Stuck Diff Bolt

[hahuang65]

Hello all,

 

Building my 420R and I got stuck on a bit here. Working on installing the diff. I've got the bottom two bolts in, and am installing the top bolt.

 

I chamfered the tip of the bolt a little to get it to align more easily. I started from the passenger side and got it thru into the diff just a tiny bit bit couldn't get it to budge in. I knew it was in the diff because if I let go of the diff, it didn't drop. But from there it wouldn't go in. I started using a ratchet to get it further and I got it all the way thru to the driver side chassis mount.

 

It's not aligned. See attachment.

 

I can't seem to push it back out, or unscrew it out anymore. I tried both unscrewing and hitting it out with a punch and hammer.

 

I don't see a great spot to lever the diff down just a bit to put the bolt through.

 

What should I do? 

[KnifeySpoony]

TADTS. Hit it with a bigger hammer. I think they call it a BFH. I think i used an impact wrench to help drive it through at some points during the install as well.  It was a real motherfucker. 

[hahuang65]

Thanks for the tip. I had used a driver as well. I guess I just need to find some leverage or bop it with a hammer. Don't want to damage anything...

[MV8]

It is good general practice to thinly grease the shank of any bolts and start ALL the bolts a few threads before tightening any. Then it won't be a nightmare to get apart in the future. If loctite is not required, grease the threads as well. Wheel bearing grease or even vaseline is better than nothing.

[sf4018]

If you don't have luck try this - remove each of the 2 shorter bolts below one at a time and replace them with narrower screwdrivers than the bolt, this will allow enough play to move the diff around a little but still hold it in place. Then you should be able to move the diff to straighten up the exit for the long bolt and smash the crap out of the long bolt to get it through. Once the long bolt is through working the diff for the shorter bolts is a lot easier, use a hydraulic jack to align the LSD to the chassis holes. Stuff of nightmares.

[hahuang65]

Thanks for the tips guys, I'm gonna try again on Thursday. Will likely take out the bottom bolts. Will post back with any results. 

[BlakeJ]

5 hours ago, sf4018 said:

If you don't have luck try this - remove each of the 2 shorter bolts below one at a time and replace them with narrower screwdrivers than the bolt, this will allow enough play to move the diff around a little but still hold it in place. Then you should be able to move the diff to straighten up the exit for the long bolt and smash the crap out of the long bolt to get it through. Once the long bolt is through working the diff for the shorter bolts is a lot easier, use a hydraulic jack to align the LSD to the chassis holes. Stuff of nightmares.

That’s what worked for me. 

[wemtd]

Fwiw - since locktite was mentioned: to disengage/remove parts that have thread locker on them all you need to do is warm them up.
 

(Different temps for different strength locker). Use a heat gun or fire & patience. 

[EdWills]

On 7/21/2025 at 11:54 PM, KnifeySpoony said:

TADTS. Hit it with a bigger hammer. I think they call it a BFH. I think i used an impact wrench to help drive it through at some points during the install as well.  It was a real motherfucker. 

I was at a small racing car shop in my city a number of years ago, and I watched in horror as a so called mechanic attempted to install the Hewland gearbox and bellhousing assembly into the Lotus twin-cam engine of an immaculate Lotus 23C.  He was using what we used to call a Birmingham screwdriver (a large mallet with steel head) to attempt to drive the input shaft into the end of the crankshaft by hammering on the rear of the gearbox.  He had not properly aligned the clutch plate with the flywheel, and the install was not going well.  The car owner was trying to help, but seemed oblivious to the damage that could be caused to his car, trusting that the mechanic knew what he was doing.  I got both their attention with a few choice words, and inquired if the clutch plate had been aligned correctly.  A check proved that it hadn't, but the mechanic insisted that he could have  jogged it into place. The mechanic did finally adjust the clutch plate.  I'm hoping that the above quoted response is meant to be humorous, but please don't try and force any bolts with a steel headed hammer.  A rubber mallet can be used to budge some components into place, but if something doesn't line up,  please find out why before resorting to brute force and ignorance. Cheers W.

[KnifeySpoony]

Have you ever installed a diff into a late model Caterham?  Until you walk in a man's shoes...

[EdWills]

15 hours ago, KnifeySpoony said:

Have you ever installed a diff into a late model Caterham?  Until you walk in a man's shoes...

Well, there is a proper way to do it, and then there is your method I guess.  What would be wrong in slightly undoing all of the bolts that hold the diff, and then perhaps using a dowel pin or a bolt with no thread and rounded off at the end, or possibly the non pointed end of a correct size drill bit, using them to align all of the holes.  Then with the correct sized bolt, slide the bolt in displacing the dowel pin(s)?  When the chassis frame is built,  there is no guarantee that the welder snugs up the jig tight enough, or perhaps moves the frame pieces slightly?  Presumably the bolts were correctly installed in the first place, so why use brute force to replace them?  W.

 

'Before you criticize someone, you should walk a mile in their shoes.  That way, when you do criticize them, you're a mile away and you have their shoes'.

[hahuang65]

36 minutes ago, EdWills said:

What would be wrong in slightly undoing all of the bolts that hold the diff, and then perhaps using a dowel pin or a bolt with no thread and rounded off at the end, or possibly the non pointed end of a correct size drill bit, using them to align all of the holes.

 

Nothing is wrong with this. I actually attempted this initially, albeit with steel rods that were NOT to size. I tested that the bolts go through the holes in the chassis, particularly the long one, and that was fine. I should have tapped the threads and cleaned the bolts, as the bottom right one didn't go through initially, so I had to drop the diff and tap the thread and clean the bolt.

 

However, once all of that was done, the bottom right hole STILL didn't go through without an abnormal amount of resistance, no matter if the diff was free to move with no bolts attached otherwise. I ended up with the assumption that the threads in the diff where just slightly angled, maybe a quarter of a degree or something. I just drove the bolt in with a ratchet by hand, powering through the resistance. A couple of turns, then it threaded with a normal amount of resistance. When the bolt popped out the other end, there were a couple of metal shavings that came off the bolt, but I was able to torque it to 81Nm.

 

The left bottom bolt went in with no issue, just like the first time I put it through.

 

I may be jumping to conclusions, but I think the top long bolt has the same issue as that bottom right bolt. It may just be a quarter of a degree misaligned or something. I also had to drive that one hard through, until I got to where I'm stuck at right now.

 

Now, maybe the issue was that I used an undersized steel rod to align everything, but the correct size one (or a dowel) is surprisingly difficult to source. None of my alignment or pin punches fit well either.

 

I'm a little less worried about the top bolt, the diff seems quite stout, as well as the mount holes in the chassis for that bolt... and since the bolt passes fully through and just nuts up with a nyloc at the end... I can re-thread that bolt if there's a small amount of damage on it.

 

That said, I think I will loosen the bottom bolts and give it a shot first, before resorting to brute force. Better safe than sorry.

 

Will report back here either way :D

 

Thanks again to everyone for their tips/advice.

[KnifeySpoony]

1 hour ago, EdWills said:

Well, there is a proper way to do it, and then there is your method I guess.  What would be wrong in slightly undoing all of the bolts that hold the diff, and then perhaps using a dowel pin or a bolt with no thread and rounded off at the end, or possibly the non pointed end of a correct size drill bit, using them to align all of the holes.  Then with the correct sized bolt, slide the bolt in displacing the dowel pin(s)?  When the chassis frame is built,  there is no guarantee that the welder snugs up the jig tight enough, or perhaps moves the frame pieces slightly?  Presumably the bolts were correctly installed in the first place, so why use brute force to replace them?

This is an initial install afaik. Nothing is being "replaced', and even so, this is not an an usual issue for late model 7 diffs. Most are seeing tolerances such that significant bushing deflection is requiring to get the bolts through. I speak from experience, it seems you are speaking from speculation. "alignment" is not possible without force, because the hard rubber bushings need to deflect. This has been for many years actually if you go look at threads on blatchat going back ~10rs. There are various strategies in terms of order of operations, but it typically requires a lot force at some point in the process. 

Edited July 24 by KnifeySpoony

[hahuang65]

3 minutes ago, KnifeySpoony said:

This is an initial install afaik. Nothing is being "replaced', and even so, this is not an an usual issue for late model 7 diffs. Most are seeing tolerances such that significant bushing deflection is requiring to get the bolts through.

correct, this is an initial install.

Also, the tolerance issues ring true. I've seen many blogs all say the same thing... and I've personally experienced a very similar tolerance issue with the front wishbones to chassis mount.

[MV8]

Not to beat another dead horse but FWIW, I just took a look at a 2014 assembly manual. The bottom bolts are to be left loose before installing the top bolt. No mention of a BFH (and it may be necessary in some cases after common sense exhausts the low hanging fruit), though every car and mechanic is different. Even bad experiences help.

 

I can tell you apprentice mechs get the smallest tools and hammers to limit damage.

 

This is in general. No judgement. Nothing I've written is referencing any member's ability/experience level.

 

 

 

[hahuang65]

11 minutes ago, MV8 said:

This is in general. No judgement.

Full agree. I think it's common sense to work your way up from the simplest/easiest/least possibility of damage solutions up to BFH, cuz sometimes that's just how it is.

 

Yeah tightening the bottom bolts is on me. Got overzealous after I got that lower right bolt in successfully. Gonna unwind this and try to get the top in. Unfortunately, I can't seem to back the top bolt out, so it's in for how far it already is in now.

[sf4018]

Regarding the BFH, on the 2nd time installing the diff (after a shaft replacement) we needed some real power to get the top bolt in. We couldn't get enough power under the car (not enough hammer stroke) so we taped a threaded rod to the long bolt head, long enough to poke out into the wheel well area, then use a masonry hammer outside the car, can really put some power into it. Good times.

[KnifeySpoony]

15 minutes ago, sf4018 said:

Regarding the BFH, on the 2nd time installing the diff (after a shaft replacement) we needed some real power to get the top bolt in. We couldn't get enough power under the car (not enough hammer stroke) so we taped a threaded rod to the long bolt head, long enough to poke out into the wheel well area, then use a masonry hammer outside the car, can really put some power into it. Good times.

You mean it didn't go in with tenderness and whispering sweet nothings? Maybe you didn't whisper softly enough...

[hahuang65]

Got it!

 

I had a friend come over to help.

 

We started out by loosening the bottom two bolts.

Then removing the top bolt. It was stuck at the end of the last session, but with my friend wiggling the diff, I was able to punch it back out. We inspected the bolt and it looked fine. I re-greased it with copper grease and inserted it back in.

 

This time, with his help wiggling the diff, I didn't need to use a power driver. I was able to bop the bolt all the way through to the other chassis mount with a mallet. We got to basically where I was stuck on last session quite quickly.

 

We found a spot where we could lever the diff downwards. This was basically pushing the diff down, against the chassis mount. We used anything that worked, from a prybar, screwdriver, or a pair of pliers.

 

We got it to a spot where it looked like the bolt should make it all the way through. We bopped it more with the mallet, but it wasn't budging. We must have just been mis-aligned by a tiny fraction of a millimeter.

 

We kept trying, and we bopped the bushing out! However, this made it a bit easier. This let us push the bolt all the way through with no resistance.

 

We simply fit the washers back onto the bolt, and then the bushing onto the bolt. We had to lever/pry the diff again to get the bushing aligned, and we eventually get it on by hitting it with a mallet.

 

The rest is just malleting it fully in, then torquing all the bolts back to spec.

 

Thanks for all the help and suggestions guys!

[KnifeySpoony]