Most big jobs under the car start with taking an undertray off: the sump and gearbox, the engine mounts, the wishbones, the exhaust. The workshop procedure for each tray is a simple unbolt, and the hard part is the bolts, which corrode.
Which undertrays there are
The workshop procedures name three trays, and owners described them in 2004 and 2005. We think they match like this, but nobody has confirmed it:
- Engine undertray (the large main section under the engine, sump and gearbox). The workshop procedure lists 13 bolts and 13 washers. An owner in 2004 described it as bolted down both sides and across the rear edge (a dozen or so bolts), with 2 heavier bolts into the subframe.
- Exhaust undertray (the smaller rear section, over the silencer). The workshop procedure lists 12 bolts, 12 washers and 2 clips. An owner in 2004 said it is bolted to the rear clam and the rear edge of the main tray.
- Front undertray, behind the front bumper. The workshop procedure lists 12 bolts and 9 washers. It comes off with the car raised and goes back before the car is lowered.
Don't confuse these with the bonded floor of the chassis, which is also the floor of the cabin and can't be unbolted. An owner in 2004 said anything with rivets and red glue along the seams is part of the chassis tub, and that the passenger floor is a single skin. The glue you can see around the edges of the trays is from the chassis bonding, and isn't holding the tray on.
Front clam removal and Rear clam removal both start with an undertray.
Taking them off
- Raise the car and support it on stands.
- For the main tray, owners said to undo the side bolts first, then the rear edge, then the rear deflectors and the front bolts. We don't know the order TIS intends, since it only lists the bolts.
- Clean and spray each bolt first, and check the socket or key is fully seated in the head before you turn it. Use an extension bar if it is stiff, because a stripped head is the main problem on this job.
- Once every bolt is out, expect the tray to be tight. In 2007 an owner had all the bolts out on a front tray that wouldn't move in any direction, and thought it was sitting in a ridge, or stuck by a foam strip between the tray and the body. Another owner pointed out TIS says it is only an unbolt job. We don't know how it ended, so go gently and look for anything still holding it.
Seized and rounded bolts
- The bolts are steel going into aluminium, and an owner in 2004 said that is why they corrode and seize. Several owners rounded off an Allen head. One owner in 2004 said the main screws looked as if they had the same red glue on them as the other body panels, but another expected they had simply corroded in.
- Two bolts in the middle of the main tray are the big ones. An owner in 2008 had one seize and round off. He ground the sides flat to get mole grips on it and replaced both.
- For a rounded head, the easy fix owners gave is a pair of mole grips on the head. The other way is to drill it out, which one said is a horrible job lying on your back.
- Check the bolts are free before you need the tray off for a job, and replace them with stainless while it is off. Two owners recommended this, saying you won't regret it.
Replacement fixings
- What owners used (2014): M5 bolts (stated as 16 mm, with one owner saying 20 to 25 mm) that screw into C-clips on the tray, with an M5 penny washer. The holes are mostly bigger than a normal washer, so penny washers are needed. A nylon washer between the penny washer and the tray was suggested to stop the bolt corroding through it. The nuts can't be held behind the tray, which is why the clips are used.
- Avoid button head bolts. One owner said they seize with time and round off. Another had changed from hex to Allen head stainless bolts because it was quicker with a key, and took the tray off as often as he drove, so seizing wasn't a problem for him. Another owner pointed out that the Allen socket would round off long before the bolt snapped. The risk is greater if the tray stays on for years.
- Quarter-turn fasteners. One owner fitted these on the rear section and said they are fabulous if you want the tray off quickly. He used plastic washers to stop rattling, and had no rattles or loose fixings.
- Copper grease or copper slip on the threads was the plan of the 2014 owner who ordered a replacement set.
- A complete kit. Speedy Fasteners sells an A2 stainless overhaul kit of 369 bolts, nuts, washers and clips (£69.99 when we looked). It covers the front and rear clams, under trays, fuse and relay box, radiator and surround, tow eye bolts and wheel arches, with Spire clips and anti-rattle washers for the trays. The page doesn't list the individual sizes, so expect some spares.
A bent tray or shear panel
A 2014 owner of a Turbo with a damaged rear tray and missing fixings was told that the panel was probably the bent shear panel, which is structural though easily replaced, and might be worth having someone look at. Owners suggested the damage was either lifting with a jack in the wrong place or off-road use, and that it can be bent back if it isn't far out of shape. See Jacking and supporting the car.
Corroded clips and a broken fibreglass mount
One owner in 2020 pulled a corroded clip off the tray and took a chunk of fibreglass with it, which left nothing to fit the new clip to. The suggested repair, from another owner, was to get at the edge of the fibreglass from below and lay supported fibreglass and resin around a new captive bolt. To reach it you would have to take off the panel below the fuel tank (with the gear and brake cables across it), which is a much bigger job than the undertray, and the instructions he gave were from memory, so don't rely on them.
Can you drive without an undertray?
These are owners' experiences, not engineering advice, and they disagree:
- One said he wouldn't, because the tray is designed to be there and he didn't want to change the airflow under the car.
- Another drove to 50 mph without it, and was nervous that the air would scoop up into the rear clam.
- Others said they were fine in the short term. One drove 12,000 miles over a year without one, and one did a track day at up to 140 mph with it off on a hot day, with no problems.
The question that started the 2008 thread was also about road salt, which nobody answered directly. Owners put taking the tray off at 10 to 20 minutes when the bolts come out easily.