TR4/4A TR4a IRS differential mount issues

[tdskip]

Hi guys - do the TR4A IRS cars have a tendency to crack their differential mounts like the TR6s do?

Any issues with using poly differential mount bushings?

Thanks!

 

[HerronScott]

Tom,

Sorry, I can't speak from experience here as I replaced the rusted frame/inner body on my first TR4A with a solid donor frame/inner body (lucky find!) and I had not gotten to that point on the second one. I would expect to possibly see the same problems where as the rubber mounts age, you get play which can cause the mounts to crack and break loose. You should certainly check it out fror cracks as you replace the mounts (and report back what you find!).

Poly mounts/bushes were not as available/popular when I did mine but I would probably use them today.

Scott

 

[KVH]

The literature out there does warn of IRS mount problems in the 4A, particularly in rust prone areas and climates.

The points of concern are the upper cross bar that supports the coil springs, and the center mounts.

Kits are sold by Revington TR, Moss and others to strengthen those areas, and an alternative Kit is available if the body is actually first removed from the frame.

The advice given is to check for rust and weakness in the areas noted to avoid problems which can be serious if a spring were to come loose. Sounds scary, but I've been reading up since I just bought an IRS.

 

[tdskip]

I'm going to pull the diff this AM and will let you know what I find...

Existing bushings are cracked/dry;

Dirty and surface rust;

But looks pretty straight under there so far;

 

[HerronScott]

Tom,

Nice shiny transmission there.

I haven't really heard of rust being an issue with the cross-member, just issues with cracking and breaking of the differential mounts. Rust in the A-arm mount area is the big issue for the TR4A/TR6 frame (and my original one was rusted through in the section extending under the trunk as well but I think it spent some of it's life near the beach).

Scott

 

[Marvin Gruber]

For some reason (maybe they used a better grade steel) I have had only one TR4A that the frame had rust and that was back of the swing arm so I repaired it. TR250s & 6's, now thats a different story. 250's the diff mounts have been pretty good. 69-72 TR6's were real bad to have rusty frames. Fixed several of them. This 4 looks pretty good. I would drive it a while and see what the diff does before changing out rubber.

Marv

 

[tdskip]

Too late Marv!

 

[RJS]

TR4A mounting pins have been known to develop cracks and become loose. Especially the front right pin. This results in the infamous clunk in the rear of these cars. So, inspect carefully.

I had my diff out last year for rebuild. Fortunately, the mounting pins were OK. I just sanded them and repainted with some black chassis paint. If they are cracked, any of the big three make repair and reinforcement kits. Welding upside down is not fun, so I am told! Also, if it is a body on repair, then access to the top of the diff bridge will have to come by cutting through the parcel shelf in the interior compartment.

When it came time to replace the diff, I chose to go with stock rubber bushings since I had heard that for street use they are plenty good and the added vibration transmitted from urethane bushings may accelerate cracking of the mounting pins. I did however, make two modifications: 1) the hole drilled in the rubber bushings were much larger than the pin it slides over. So, I sleeved the pins with a length of poly vinyl tubing (1/2” ID x 5/8” OD) to remove all play. This allowed the bushing to fit perfectly over the pin with no play, 2) for the rear mount washer (the big rusty washer in your first photo), I fabricated custom differential mount stabilizer cups with stainless steel. Basically, I welded a 1/2" high wall around the edge of the washer. This acts to "cup" the rubber bushing and remove some of the lateral squirm.

Bob

 

[Marvin Gruber]

Skip

You are a glutton for punishment. Now that you are in there think about putting some reinforcement welds in.

Marv

 

[tdskip]

Now that you are in there think about putting some reinforcement welds in.

Yeah, I agree Marv. I have the reinforcements from TRF in hand already. Can't say I'm ugly thrilled about the cleaning part, the welding is OK, but I really dislike the cleaning to a point where it is ready for the MIG. Oh well, figure if I do it right it should outlast me anyway....

I'm going to drop the diff off for seals and bearings since now is the time to do it.

 

[tdskip]

When it came time to replace the diff, I chose to go with stock rubber bushings since I had heard that for street use they are plenty good and the added vibration transmitted from urethane bushings may accelerate cracking of the mounting pins.

Hi Bob - thanks for the note. I hear you on the rubber vs poly, but after having some rubber bushings start cracking and getting crumbly in less than 24 months while a TR6 sat in the garage I'm spooked on the quality of the rubber bushings these days.

That said, I have been toying with the idea of poly on the rear and rubber on the front. I know some TR6 guys do that. Anyone been there and done that?

 

[HerronScott]

Tom,

Did you find any cracks in the front right mount?

Scott