Transmission Brace 91 Bricknose

So while doing some inspections under my truck I found a rather impressive failure of my transmission brace. I'll also clarify I've already changed the two motor mounts which were indeed bad and likely caused this. I also have a brand new transmission mount to put in.



I've not sourced a replacement brace so my original plan was to remove it, drill all the cracks I can find. Plug weld the drilled out parts. Then grind out and weld the rest. Grind all the welds smooth so the new trans mount rubber sits as it should. On the bottom fully weld in a piece of 3/8 plate I have one hand using stich welds so I don't turn the thing into a pretzel.

Anyhow I mentioned how I intended to repair this to a friend of mine and he strongly felt I needed to drill the cracks and only plug weld the holes, leaving the rest go. Then add the plate as I planned but not fully welding it only doing 1" stitches with say a 1" gap between. He was pretty sure the brace itself needs to flex or it will fail again.

I thought this was a neat discussion topic so I thought I'd drag it over here. He has far more experience doing fabrication than I do and I plan to follow his advise.

All the same I was curious if anyone else had to do any repairs like this and what their thoughts or experiences were.

It's one of those things where I see it could have a few schools of thought to apply which might make an interesting discussion.

My bet is that was likely caused by leverage on the crossmember from the mount. It is very possible that bad motor mounts could torque the transmission around, turning the mount into a lever. I can definitely see why this happened.

I'll let those more well versed in engineering explain whether or not the transmission crossmember needs to flex.

I can say though, there's no way the one in my 2003 Ranger can flex with the way it's built. But that's a different type of frame (boxed front), so that may play a part.

the rear mount/ transmission mount does NOT need to flex. it is a rubber mount to reduce vibration transfer and the relative noise from it. the front two mounts are the same. three-point mounting keeps the engine planted. having any one of the three points broken will cause amplified stresses to the remaining mounts. if all three were solid mounts the likelihood of any being broken goes down drastically yet the driver may feel the engine pulses. the cracks in your crossmember are unusual the way they go all the way across the bolts and on both sides. the red dusting coming from the cracks shows that it has been that way for some time.

Great info. My initial thought was to go as soild as possible with the repair like you are saying. I agree there's rubber at all three points so I wouldn't see why the steel it's mounted to would need to flex.  

I'll try to give some back story

You are correct if I were to guess I broke the mount about 2 years ago. I pulled another vehicle out of mud being a little harder on things than I would have liked. I used a chain and I assumed the sounds I was hearing were the chain moving under tension but one of the pops may have been this mount.

It was when pulling hard with the truck like that (which I hadn't done to that extent in a long time) I was noticing my shifter moving a bit more than I felt it should which lead me to motor mounts last year.

The truck sees intermittent use so last fall while I was under it checking other things I found the trans mount issue after double checking my work on the motor mounts.

I periodically get under the truck I don't think I would have missed the broken mount since it was glaringly obvious last fall but you never know.

Sorry if this got a little wordy. I'm thinking more solid is more gooder with the brace. I was fairly certain of that before I talked with my friend which threw me off a bit. I guess it leads me to wonder if older rigs needed more flex? I realize a 91 is older but I mean more 60s and 70s which he would have wrenched on quite a bit.

there is a rather simple means of fixing this. welding of course. we run a welding shop. but basically, if you were able to remove the two nuts and grind the surface surrounding the mounting points (not really concerning yourself with the path of the cracks) and place a heavy washer over the bolt then put the nut back to hold in place snug. then tack around the circumference at four points. then remove the nuts and lift the trans off for final weld so there is little heat transfer. this will be a substantial repair, yet the only issue will be compatibility with another trans if you did a swap.

Do you think there's not much concern of the cracks migrating in the future? If the truck were to get another trans it would likely be a replacement M5OD like it had stock so this repair should be good.

Would you think a washer around 1/4" thick would be an adequate thickness? I didn't measure the base metal but if asked to guess I would guess somewhere around 1/4".

I do plan to remove this bracket to help to install a full all stainless exhaust including a high flow cat. It seemed logical to try and fit the two projects in with each other. So the bracket will be getting removed in the near future.

the cross-member steel is only about 1/8" thick. that's the reason for them being formed into corrugated form. like the difference between paper and corrugated paper. one has much more rigidity. another example would be the body line from side marker light to tail light. this is for added rigidity across the door handle area and carried the full length as a style line. or the lower one along the lower part of the truck. if concerned about the crack continuing on, drill a hole at the end where the two converge. big enough to be clean and deburr it.