensions tell me i can, but there may be some different problem thrown into it? If i cant, i really dont want to do cutting/welding on the mounts, as that sounds sketchy to me, so is there other mounts from a different car that would bolt in with no modifications?

Thanks.
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wayfarer
Posted 2020-01-13 12:41 PM (#592864 - in reply to #592841)
Subject: Re: 318 Poly/ big block motor mounts interchange?



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The brackets may well bolt to the RB but I think that you will find a variation in the width of the block so that using an A engine mount will not land on the cross member where you want it to.
If any following posters do not have any definitive measurements I can check for you...I happen to have both engines in the shop right now.
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mstrug
Posted 2020-01-13 1:03 PM (#592865 - in reply to #592841)
Subject: Re: 318 Poly/ big block motor mounts interchange?



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They are different. Big Blocks had their own mounts. Some on Epay right now... or cut off the ears and have a friend or shop weld them back on where they need to be. None close by me. I believe some people have used them with spacers, but that just isn't right, too stressful.
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mstrug
Posted 2020-01-13 1:05 PM (#592866 - in reply to #592841)
Subject: Re: 318 Poly/ big block motor mounts interchange?



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Posts: 5943
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Location: Newark, Texas (Fort Worth)
https://www.ebay.com/itm/1957-59-Plymouth-Desoto-Dodge-361-383-440-M...
/274101493023?hash=item3fd1b8dd1f:g:8m4AAOSwTh1dxgju&vxp=mtr
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58coupe
Posted 2020-01-14 11:55 AM (#592922 - in reply to #592841)
Subject: Re: 318 Poly/ big block motor mounts interchange?



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Location: Alaska
I had the same problem many years ago. I was installing a 361 into a 57 Fury (bought minus the engine) and had to use big block mounts. No problem with the width though.
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58Jackie
Posted 2020-01-14 5:15 PM (#592944 - in reply to #592922)
Subject: Re: 318 Poly/ big block motor mounts interchange?



Veteran

Posts: 123
100
Location: Southern Idaho, USA
So even though the width of the mounting tabs is correct, the big block mounts are longer? So could i possibly shim up the 318 mounts to make them the desired length?
I really dont like cutting/welding the mounts, especially motor mounts for the 440 cause that thing weighs close to a 1,000 pounds, as i think doing that weakens the mounts and creates stress points on the mounts. I've actually seen this done; i had a car with a small block chevy in it with fabricated motor mounts. After driving it for over 50k miles, pulled the motor and where the mounts had been welded they were about to break by stress cracking.
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58coupe
Posted 2020-01-15 12:09 PM (#592976 - in reply to #592841)
Subject: Re: 318 Poly/ big block motor mounts interchange?



Expert

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Location: Alaska
I believe the 440 weighs a little over 600 lbs. The Chrysler hemis weigh about 100 lbs. more. I think you would be surprised if you weighed an early 318 to find it is closer to the same weight as a 440.
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58Jackie
Posted 2020-01-16 4:44 PM (#593047 - in reply to #592976)
Subject: Re: 318 Poly/ big block motor mounts interchange?



Veteran

Posts: 123
100
Location: Southern Idaho, USA
58coupe - 2020-01-15 11:09 AM

I believe the 440 weighs a little over 600 lbs. The Chrysler hemis weigh about 100 lbs. more. I think you would be surprised if you weighed an early 318 to find it is closer to the same weight as a 440.


I was only guessing that it weighed a 1000 lbs due to it dented my pickup bed when i got it. I thought it was kinda funny.
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