Cracked diff mounts?

[Nick Mathias]

I would like some help please to diagnose a problem under the rear end of my TR250. 
I had a loud clunk recently, and since. On inspection most things are in good order. However, the top of the front diff mounting pins (both left and right) look to be cracked.
I have no experience of this issue. Any advice about how to proceed would be very welcome. The idea of separating body from chassis is daunting (never been done to this car).  
Attached are two photos, showing hairline cracks around the top of the pins, and a worrying hole on the right side pin. 

[RogerH]

Hi Nick,

what you see in you photo may simply be where the original weld has not penetrated from beyond the top surface
into the bottom of the skin.

You need to get a good view of the top surface.

I believe that the front right corner is usually the first to go.

Separating the body and chassis is the ideal way forwards.  But owners have done it with body still on the chassis.

The whole area needs a serious good clean before any welding - main ti see where the cracks are.

 

Good luck

Roger

[trchris]

Looks like a poor repair to one side you will need to remove diff assembly clean everything  weld the cracks then either make your own re- enforcing plates or buy from the usual suppliers 

Chris

[harlequin]

One of our local repair shops cuts a section of the floor out over the differential to get good access for the dif  pin repair, once completed he welds the panel back in. Its a little bit of lateral thinking but he is a craftsman and makes an excellent job of it.

George 

Edited December 15, 2025 by harlequin

[RogerH]

1 hour ago, harlequin said:

One of our local repair shops cuts a section of the floor out over the differential to get good access for the dif  pin repair, once completed he welds the panel back in. Its a little bit of lateral thinking but he is a craftsman and makes an excellent job of it.

George 

I was going to mention about going in through the floor panel in the rear seat area.
However I thought I would let somebody test the water temp before I dived in.

You still need the surfaces to be clean and this could be the hard part of the job.

 

Roger

 

Roger

[harlequin]

1 hour ago, RogerH said:

I was going to mention about going in through the floor panel in the rear seat area.
However I thought I would let somebody test the water temp before I dived in.

You still need the surfaces to be clean and this could be the hard part of the job.

 

Roger

 

Roger

Yes it's a little bit of a radical approach, but then why not?

G

[Nick Mathias]

Many thanks RogerH, harlequin and trchris for your responses. The thought of cutting through the panel leaves me cold, but I guess that might be part of the answer. (The body panels are untouched so far). 
Is this a sensible idea: to cut off the whole front bridge, and weld on a new bridge (I have no idea if they are available) or weld back on the repaired original? My welding skills are non-existent so I will have to find someone to help locally to me in Shropshire.

 

[trchris]

Hi Nick the front bridge use to be available and isn’t too bad a job to replace,looking at the pictures the worse part would be removing the bad weld that someone has attempted once that’s out the way l see no reason why a strengthening plate cannot be fixed in place perhaps your local group may have member who can tackle this sort of repair or point you in the direction of a reputable garage ( who can do the job correctly) maybe ask here if anyone can help
Chris

[PodOne]

Yes the bridge is available but avoid from the usual suspects as they are poorly made from thinner steel. The upper spring seats are in two pieces and need an additional donut plate of 2mm to be welded onto the top or they split. Also the cut out for the diff at the rear is on the wrong side. Only good bit is the pins are already boxed in but I would still add a top plate of 2mm across both pins. Just my experience here but mine was rotten.

Better to perhaps repair what you have once removed and re-enforce or find a good used item and do the same. That said it can be done with the diff removed and new pins used see here 

 

Just for completeness somebody had cut the panel in mine in the past to attempt a repair and failed and fibreglassed it back in place - bodge job.

Andy

[had17462]

Unless you take the body off which will lead into more work than expected get the floor cut by an experienced welder to do it once and forget about it 

Nick

[Nick Mathias]

Thanks again for such quick supportive comments. They are all useful. I realise that cutting off the whole bridge and repairing it will have an added challenge (trying to maintain alignment when putting it back). So in conclusion, the sensible answer is to weld the existing pins, and reinforce the top side of the bridge through a hole in the floor. 

[Tom Fremont]

A friend in his 9th decade with (3) souped-up TR6s including one with S/C giving circa 220-230 BHP, who thrashes them without mercy ( 6800 rpm with stock rods ) doesn't bother with reinforcing his diff mounts like the rest of us - he simply stuffs a slotted hardwood block into the RF bracket to prevent it from " rising to the occasion " under shock loading to which he subjects them with regularity, i.e. spinning the wheels from a standstill with 215 section tyres.

My operating philosophy couldn't be more different but his commands respect nonetheless. 

My own trick engined TR250 has her diff mounts boxed in and reinforced albeit not to the extent of adding weight to speak of.

Cheers,

Tom

[Nick Mathias]

Many thanks to everyone for your thoughtful responses. Really appreciated. 
My welding skills are non-existent but my woodworking skills are OK so a block of hardwood is very appealing - thanks Tom.

[dblenk]

On 12/16/2025 at 9:28 PM, Tom Fremont said:

A friend in his 9th decade with (3) souped-up TR6s including one with S/C giving circa 220-230 BHP, who thrashes them without mercy ( 6800 rpm with stock rods ) doesn't bother with reinforcing his diff mounts like the rest of us - he simply stuffs a slotted hardwood block into the RF bracket to prevent it from " rising to the occasion " under shock loading to which he subjects them with regularity, i.e. spinning the wheels from a standstill with 215 section tyres.

My operating philosophy couldn't be more different but his commands respect nonetheless. 

My own trick engined TR250 has her diff mounts boxed in and reinforced albeit not to the extent of adding weight to speak of.

Cheers,

Tom

Now thats a project i would love to do on my USA 6 a SC engine with220-230 BHP (i have the 7.5:1 head)

David

Edited December 20, 2025 by dblenk

[Malbaby]

Hi Nick,

From your story, I would have expected to see more damage than what is shown in your pics....Perhaps something else is happening.

I would not take a radical approach just yet.

Remove the diff, and clean the area thoroughly to identify any cracks/damage.

Then get back to us on what you have found.

Edited December 21, 2025 by Malbaby

[Nick Mathias]

Malbaby thanks for the suggestion, I think you are right. I have been planning to strip down and rebuild the front and rear suspensions. Doing this first, then checking the diff, and all the time looking for clues is the plan. 

[keith1948]

The floor is not a structural feature because there is a chassis. Would it be possible to cut out a section of floor and then weld a flange around the edge of the removed section. It could then be bolted back in place leaving access if needed in the future. I did something similar to the floor of a MK4 Ford Fiesta to enable access to the fuel pump and sender unit which is located under the rear seat floor. I removed a circular piece of floor above the pump/sender and fitted a flange. It is now bolted to the floor and provides access for any future need. I had a 3 series BMW that already had a floor hatch for the same reason. Before I did the Fiesta, the workshop manual said the fuel tank had to be removed to access the pump and sender.

Has anyone done this mod on a TR to access the diff mount area rather than rewelding the floor section back in place?

Keith

Edited December 22, 2025 by keith1948

[Dave McDonald]

I had diff hanger pin pull out completely from rear bridge left side on TR6 race car. 
Cut out an opening in rear floor bench/ spare wheel well under fuel tank area. 
Welded pin back in and plated it over top of bridge with a hole bigger diameter than pin diameter then flooded that hole with weld to plate and pin for extra security. 
Rather than weld patch back in floor, I pop riveted an aluminium plate back over it with silicone sealant all round, to make it easier to access should I need to be back in there again. 
See photos. 

 

[Z320]

Hi,

on the diff bridge all surfaces are plain and 90° to each other.

I made a quick and "temporary" repair there to bolt in 2017 - it's still there, does a great job and cost me an afternoon.

Just 4 big washers, 2 small centering washers, 3 UNC 3/8" bolts (on two the head off) and 2 pieces 13 mm hex out of the lathe.

Just to test I made it with a M10 threaded bar and M10 long nuts, that would also work.

Only a bit more difficult without lathe is to cut the PU insulator out for the big washer.

But also this can be ignored if the car has to go back on the street.

Ciao, Marco

Edited December 25, 2025 by Z320

[harlequin]

On 12/22/2025 at 11:03 PM, keith1948 said:

The floor is not a structural feature because there is a chassis. Would it be possible to cut out a section of floor and then weld a flange around the edge of the removed section. It could then be bolted back in place leaving access if needed in the future. I did something similar to the floor of a MK4 Ford Fiesta to enable access to the fuel pump and sender unit which is located under the rear seat floor. I removed a circular piece of floor above the pump/sender and fitted a flange. It is now bolted to the floor and provides access for any future need. I had a 3 series BMW that already had a floor hatch for the same reason. Before I did the Fiesta, the workshop manual said the fuel tank had to be removed to access the pump and sender.

Has anyone done this mod on a TR to access the diff mount area rather than rewelding the floor section back in place?

Keith

That's a good idea.

I would suggest that even though the floor is not a structural item you have a good radius on the corners of the cutout to prevent cracks developing 

George 

[RogerH]

Remember the Comet Windows - OMG

 

Roger

[harlequin]

I worked as a Structural designer in the offshore industry and following the Alexander Kielland disaster (https://en.wikipedia.org/wiki/Alexander_L._Kielland_(platform) the first thing we had to look at was crack prevention 

George 

[JohnC]

Steady on chaps! The Comet and Alexander Kielland disasters were crack propagation in stressed and critical components. Not that it wouldn’t be embarrassing, but crack propagation in the floor panel would at most be drafty and wet 🤪. But radiused corners are good engineering practice in any case.

JC

[RogerH]

27 minutes ago, JohnC said:

Steady on chaps! The Comet and Alexander Kielland disasters were crack propagation in stressed and critical components. Not that it wouldn’t be embarrassing, but crack propagation in the floor panel would at most be drafty and wet 🤪. But radiused corners are good engineering practice in any case.

JC

Fifteen or so years ago on the then Welsh Week-End the convoy of TR's swept around a decent sized country road bend.

As I swept around I was confronted by the back of the precediong TR. Brakes on sharpish.  Behind me came Keith Brown in his BRG TR4 with tyres squealing.  Thankfully we all stopped without changing the shape of anything.

The problem was a horsebox.  Where the horse had been p*ssing on the plywood floor it had become rotten.

At this bend, on the on-coming side of the road, the floor had collapsed and now (I had a vision) the horse was galloping along hindered by the floor remains.  I imagine it was quite serious as it took some time to get things sorted

 

Happy days

 

Roger

[JohnC]

Good point. Best not to put a horse in the back then. And to radius the corners