MGF & TF Subframe Corrosion

The Question
I have an MG TF 2005 which in my opinion is in good condition and well maintained. However, I was surprised when the last MOT said the following: "Offside Front Inner Subframe mounting prescribed area is corroded but not considered excessive and recommended to replace soon". There is a rust hole on the inside of the subframe in the back right corner as the picture shows. The rest of the subframe is without serious rust. Is it allowed to weld subframes? What rules apply to subframe repair? If I have to change the subframe, who can do this job? How much will it cost to replace the subframe? A lot of questions, but for me it is unclear what is possible to repair on the subframe.

A good example of MOT failure front subframe corrosion.

The Answer
The subframes on the MGF and TF are modified Rover Metro subframes, and related to the original Austin and MG Metro, so have been around for decades.  They’re substantial heavy steel structures for the ease of mounting all the major mechanical items of the cars that is then bolted up to the cars body.  They can be treated in exactly the same way as the chassis of an older MG such as the MGA so when needed can have welded repairs in exactly the same way a chassis or body welded repairs are done.

Front subframe after shot blasting and powder coating.

They may be substantial but they originally only had a thin coat of paint that doesn't take long for that to degrade and be lost, and then the mild steel starts to rust.  Being very thick steel it takes many years for the rusting to become a problem, but I have seen cars suffer more accelerated rusting that develops holes in the same area as your car has suffered from specific external influences.  This is most common where cars 'live' in seaside areas that are regularly bombarded with onshore weather carrying salt in the air/rain. 

Generally you see more problems with front subframes than rear for several reasons and not just the obvious one that the front faces more weather.  The biggest influence is that there is no engine heat to rapidly dry it out after driving in rain and radiator heat doesn’t offer much help.  Additionally there is also no helping hand anti-rust influence from engine or gearbox oil leaks as they are at the rear, although MGF and TF do not usually suffer significant oil leaks.

Addressing the current problem is no more complex than having the rusted area of the subframe cut out and a similar gauge steel patch welded in, a job that will present no problem for a good garage who can lift the car on a ramp to gain good access from underneath.  What may be a problem is if full access to the rusted area and if that is the case then the subframe may have to be removed to do the repair, and that significantly complicates the job and makes it much more expensive in labour.

A related point is that I would hope that the tester would have noted any degradation in the front subframes two front mounting brackets to the body, as this is a far more common corrosion problem, so do check or have these specifically checked to confirm their condition. 

The front mountings between the front subframe and the cars body on MGF Trophy and all TFs are more vulnerable to corrosion than earlier MGFs, very severe on this TF.

With these mountings there is an interesting contrast between the MGF and TF mountings as the mainstream MGF subframe mounting doesn’t suffer this corrosion problem as they have a rubber infill that is integral part of the mounting.  The MGF Trophy 160 SE introduced the solid subframe mounting layout for more precise subframe to body anchoring, helping to provide more precise suspension and steering operation.  The original mild steel mountings may rust but this can be cured by replacing them with stainless steel alternatives that are obviously very much more resistant to corrosion. 

Corroded TF front subframe mounting on left, new standard mild steel painted item on right.

KGE000110SS - MGF Trophy & MGTF Stainless Steel Front Subframe Mounts

Looking at the repaired subframe to achieve better medium to long term protection you need to consider additional protection after the welding repair is completed.  Here where the subframe remains on the car I would suggest that this starts with a wire brushing and cleaning of the accessible external surfaces of the subframe.  It can then be painted, as best as the available access gives, with a more resilient paint such as Hammerite and then spraying with an anti-rusting treatment, such as Waxoyl.

A001 - Waxoyl Members Pack | 5L Clear Waxoyl, High pressure sprayer with probe & grommets

Ideally this sort of anti-corrosion wax product should be sprayed into the internal box sections of the subframe, using the common small tube attachments found in the ‘applicator kit’.  This can be inserted through already present gaps and holes and the atomised spray is able to achieve excellent penetration and coverage, although use of compressed air does give the best results. 

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Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.

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