codyv17

so i have a 92 cheorkee, 4.5" lift, with 33s, stock gears, one of my motor mounts broke not to long ago. two if the bolts broke off in the block i cudnt get em out and was getting frustrated so i just took it to my buddys shop to have him do it. he couldnt get em out either so he ended up welding on studs, now the motor mount broke again shortly after. Im thinking maybe chaining the engine down so the engine doesnt move/twirk as much on hard acceleration, which im guessing is the reason im breaking the mount because both times i broke it i wasnt even wheeling it. If anyone has any ideas please help..

ktmracer419

he welded studs to the cast iron block?

what broke the second time? where the studs meet the block?

any good mechanic should be able to get the broken bolts out. but sounds like you might have to look into the brown dog heavy duty block bracket kit.

http://browndogindustries.com/S2572P...-Brackets.aspx

Tom95YJ

^ This X2

hankthetank

Hope not...that's retarded. Doing a helicoil would be better than that...

cruiser54

This is why the mount bolts break:

From Jon Kelley aka 5-90 @ www.kelleyswip.com
　
　
This is a known issue.

1) Unless you're going to get silly with the skinny pedal, 2xSAE8 screws on one side won't be a huge problem. You'd be better off with three, but if you don't get silly you'll be okeh.

2) The Brown Dog engine mounts use two or three additional holes on each side, and spread the mount/clamping force over more of the block. This is invariably a good idea, and I'll probably be doing it as I refit my 88.

3) The primary reason that the screws snap is because, sometimes, the screw holes in the block aren't drilled & tapped deeply enough. This causes the screw to bottom out in the hole, so you end up torquing against the bottom of the hole instead of stretching the screw (as you're supposed to do. Not your fault - you aren't doing anything wrong.) The screw is not stretched properly, and the head is not butted up against the bracket, so there's some room to move there. Vibration then causes wear cycles on the screw, which generally leads to rupture.

The easy/cheap fix? When you replace the screws (3/8"-16x1.25", as I recall,) put two flat washers under the head before you screw it into the hole.

The check? Take a feeler gage (.003" to .005") and try to slide it under the head of the screw. You'll be able to get under the corners (look at the hex head, and you'll see that the surfaces curve toward each other slightly) if it's a standard hex head - if it's a flanged hex head, you should not be able to get under the head anywhere. If you can slip the gage under the head, you have a problem. If you can touch the shank of the screw, you have a big problem!

In no case should you reuse the screws after you take them out - they'll be stressed under the head, and you'll have a significant reduction in strength. Replace them outright, putting washers under the head as I mentioned before. The washers will make up for the slight lack of depth in the hole (two of them will be about 0.125" or so,) and allow the screw to be preloaded properly.

hankthetank

What's the torque spec on the mount-engine bolts?

hankthetank

And SAE8 is a fitting designation, unless he means 3/4-16...??? Then he says 3/8-16....which is equivalent to M10 x 1.5 ????

xjmarc

Do you have a t-case drop? They tend to put more of a strain on motor mounts since the engine sits at an angle with the drop and causes the rubber to wear faster but doesn't usually break them.

freegdr

how trans mission mount ?

cruiser54

I think he means Grade 8, 3/8 X 16.

5-90

Engine mount screws breaking - common issue. I've covered this in the last couple of weeks, so I'm not going to do it again. However, the answer is on the board, so a brief search should turn it up (explaining both the cause and the correction. It's cheaper than you think, once you get the holes cleaned out.)

cruiser54

Jon, it's in post #5 of this thread. I saved it from a previous thread.

5-90

SAE5 and SAE8 are strength designations (SAE5 is a nominal 150ksi min, SAE8 is a nominal 180ksi min, as I recall, loaded in tension,) similar to the ISO property classes (4.8, 8.8, 9.8, 10.9, and 12.9 being common. Yes, there is an SAE9 and SAE L9, but they're difficult to find.)

3/8"-16 is a thread pitch specification (I believe the engine mount bracket to engine block screws are 3/8"-16x1.25" HHCS or FHCS,) which has nothing to do with material or strength (a 6/6 Nylon screw threaded 3/8"-16 will fit in the same place as an SAE8 screw, 314SS screw, 416SS screw, half hard marine brass screw, titanium screw, aluminum screw, or whatever...)

I think they're actually flange head screws, so the proper callout would look something like [3/8"-16 UNC x 1.25" SAE8 Class 2A FHCS] - 3/8" nominal diameter, 16 threads per inch, 1.25" length under the head, H&T to SAE8 specifications (180ksi minimum strength loaded in tension,) thread fit class 2A, flange head capscrew.

No such thing as "equivalent" sizes between SAE (inch) and ISO (metric) - and 3/8"!=10m/m. 3/8" = 0.375", 10m/m = 0.393". And, 16TPI isn't anywhere near 1.5m/m per thread. (The only close case I can think of is 5/16"-18 and M8-1.25. They're often confused, are visually similar, and one can actually reshape the threads to the other and just feel like you're working with really dirty threads. Examine the marks on the heads of the screws if you think they're mixed up - SAE screws are identified by hash marks for the fastener grade, while ISO screws have a numeric "property class" marking, I've listed some above.)

SAE8 or SAE5 has nothing to do with fitment of the screw - you can have a 1/4"-28 SAE8 screw just as easily as a 5/8"-11 SAE8 screw, the difference is in absolute minimum strength (both will be H&T to min. 180ksi ultimate tensile strength, the absolute strength is determined by using the minimum diameter to get the shank area, then multiplying that by the min. 180ksi.) Obviously, the larger screw will be absolutely stronger, but both are the same nominal relative strength.

But, cruiser54 did save you the searching - this is a known issue, and it's been around for years. One of these days, I'll be doing the FAQs for the tech section of my site, but I figure that actually typing it all out once every two or three months is enough. Honestly, I do get tired of answering the same question over and over again, but I don't usually do a "search, n00b!" post unless I know the answer is out there. Usually, I know this because I put the answer out there - as cruiser54 just ably demonstrated. (This is why I need to do a Tech FAQ section - instead of typing it up, I can just link to it...)

hankthetank

Grade 5 is 120, grade 8 is 150. Those are minimum tensile strengths. 60% of that is its shear rating...

5-90

Ah. I wasn't looking at a reference chart, which I invariably do when I'm designing something.

Gotta stop overestimating tho, I was probably keying off of the "5" and "8" in the spec.