Help with rear differential getting loose
 

My backdraft #990 rear differential 14mm bolt keeps getting loose and breaking when I race. I am a drag racer and after about three passes I have to bring it back and tighten the bolts that support the rear end. I have broke it and luckily been able to use a point chisel and remove it. I am using a grade 8 bolt and lock washer and nothing helps. I need some way to keep this tight. I don't want to use lock tite cause if it breaks I will never get it out. Please help me.

hmmmm, i think your getting off lucky myself. when you break a half shaft and it comes up thru the car you'll really be pissed.

I have broke both of mine. Luckily they did no damage. I got the performance half shafts from backdraft installed now

Loose bolts
 

I have a 9" ford rear in my Cobra and the bolts always bome loose. I got rid if the nylon lock nuts and bought stover nuts and use locktite with it....so far it's working .....
Jon

Thanks for the info. I am still running the independent rear suspension that comes with the BDR cobras.
Has anybody heard any info on the support plate from this company. I hope this helps me launch off the line at the strip. Active Autowerke BMW Differential Support Bracket E36

Are you tightening it or torquing it? If things are torqued correctly, they usually don't come apart under normal conditions but it sounds like you are not necessarily using it that way so the reinforcement plate may be the way you'll have to go to keep it together. Until it finds the next weakest link...

Use Loctite. Loctite is easily removable by heating to 500F. If you need to remove just use a propane torch for 45 seconds on the bolt and it will melt the loctite. The only time loctite has the ability to break a bolt is if it is .25" or smaller. If you are concerned use loctite 242 as it is removable with hand tools.

Grade 8 bolts are very strong in tension, but can be brittle, and will break when exposed to lateral tension. A grade 8 should only be use in double shear, or when a locating dowel is used.

I would start by switching to a grade 5 bolt, which is more forgiving in single shear applications.

A split lock washer only works when torque has been lost, and the ends are allowed to split apart. If you're using rubber or poly bushings, a split lock washer is a good choice. Some people make the mistake of putting another flat washer under the split lock, but that defeats the purpose. The nut will still rotate and come loose. The split lock washer needs to dig into the nut and the base material.

It's hard to imagine that a nylock nut would come loose, but it does happen upon rare occasion. If you have solid bushings, I would suggest an internal star washer and a nylock nut. If it still comes loose, use two nylock nuts.

Agree with Bob, Grade 5 bolt, star washer, and some safety wire.

Paul T.

Do you have a picture of the bolts coming loose?
 

427FAST I am not sure which bolts are are talking about but here's a couple of things I have learned over the years
#8 bolts are stronger and more brittle. They have no to little give. Grade #5 is on 90% of my car.
3 ways to stop a bolt loosening, Locktite is first. Start with the blue, if this doesn't hold then goto the red. IF the hole is clean, it will hold.
If this is a bolt that looses, drill an 1/8" hole through the head and safety wire tie it to another bolt or mounting plate.
last is to tack weld the bolt head to the plate in 1 or 2 spots. You will have the grind the weld to make repair but the bolt will not loosen.
Last point, If the bolts are coming loose, re check the trueness of the rearend and mounting plate and that the surfaces are both flat. There sounds like an alignment problem between frame and rear diff surfaces. Road racing, drag racing, auto cross, all put a heavy load on parts and mounting locations. I have been racing for better than 30 years and have not had this kind of problem when using locktite or wiring unless mounting surfaces where the problem. Rick L.

Thank you for the info. I will change the bolt to grade 5. Would these solid diff mounts help from backdraft also? Backdraft Parts - Solid Diff Mountings - Performance Suspension - Suspension - Products

Need to see a picture of mounting system
 

FAST427 First name would be nice. Solid mounts have good and BAD issues in cars. Here's what I have done to my ERA car rearend. I went to solid poly bushing for lower control arms. I slotted them for greasing of them. They are 14 years old and still tight. The rubber cage mounts are still tight and working good. Kept them clean and dry with light coat of grease to protect from weather. My motor mounts are the oem ford units with bolts going through them to act as a solid mount with just a little give. Trying to limit the vibration in the car between drive train and frame and body of car.
427 here's the thing, if you want to drag race your car, an IRS is not a good unit for this. I am not saying it isn't done. A solid 9" ford would be the way to go with a 3 or 4 link setup. I built my Jag setup for 600/600 hp and torque. Chrome moly shafts, locker for dana 44, carrier cover to support carrier caps, and watch the tires and rpm from a stop.
5,000 rpm launches are great if you have the correct drivetrain. When I do drag race the car, I leave at 1,400 rpms and roll onto the throttle. This stops rearend walking and schutter in drive train. I run a twin disc and it locks up quick. These car where never ment for drag racing with IRS rearends. I can see SB cars with good HP and not alot of torque. BB motors with 400+ ft of torque is a different problem.
Last note I run a soft setup for the rearend of my car with springs and shocks. I get about 1-1.5" of squat and leave, no tire slipping. Ran about a 1.8 second 60' time. Tire pressure in the high teens and a little higher on l/s rear because of the fat driver. Car goes straight. Wheel alignment and thrust angle are also very important for straight runs too. Good luck. Rick L. Ps would still like to see a couple of pictures of the rearend mounting system. Any chance of going to a larger bolt setup? 16mm or American size?

Using a Grade 8 or a Grade 5 I would do the following as this is what I have done on my car after breaking multiple bolts.

Drill bolt head for safetywire and drill a pilot hole into the last 1/2" of the bolt.
Put bolt in with a little (not much at all) anti-seize and then safetywire bolthead. Bolt will NOT backout due to safetywire and if bolt snaps you can pull the shank out the backend of the diff using an easyout.


Kevin.....

427F, have you gone to the solid diff and subframe bushings? If not, I would suggest it. If you still have all of that rubber in it I'm not surprised that things are breaking with all that flex.

I am ordering the solid diff and sub frame busing now. I hope that does it