Topic

I have recently bought an MG TF 03 as a bit of a project and with the help of my neighbour (who knows what he is doing) we have done a lot of work on the front of the car - sub-frame mounts, wishbones, ball joints, TREs, discs and pads.

The final thing we needed to do was the rear sub-frame mounts. The first of the mounting bolts sheared off. Ludicrous design and size of bolt for the job. After much swearing and scratching of heads we packed up for the day.

I have read the excellent how to guide on what to do next with drilling out, re-tapping and a helicoil kit. I have a couple of questions:

1) The guide makes it sound really easy. My gut feeling says it isn't! If anybody has done this, please can you advise how difficult it was and how long it takes? What size drill bit did you use. I am thinking using left handed drill bits - initially a 4mm and stepping up to a 6mm and then 8mm if necessary. Optimistically hoping a left handed thread might pull the bolt out

2) How on earth do I get the other 3 bolts out without the same thing happening? We gave it a spray, a bit of tapping with a hammer and heated with a blowlamp. Any other advice?

3) Has anyone put the main sub-frame bolts in the opposite way round (i.e. nut inside the mount cup) so that you don't have to drop the sub-frame? This looks like it will work although pretty tight to get a socket on the nut to stop it from spinning when torquing up. I have replacement bolts so can angle grind the originals out. Anyone any experience of this?

Many thanks in advance...

I have no direct experience with subframe bolts snapping but have dealt with a number of other snapped bolts. the secret is good quality cobalt drill bits and take your time. Start with a really small bit, maybe 3mm, and increase by 1mm at a time.

A good soaking with Plusgas or similar for a couple of days prior will always help with the rest.

Hi and welcome to the forum

I have just been doing work on my subframes.

I don't know about the rear subframe but on the front subframes the bolts on the rear mounts only go one way.

James

Yes you can put your rear longitudinal bolts in the other way about, I cut the old ones so they could come out without dropping the sub frame and you just have enough room to get a socket in to tighten the nut up.

I had to drill and tap a couple of the sub frame mounting bolts out. They are quite hard, use cobalt drills. Once you have a pilot hole, they will drill out ok.
Tapping and helicoil is no big deal. You could also try to use stud extractors to get the threaded shank out after you have drilled the pilot hole, but chances are you will need to drill out to the helicoil tapping size and fit a coil.
Took me an afternoon to change my mounts over with helicoiling two holes.

Sundance

It's all due to the fact that in their wisdom MGR used self-cutting bolts in to existing captive nuts.... It SEEMS a good idea to ensure that threads are cleaned out by the cutters - but in fact all it does is allow water to seep down the cutting line of the bolt, and rust the bloody threads together! GRRRR Plus of course the bolts are made of butter and so fall apart at the slightest twist!

When I replaced my front subframe bolts and mounts I went with 'Lidl Rust Dissolver' - truly the most amazing penetrating liquid I have ever used, and kept popping a quick spray on each bolt (top and bottom and in cavity areas) every day for a week before I started the job. Then I did a tiny turn back, then forth and went like that all round... the cutters atcually helped as they cleaned the threads back out as i wert back and forth, and not a single bolt snapped (although one got pretty close I seem to remmeber!).

The rears in theory shouldn't be any worse, but the problem you have is the captive nuts live in box sections that are not remotely waterproof - so the water and crap gets in there and then seeps down the thread cut even more! I didn't even think about cutting the big cross bolts and then replacing them - what a great idea! I am gonna take the whole rear lot out at some point, so will definitely go that way I think. I want to redo the compliance bushes as I have some poly ones to go in (although my Ikea chopping board washers have passed MOTs and stopped the clanking for years so far!), and replace a load of the other bushes while I have it all out - maybe even swap subframes if I find a good cheap one!

The best way to avoid broken bolts is to remove them using an impact wrench. The reason is that the hammering effect of the impact wrench does a better job of loosening a 'frozen' bolt that the constant pull that you put on the bolt. The impact wrench more gently 'jars' the bolts loose. Also, I believe it is helpful to first 'tighten' the bolt with the impact wrench and then remove the bolt.

The first time I had to remove sub-frame bolts I used a socket and racket and one of the four bolts holding on a front mount broke. Since then I have used an impact wrench on 12 front sub-frame bolts and none have broken. When reassembling I do apply anti-seize to the bolts.

Thanks for all your advice and confirming that turning the sub-frame bolts round does work with enough space for a socket.

I took the car to a local garage yesterday to get their advice and see if they could get the bolt out. Unfortunately, they thought getting the bolt out and loosening the other 3 was going to be a big job and would have to charge what they considered too much.

So the mechanic offered to lend me his left handed drill bits and easy outs so that we could do it ourselves - what a great guy! So when I get back from holiday, we will drill out and re-tap/helicoil if necessary.

Will spray penetrating oil on (although not convinced this does much as it is a hidden captive nut and gravity isn't helping). Also going to get a better blow torch to give the remaining 3 bolts a blast before attempting.

Interestingly didn't have any issues with dropping the front subframe. I think we were lulled into a false sense of confidence!