1995 XJ 4x4, 4.0, auto trans, 4-door, 1.5 lift front & rear, otherwise all stock.

Back on 6/10/24, I pulled into a parking lot, hit a small pot hole, heard the “pop”, then heard the fan clutch whining. The fan was into the shroud because all three of the passenger side motor mount bracket bolts had sheared off in the motor block.

On 6/12/24 I collected tools, E-Z outs, drill bits and attacked the repair. I got all three of the 3/8” x 16 broke-off bolt remains out of the block in a few hours and replaced them with 1.25” long 3/8 x 16 GRADE 8 bolts. I also replaced two of the three 3/8” x 16 bolts in the driver’s side bracket with grade 8 bolts.

Then I started reviewing the event and the repair in my head. While drilling / extracting the original broken bolts, I noticed the bolts were NOT hard. I drilled holes in them easily with common drill bits.

Now I’m questioning my use of grade 8 bolts. Are the original bolts designed to fail before the motor mount bracket jerks a chunk out of the side of the engine block??. Should I swap the grade 8 bolts for common / grade 5 bolts??

 

You're fine. Just make sure the bolts didn't bottom out in the holes. You want them clamping the bracket not taking the shear load, which is what happens if they get loose and then fail.

 

Hello
Same happened to me.
the short answer is no. Grade 8 has a snap pressure in the thousands.
Harder bolts are brittle. Bolts are designed to streach (sp)
Replace them with brown dog engine mounts and they will never break again.
It comes will proper bolts and no more problems . Cj at brown dog is a great guy and will walk you thru
Good Luck

 

I can attest to going the brown dog route. I fortunately have never experienced the engine side mount braking off any of my jeeps but am grateful to not have a hand in that ordeal.

Their technical support was very helpful when it came to getting the alginment correct as well.

 

I'm well aware of Brown Dog's reputation - but if the problem is the bolts how will replacing the mounts help?

 

I don’t plan to change out the mounts. The ones on the vehicle are in remarkably good shape to be 29 years old, even the rubber. I might go with some not-as-hard bolts since the originals were not hardened bolts. I think one or more of my bolts have been broken for a while. The rumble & roar I’ve been getting during hard acceleration is gone now that the mount is solid again. I think my exhaust has been touching the cross member at the rear transmission mount during acceleration / torque, and I didn’t know what it was. While preparing for the repair, I took the trans mount nuts off the studs and noticed the pipe laying on the x member. It had good clearance after the bolt replacement.

Thanks for responses.

 

OEM brackets bolt to the engine block with 3 bolts. The Brown Dog "Super Brackets" use 7 bolts on the driver's side, 8 on the passengers...

https://browndogindustries.com/S2572...ts-Header.aspx

 

I have been running grade 8 bolts in the motor mounts brackets between both my jeeps for over 30 years now. Several hundred thousand miles with 2 strokers. If they are installed correctly, they do not just break because they are hardened.

 

I always laugh when the grade 5 versus grade 8 bolt debate comes up. A grade 5 will have deformed, stretched and broken well before a grade 8 would begin to deform, regardless of shock loading or load cycling. The failure here is simply due to the bolts becoming loose or stretched, allowing the bolt to take a side load. If properly torqued, the friction of the mount clamped to the engine takes the weigh of the engine. This need to firmly clamp the mount to the engine is why it's critical to ensure the bolt isn't bottoming out.

There are valid reasons for choosing a 5 versus an 8 though. Mostly where you want the bolt threads to fail first, or where the different material is less likely to gall or seize. Or cases where the bolt is likely to fail first and you want it to be easily to drill out.

 

So they do, now I know a little more about the product than just their reputation.

I notice they include grade 8 bolts.