The penguin's motorcycling and Jeep blog

Friday, December 24, 2010

Motor mount woes

First, let's review what a motor mount does in a Jeep. The job of the motor mount is to a) isolate the engine from the frame so you don't get your teeth vibrated out, and b) hold the engine on top of the frame. There are two motor mounts, one on each side of the engine about 2/3rds of the way towards the front, then the back of the engine is held up by the transmission. The transmission then has a rubber mount at the back of it that holds it to the cross member / belly skid plate that runs across the belly of the Jeep. So there's three pieces of rubber holding the whole engine / transmission / transfer case assembly off the ground, and three points determine a plane so these three pieces of rubber determine the location of the whole assembly.

Here is what the motor mount looks like on my Jeep:

There is a bracket welded to the frame that this mounts to. The part of the bracket closest to the outside of the Jeep has a nut welded into it and you go down from the top through that small hole in the motor mount. On the part of the bracket closest to the Jeep, there's just a hole. A stud comes down from the motor mount that must be pushed through that hole, and then you put a nut on the stud from the bottom. You can't come in from the top because the stud is below that big rubber bushing you see. Then the engine has a forked bracket that comes on either side of that big rubber bushing, and a crossbolt goes through the forked backet and through the big bushing in the middle of the big rubber bushing, and that's what holds the engine up off the frame, nicely isolated by a big rubber donut all around. Here is what it looks like installed in the Jeep (sorry I didn't get a better picture): So anyhow, it's been raining here in the Silly Cone Valley for 40 days and 40 nights, well, feels like it. I decided to take a trip in my Jeep for Christmas. Before taking any trip, I inspect my Jeep to make sure all the fluids are okay, all the bolts are nice and tight, none of the u-joints or tie rods or anything waggle when I whack'em with a rubber mallet, and so forth. And so things were going along fine until I got to the passenger side motor mount on the underside, looked up to where there's supposed to be a stud and a nut holding the engine side of the mount down, and... err. Yeppers, the blasted stud snapped right off at some point in the recent past!

That's from right after I pulled it. To pull it, you first remove the nut from the stud. Well, if there's a stud :). Then you remove the bolt. Then you put your floor jack and a piece of 2x6 under the oil pan skid plate and jack up the engine until the mount is just barely above the frame. Then you remove the crossbolt and the motor mount slides right out.

So anyhow, thanks to Christmas I can't get a new motormount until January (because all the suppliers are on holiday). So I rednecked a temporary fix: I drilled out the remnants of the stud, and jammed a nut on top of it, grinding out just enough so I could tap in the nut with a drift to retain it. This is a flange nut, which has a serrated bottom, so once there's pressure on it from a bolt pulling it towards the frame, it's not going anywhere. But there has to be enough friction to allow it to get pressure on it. Thus why I had to barely grind out enough to be able to force it in there with the drift and 3-pound hammer:

Once I did this, I then had to properly space out a bolt so that it wouldn't bottom out on that bracket part that's above the nut, yet would grab enough threads to not strip out. That required trial and error with various washers and nuts lying around. But I found the right combination, and now it's all installed again, waiting for a new mount but quite usable in the meantime.

As for why the stud broke: It was a metric grade 8.8 bolt welded to the motor mount before the top part of the motor mount was fabricated. Metric grade 8.8 (not to be confused with SAE grade 8, which is the equivalent of metric 10.9) is barely above compressed oatmeal on the hardness scale, but normally won't break in this application. It does bend fairly easily if not fully retained by a torqued nut, however, and also stretches fairly easily if overtorqued or given a sudden shock in the longitudinal direction. What I'm suspecting is that the bolt stretched, perhaps from the oil pan skid plate getting whacked by a rock, letting the motor mount jump around bending the stud back and forth until it work-hardened and finally snapped off. I didn't catch it because I was testing the nut by putting a wrench on it and seeing if it would move, and of course it was corroded in place and wouldn't move -- and unfortunately the place it was corroded into did not put enough pressure on the now-stretched stud to keep the motor mount from moving around.

All of this is compounded by the fact that I can't get a torque wrench onto most of these bolts due to lack of access -- all my torque wrenches are too big to fit into these cramped quarters and it's problematic running long extensions thru u-joints and expecting to get the right torque reading on a tork wrench -- so I'm just hoping that I'm putting the right torque on them. It may be that I had already put close to too much torque on the bottom, and whacking my belly pan on a rock was just enough to finish it off.

So anyhow, needless to say I'm not happy here, I think they should have gone with at least a metric 10.9 grade bolt for the stud because they don't stretch as easily, but at least I'm on the road again. And that ends today's adventure in Jeep wrenchin'...

-- Badtux the Wrenchin' Penguin

No comments: