Motor mount issue

[dyeager535]

I know, I know, picture fail. I can get one later.

Years back I installed poly motor/trans mounts in the truck. I recall back then they were a real bear to cram into the clamshells, and then another real pain to get the mounts to line up to get the bolt in.

Had no problems though until hunting this year (along with everything else that went wrong in that week...heater failed and melted fuse panel for the second time, windshield randomly cracked all the way across, massive oil leak, etc) when I happened to look underhood and noticed both ends of the 7/16" bolt that holds the engine clamshell to the frame side of the motor mount, hanging out, bolt obviously broken.

Tore that apart last night, and the motor mount "puck" is damaged. There are four metal tabs on the puck that engage the clamshell on the engine, shown in this random pic:

On mine now, the tabs on the left (lets call them the "lower" tabs), on both sides are smashed into the mount. Bent at pretty much a 90* angle.

I think I used the wrong 7/16" bolt (threaded almost the whole length). In any case, it snapped in the middle. Now I have two questions. Without looking at how everything interfaces closely, is it a bad idea to re-use the puck with the bent tabs, and are the bent tabs a result of the bolt failing, or did whatever caused the tabs to fail result in the bolt failing?

While I recall the mounts being a real bear to install in the truck, I don't *think* my install would have bent them, but it's possible I suppose. Just curious what is the best way to proceed.

 

[Blue85]

Any chance you installed them backwards? The insert looks kind of symmetrical, but it has wide tabs and narrow tabs. They fit in the clamshell fine either way...until you bolt the clamshell into the truck. I'm not sure if you can get the stock bolts started with the insert backwards, but if you did, you would crush the tabs as you drew the bolts tight. They're like limit tabs to prevent the motor mount from moving very far and they're aligned in close proximity to the engine and frame brackets when installed. Perhaps that broke the bolt, but I think it's more likely the through-bolt broke first and that let the motor mount move enough to bend the tabs against the brackets. Why would the bolt break? NOS? More likely it was never tight.

 

[dyeager535]

My buddy was saying you can't really install them "wrong" since both tabs engage the clamshell in a specific location, you'd have to destroy all the tabs to make it fit. From a quick glance that appeared to be true. When I pulled the mount it was obvious the upper tab was aligned with the right notch in the clamshell, so I think that doubly indicates no problem with how it was oriented on install.

I think the bolt was wrong, it was threaded almost the entire length, I don't think that is stock. It was an old used bolt though, I just figured it wouldn't matter. I am normally pretty good about not overtightening bolts, but it could have been overstressed from tightening, I'm sure I didn't use a torque wrench on it.

 

[Hogback Fabrication]

The stock bolts are threaded the whole way

 

[dyeager535]

Well in that case, correct bolt! Seems odd GM did that, wonder if that allowed them to buzz it in with an air ratchet or something on the assembly line with having to worry less about alignment?

 

[diesel4me]

I have seen both fully threaded bolts and "regular" bolts from the factory on GM motor mounts,on my own vehicles,and ones we took apart at the junkyard...

 

[Blue85]

A broken bolt certainly would allow the tabs to get mashed, so it's reasonable to assume that's the initial failure mode. Didn't you hear some weird noises? I suppose with only one side broken and the other tranny mount good, it might not thrash about too bad, especially if not driven too hard.

 

[kennyw]

Does that mount have a sleeve all the way through the bushing or is it hollow inside?

 

[dyeager535]

Does that mount have a sleeve all the way through the bushing or is it hollow inside?

Yep, sleeve all the way through.

No noise, the bolt was still wedged in both holes, just chewed up the sleeve at either end a bit.

Something else is wrong, not sure if bent frame, bent hat, or bad motor mount perch. I can see now why it was hard to put together. Pictures don't particularly do it justice:

Kind of hard to tell, but with the mount removed from the perch, the bolt going into the hat is crooked. If I snugged it down, it would have to pull the hat "straight". The perch is nowhere near aligning in this position, the bolt holes are offset front to rear, with the bottom being off a greater amount than the top. I don't think I have a spare hat, but I think I have a spare perch to swap and see. I think the problem is in the hat though, since the bolt isn't straight. The upper tabs just don't look like they would fit with that misalignment (wrong angle in pic, but if you think about it, the lowers and uppers should either both fit, or not), and may have been crushed on install since I had to fight it so much.

Maybe the bolt snapped because it was bending from misalignment?

 

[diesel4me]

Is this the drivers side mount that gets all the torque when going forward ?...
I'd assume that the drivers side mount gets the brunt of the forces.and that this is a square body truck..

Does the perch have multiple bolt holes for the hat to bolt too?..maybe you have them "off" by a hole ...

 

[dyeager535]

Yes, square body, drivers side.

You can barely see both holes in the perch in the lower photo I posted. I can't recall what the upper bolt holes look like, but the lower is so far off (at least an inch) that I don't see how it could be tweaked that far and still go together.

I did compare against the passenger side, and the passenger side does not seem to have any of the issues the drivers side does. But when I assembled, it probably went together first.

 

[Blue85]

Could the tranny mount be broken, letting the whole drivetrain sit crooked?

 

[dyeager535]

No, it's the later "short" adapter with a matching poly mount that is very solid, unfortunately.

I had to drill the frame to fit the crossmember, it could be misaligned, although my measurements before indicated it was not. I will probably unbolt the trans and see if that changes alignment.

The passenger side mount doesn't seem to be "bound up" like the drivers side is though, and I would think it would be if the trans was offset.

 

[ramack]

Shortly after buying my 79 K5, up in the high country, my clutch linkage kept popping off while crawling around some trails. Once I was back home I looked at the motor and trans mounts, they were all shot. I replaced all three mounts, haven't had any issues since. I don't recall having problems while assembling and installing new mounts. I think I did have to clamp the clam shells in a vice then riveting the halves together. I'm including before/after of the mounts, doesn't look like I have any shots of them on the K5.

 

[dyeager535]

I know one of my clamshells was much harder to put together than the other. They were cheap Indian clamshells though, so who knows if they were up to spec either.

Got my hands on two sets of SBC motor mount "hats", and have a replacement poly puck en route, once I get those I am going to mock everything up and pay close attention to how it all fits together to determine if something else is wrong in there. I'm optimistic though that the hat is the problem. If it's not, the problem is going to be a lot more serious. I'm hoping that the hat was bad when I initially put it together, and not that the hat has been deformed due to something else not being in spec, like the frame or motor mount perch.

 

[Blue85]

That's maybe the scariest tire I've ever seen. On Craigslist, that's probably "a lil weather checking".

My mounts looked pretty much like that. I put the clam shells in the vice and tack welded the two halves together in the old rivet holes. The inserts came with big zip ties, but I thought they might get in the way of stuff during the install. Plus, free zip ties.

 

[dyeager535]

Heck, if you look at my picture you can see the original zip tie used to hold the clamshells together. It lasted longer than the mount!

But in any case, the one that was harder to assemble couldn't be held together with the zip ties, they stripped out. Welding would have been the smart choice, I'll do that if this replacement is that difficult to hold together. Buddy said he did the same, so it must be pretty common.