Replacing a rear crossmember in a Triumph TR-6 (long with pics)

My fathers car developed a clunking problem when moving from a stop. I think I found out what was causing it.
Here is the condition of the drivers side rear crossmember:



and passenger side:



I also noticed how much the shock brackets were moving by the witness mark on the frame.



Here are two additional pics, these cracks are on the foward side of the CM.

Drivers side:


Pass side:


I can grab a hold of the shock mounting bracket and move it back and forth more than 1/2" and watch as the CM flexes at the crack. I'm glad I caught this before driving it further.
As it turns out this is a known problem on 6’s that have switched from the lever shocks to tube shocks.

The old CM is out. After I dropped the diff it was very apparent just how bad the CM was. There was only about 3" of metal still connected to the frame on the right side, less on the left side.



I have cut some metal to see if I can make a replacement.


I got the basic shape of the new cross member welded together.



While this does add weight, it's much beefier than the original, so it should hold up better.
Next, cut the pins on the lathe:

Turning down the 5/8" 304 Stianless rod:



bushing and threaded section at correct diameters:



Set up for threading:




Next parting it off (neat shot, the chip is hanging in mid air :


and finally the finished pin:


I am leaving the part that passes through the CM at 5/8" do I have a beefier section to weld. In order to not create a stress riser there is a radius between the 5/8" and .555" section

Before welding the lower portion of the cross member I used the exposed edge (circled in red) to locate the center for the purpose of milling the holes for the pins.

Here is the CM in the mill with a center finder getting everything set up.


Center drilling the exact center of the CM. I’ll use this as a reference point in case I need to take the part out of the mill, or flip it over.

Center drilling the pin holes.

After center drilling I used a 3/16” drill to start each hole. Next, using a 5/8” end mill I drill the hole for the pin.

Next, I set up the mount reinforcement plates and drilled a 5/8” hole with the same end mill. The pins and reinforcements are 304 stainless.


With all the parts cut out, next step is to weld it all together. Here I am welding the reinforcement plate. The plates were tacked first with the pins in place to ensure alignment.


Here’s a pic of one of the welds on the reinforcement


After the reinforcements were in place I welded in a section to cover the exposed ends mentioned above. Here is a shot of the welds getting smoothed out with a grinder. I highly recommend the Bosch grinders, this is the nicest I have used.


And a shot of the work completed so far. Note the pins are not welded in place yet.






I cut rough cut the legs with a cut off wheel on the angle grinder


And finished them to the right length on the mill (no good photos)

Here’s a shot with the old and new

I got the driver side frame rail cleaned and prepped for the reinforcement plate


Next the plate in place ready for tac welds



Plate welded in place



When I put the cross member on the diff to make sure everything was lined up properly my brother pointed out that the stock CM has a relief cut out for the fins on the diff cover. I thought I was finished with it, but...

It doesn’t appear that mine will have an interference problem, but I decided to notch a section out. I drilled out a piece of stainless on the lathe (it was pretty hot, that's WD40 smoking off the drill and piece.)


And notched out the CM, welded the SS tube in an cut off the excess



I also added some gussets to the rear of the shock mount.


Please excuse the poor looking welds in the above pic.

Hopefully I can get it welded into the car next weekend.

Ryan

well, I am impressed
that came our really well

bob

Wish I could do that kind of work, looks great. 930

Great job ! Dad be proud !!

930dreamer said:

Wish I could do that kind of work, looks great. 930

Click to expand...

+1, thats good looking fab work

Excellent fab work I'm impressed!!

Wow, you have some serious skills! Great work...I too wish I could do that.

I bet having all the right tools for the job helps a lot, nice job.

Very impressive keep up the good work!! It helps to have a lathe,Mill and a welder.

Nice job man, the welds look pretty good. Ime sure its nice having all the right tools.

Frikken WOW!!!

What tig welder are you using? And how well do you like it. I'm looking to purchase one and need some feed back, Thanks.

Thanks for the compliments guys. The welder i am using is a Hobart Tigmate. It has performed flawlessly through all of my projects accept one. It is not good with very thin materials, the current does not go low enough and I am not good enough. When I say thin think of one of those spun aluminum or stainless water bottles that are now all the rage.


Ryan

Great work, Can't wait to see it back in the car.

Very nice work! Your father is very lucky ot have a son who can do what you can do. I've always loved TR-6's a friend of mine in high school father had one he totally rebuilt / hot rodded, he shoe horned a Jaguar V12 326cid and trans into one, what a sleeper. You knew by the sound that it was something special, there wasn't a Vette that could catch it.

TheGrooveking

Sweet looking job, keep up the good work!
________
Cat wheels

A friend had a GT-6.

Beautiful work. My uncle's TR6 has a clunk in it too, although it's been converted to tube shocks, the damage may have already been done. Your repair would be a heck of alot easier then replacing the frame. There is a company that advertises in Classic Motorsports, their frames look nice.

You're dead on that the upper tube shock mounts move at a slight arc, causing the cross member damage you found. The problem is that the TR6 trailing arm and the aftermarket tube shock conversion you have don't move uniformly together - as the trailing arm rises, the lower rear shock mount moves in an arc upwards and towards the front of the car, which inclines the shock rearwards, forcing the upper mount to move backards also as the shock compresses.

On installing that type of aftermarket tube shock conversion a small bracket should be welded to the top of the tube shock bracket that allows the bracket to be held stationary through another bracket fixed to the inner wheel arch. This keeps the shock bracket from moving, and no cracking / bending of the lower cross member occurs.

TR6 rear clunks also originate from broken passenger side differential mounting pins, heavily worn ujoints on the rear axles, excessive sliding spline wear on the rear axles, and worn out diffs.

The better tube shock conversion is from Revington in the UK. It is a proper shock in coil arrangement, where all of the components move in a more complimentry manner. There is a knock-off of the Revington kit made here in the USA - I hate knock off garbage.