Engine Mounts Nightmare

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Michael O’Harran

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So my mechanic gave me an engine mount to remember how far we came. I had an original build in 2009 from a rolling chassis and they built a 351M and put it in. I found that I kept tearing and having to replace engine mounts. A different mechanic said it was because of how the 351C engine mounts angled under the 351M motor so he tapped new bolt holes and “custom fabricated” engine mounts for me to fix the problem. Here I am 10 year later doing a full resto with 351C built and being put in right. But wanted to share what “custom fabricated” engine mounts look like in case anyone is in the market.
 

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I've done worse. Okay, just kidding. Fortunately I can't burn metal and now that I am old and only have one and a half good eyes I will never get the chance.

At least you have a Mach 1. The convertible 351C engine mounts are unique and, last I checked, unavailable aftermarket. I have a pair in the attic in case I need them.

So it had a 351M? Are you swaping the tranny too because the 351M and 351C bellhousings are not the same. (You probably already know this)
 

Michael O’Harran

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I've done worse. Okay, just kidding. Fortunately I can't burn metal and now that I am old and only have one and a half good eyes I will never get the chance.

At least you have a Mach 1. The convertible 351C engine mounts are unique and, last I checked, unavailable aftermarket. I have a pair in the attic in case I need them.

So it had a 351M? Are you swaping the tranny too because the 351M and 351C bellhousings are not the same. (You probably already know this)
So I have a mechanic doing the whole build, but I'm involved at most of the steps along the way and I am learning a lot, just lack the time and actual mechanical knowledge, but yes your right this came up to be an issue. So this 351M block was bought new and built with a tremec 5 speed trans. My car had a violent vibration when you'd hit 70 mph. Not when up on the chassis dyno, but on the road. It had a new driveshaft. It was a nightmare so beyond never driving it past 90 without fear of coming apart when reentering earths atmosphere, I was shearing engine mounts. A couple mechanics looked for the issue without resolution. Opportunity with resto to prevent this again. 1972 351C found and rebuilt new, proper engine mounts, same tremec 5 speed and an original bell housing. My mechanic said he new a guy once that couldn't find the source of a heavy vibration once and after trying everything else, they got rid of the after market bell housing, went back to stock and it resolved, so we went the same route as for the bell housing. That's as best I understand where I am at now. I don't recall if some conversion piece was needed to make the stack bell housing fit or what. But drove the car since 2011 till 2019 when I started this resto and over that whole time I never drove it over 90 mph and less than 1000 miles. Constant issues to contend with. Man am I excited to do it all over again beyond right and get it turn key ready, fully done and enjoy like driving it off the line brand new!
 
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I've done worse. Okay, just kidding. Fortunately I can't burn metal and now that I am old and only have one and a half good eyes I will never get the chance.

At least you have a Mach 1. The convertible 351C engine mounts are unique and, last I checked, unavailable aftermarket. I have a pair in the attic in case I need them.

So it had a 351M? Are you swaping the tranny too because the 351M and 351C bellhousings are not the same. (You probably already know this)
Convertible and Mach1 engine mounts are the same. 351C and 302 mounts are the same also. 351M is different beast all together, don't bother with them. I think he was saying someone previously had put a 351M in (dummy) and now he is getting a 351C put in.
 

Michael O’Harran

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Convertible and Mach1 engine mounts are the same. 351C and 302 mounts are the same also. 351M is different beast all together, don't bother with them. I think he was saying someone previously had put a 351M in (dummy) and now he is getting a 351C put in.
Well I had 351M motor and some kind of mounts that were cut, welded sloppily, and hacked half off. Really scary stuff that that was the paid solution. I now have the proper 351C with the matching and proper mounts.
 
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I don't know about 71 and 72 but in 1973 the 351C engine mount for the Convertible is unique. Give me a bit and I will try to dig up the info on it.

Edit: Found this thread, there is a PDF attached with some interesting motor mount info. https://7173mustangs.com/threads/do...agazine-issue-02-from-2013.38617/#post-391678

The 73's are unique and so is the convertible for 73 but it might just be the 'body bracket' that is different. I will have to poke around some more. I have a set of used but good 73 convertible engine mounts in my parts pile.
 
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Michael O’Harran

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1972 Mach 1 Mustang 351C
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I don't know about 71 and 72 but in 1973 the 351C engine mount for the Convertible is unique. Give me a bit and I will try to dig up the info on it.

Edit: Found this thread, there is a PDF attached with some interesting motor mount info. https://7173mustangs.com/threads/do...agazine-issue-02-from-2013.38617/#post-391678

The 73's are unique and so is the convertible for 73 but it might just be the 'body bracket' that is different. I will have to poke around some more. I have a set of used but good 73 convertible engine mounts in my parts pile.
Its amazing the science that goes into the design and evolution of many small parts. Thanks
 
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Well, I'm no mechanic by a long shot, nor am I an expert at anything, but my basic knowledge suggest to me that the entire drive line angle was way off, engine, trans, drive shaft to back axle. Heck even the axle may be incorrectly installed.
I remember lots of posts in the past on engine mounts, so might be worth a search.
Good luck with the new build.
 
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The '73 motor mounts are different (thicker) due to regulations requiring motor mounts that can't separate, which allowed the engine to tip over. The hot rodder's solution prior to '73 was a piece of chain run from an exhaust manifold bolt to the frame.
 

Michael O’Harran

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Well, I'm no mechanic by a long shot, nor am I an expert at anything, but my basic knowledge suggest to me that the entire drive line angle was way off, engine, trans, drive shaft to back axle. Heck even the axle may be incorrectly installed.
I remember lots of posts in the past on engine mounts, so might be worth a search.
Good luck with the new build.
New engine, new mounts, rebalanced drive shaft, new original bell housing, new suspension all around, rear end opened and resealed, all metal is solid and locked in as intended, full disassembly and reassemble. Man I sure hope we will have replaced the problem by now. There's only one moment we'll know for sure and then when we open her up.
 
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New engine, new mounts, rebalanced drive shaft, new original bell housing, new suspension all around, rear end opened and resealed, all metal is solid and locked in as intended, full disassembly and reassemble. Man I sure hope we will have replaced the problem by now. There's only one moment we'll know for sure and then when we open her up.
Michael, I hope so too. It is soo frustrating when we get these issues and can't figure out why.
Not to say that there is anything wrong with all the new stuff you've put on, but If it were me, I'd want to know that everything is inline, including the pinion angle of the axle. At the end of the day, it could just be that 'truck engine' that was in there. Anyway I wish you luck.
 

droptop73

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I had a similar issue back when I installed the T5 in my convertible. At about 60 - 70 mph it had a harmonic vibration that felt like an out of balance rear tire with a bad set of rear shocks. The whole car would shake. I ended up putting wedge shims between the axle and the springs to resolve it. My car is lowered a bit which contributed to the issue. Drive line angles matter...
 

Michael O’Harran

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I had a similar issue back when I installed the T5 in my convertible. At about 60 - 70 mph it had a harmonic vibration that felt like an out of balance rear tire with a bad set of rear shocks. The whole car would shake. I ended up putting wedge shims between the axle and the springs to resolve it. My car is lowered a bit which contributed to the issue. Drive line angles matter...
Fingers crossed on a successful rebuild
 
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I picked up a small driveline vibration from lowering my 351C a bit in order to get my taller Blue Thunder intake to work under the Ram Air system. I used the shorter aftermarket mounts for a 71-72, but also kept the shorter 73 brackets. Lowered the engine about 5/8" but picked up the small vibration around 45mph. I have not yet done anything to rectify it. I'm just living with it right now.
 

Hemikiller

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Convertible and Mach1 engine mounts are the same. 351C and 302 mounts are the same also. 351M is different beast all together, don't bother with them. I think he was saying someone previously had put a 351M in (dummy) and now he is getting a 351C put in.

The convertibles going from 72 back to 68 used a different mount from the coupe/sportsroof. The 71-72 brackets on the frame were different as well, which changed the angle of the engine. Common understanding was it was done to help address a vibration specific to the convertible platform.

73 continued the same idea, but with the captive style mounts.
 

Michael O’Harran

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I picked up a small driveline vibration from lowering my 351C a bit in order to get my taller Blue Thunder intake to work under the Ram Air system. I used the shorter aftermarket mounts for a 71-72, but also kept the shorter 73 brackets. Lowered the engine about 5/8" but picked up the small vibration around 45mph. I have not yet done anything to rectify it. I'm just living with it right now.
I tried to do my research on the intake and believe it will clear with the ram air system so hopefully there won't be any adjustments needed there for me. I hope yours remains fine for you. Thanks
 
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Well I had 351M motor and some kind of mounts that were cut, welded sloppily, and hacked half off. Really scary stuff that that was the paid solution. I now have the proper 351C with the matching and proper mounts.
Thank you for the info. I was not aware of that. Actually this is the first time I have ever heard of this being a problem. I have a 73' so I didn't face this issue.
 

steves73

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Like droptop73 said, I have a 73 cougar with an Fmx, it was a V8 type of harmonic shake. I narrowed it down by driving at the same speed but in different gears. The vibration was based on engine speed, not road speed. The rubber in the trans mount had degraded.
 
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