Loose Motor Mount Bolt; Mount to Block

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kiko619

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Hey guys! I just replaced the motor mounts on the 351C and "bolts" from were acquired from Johnsmustang.

image.png

Of the 4 bolts that screw into the block. 1 bolt would not catch at all if it did. It would spin freely it is the right size because pitch and length match, however when the bolt was being tightened down it just wouldn't securely tighten. so I reverted that 1 screw back to the original and now its not catching. My only guess is that these bolts are supposed to be stretch bolts. can someone please confirm this? If I need to upgrade to I higher grade of bolts, noted, I just need some expert advice to know what may be going on here.

 
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Well the picture you show has grade 5 markings on it.  Not a stretch bolt or anything special..

A couple questions.  1)Are you sure that you are starting in the block hole.  2) is the motor sitting down on the mount, or is there a gap.  3) If you pull a bolt back out, are you getting metal chips?  In other words are either the block or the bolt getting stripped out.?

There should be no special trick to this fastener situation.  It sounds like something is not aligning correctly.

If the block threads are stripped you should be able to helicoil it.

Another option is to jack the block up about 4 inches.  Remove the cross bolt that ties the upper and lower motor mounts.  Fasten the upper half to the block first and torque in place on both sides.  Then set the motor back down and replace the cross bolt.

kcmash

 

kiko619

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I started it off from second approach you suggested initially, its just easier imho. However, when I was torqueing the bolt into the block that one bolt would just go in like normal, no misalignment or cross threading, but kept on going until it bottomed out and just wouldn't tighten.. and there's no stripping of the threads on the block or the bolt itself. as for the bolt type itself I figured the grade 5 was a sufficient grade. 

 
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Was the motor mount material a different gauge / thickness on your old engine mounts?  If the new ones were thinner that may explain the fastener bottoming out.

Personally I’d try the “shell game” with your fasteners and shuffle them around.  That way you could identify if you have a bolt that is damaged or a threaded hole in the block that is the problem child.   Could also be something like the holes in the new mounts not being in exactly the right spot.  Also, are you trying to bolt them onto the block while they are supporting the weight of the engine?  That could be adding to your frustration.

 
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I wasn't going to comment on this, but reading that the treaded hole is NOT stripped, nor is the bolt bad. It is possible that the hole was not tapped as deep as required, therefore if your new bolts are a tad longer than the factory ones, it will bottom out without tightening to your bracket. 

My first question is; is the motor out of the car so you can see exactly what you're doing? 

2) is it just one bolt that is causing problems and just on one side? If you're absolutely sure the threads are good, you could shorten that 1 bolt to achieve the torque needed. You don't need the entire length of the bolt for it to achieve the correct torque. Or you could use a second flat washer to take up the difference.

When I did mine, I bought a new set of motor mounts from NPD only to find they were made in India, the steel appeared thinner and not only that, the mount surface was curved, not flat. I could have used a press and flattened those areas out, but I decided that as the original were still in excellent condition, I'd use those again until I could get a better set. They are still on the car 5 years later.

 

kiko619

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Was the motor mount material a different gauge / thickness on your old engine mounts?  If the new ones were thinner that may explain the fastener bottoming out.

Personally I’d try the “shell game” with your fasteners and shuffle them around.  That way you could identify if you have a bolt that is damaged or a threaded hole in the block that is the problem child.   Could also be something like the holes in the new mounts not being in exactly the right spot.  Also, are you trying to bolt them onto the block while they are supporting the weight of the engine?  That could be adding to your frustration.
motor mounts are same thickness, nothings different. I am bolting the mounts while the engine is sitting on the stand and the bolts just keep spinning in place. 

Is it goes all the way in but wont tighten or take any torque my bet is either stripped or maybe someone broke one before and drilled/tapped it bigger at some point. 
they go in all the way and just spin. there's no presence of deflection either. it just spins 

I wasn't going to comment on this, but reading that the treaded hole is NOT stripped, nor is the bolt bad. It is possible that the hole was not tapped as deep as required, therefore if your new bolts are a tad longer than the factory ones, it will bottom out without tightening to your bracket. 

My first question is; is the motor out of the car so you can see exactly what you're doing? 

2) is it just one bolt that is causing problems and just on one side? If you're absolutely sure the threads are good, you could shorten that 1 bolt to achieve the torque needed. You don't need the entire length of the bolt for it to achieve the correct torque. Or you could use a second flat washer to take up the difference.

When I did mine, I bought a new set of motor mounts from NPD only to find they were made in India, the steel appeared thinner and not only that, the mount surface was curved, not flat. I could have used a press and flattened those areas out, but I decided that as the original were still in excellent condition, I'd use those again until I could get a better set. They are still on the car 5 years later.
motor is out of the engine compartment. the bolt that is causing issues is just on one side. and the threads are not stripped on the block/bolt itself. I misused the term bottoms out, I meant to describe the bolt goes in all the way but will not tighten. it just keeps spinning in place once all the way in.

 
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Did you try one size larger bolt? It's possible that a previous owner stripped it and rethreaded it one size larger. Or, it was threaded for a helicoil and the coil pulled out when the bolt was removed.

 

kiko619

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I'd be lying if i told you there ever was one and if it came out during bolt removal. II didn't try a longer bolt. I mentioned previously that all the bolts matched the originals in legnth thickness and pitch. So much for upgrading the bolts to throw out the old ones. ill try the longer bolts/helicoil method and see what i get from it.

 
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Like Don said it may have been drilled and tapped to a larger size.  If a helicoil was used and it backed out you will find the thread pitch is the same as the original bolt, but the diameter is larger.  May be worth a trip to pick up a low quality thread pitch gauge and caliper from Harbor Freight or similar retailer to confirm what you have.  If it ends up being a backed out helicoil you would need to track down an new insert and insert install tool.  
 

Take the time to measure what you have.

 
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Try one size larger diameter, not longer. You say it doesn't look like it's stripped, which means it has threads in it, so it may be one size larger. I believe the original size is 3/8"-16x3/4". Try a 7/16"-14x3/4".

Edit: Bentworker made a good point, it could be the correct thread pitch, but too large of a diameter. Good idea for getting a thread pitch gauge.

 
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kiko619

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thank you both for the help ill check it out and follow up when I get back in town.

 
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I'm with Bentwork, It sounds to me that there was previously a helicoil that has gone missing.  The only other way I can think of for a bolt to just keep spinning, is it's too small a diameter and not fully engaging the threads in the block. 

 
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