Welding precautions

When garages weld on new sills do they remove the petrol tank or is this considered unnecessary? Colin

Not sure, but full tanks are safer than empty.

regards Martin

Not usually, suppose it depends on what vehicle it is and where the tank is located etc.

Bit of a thing of the past now welding sills on, but did loads in days gone by, never took tank out and lived to tell the tale.

Doesn't it depend on the car?

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The tank my need shielding if it's plastic and you weld very close to it. Taking the battery leads and the alternator lead off is much more useful insurance on modern stuff.

Plus 1 for battery and alternator leads if you are electric welding. Of more concern with sills is the interior trim/carpets.

My Daughter had her old Metro welded by a garage to pass an MOT and it set the trim on fire.

Many years ago ,a friend owned a garage,I got one of his men to do some "cash" welding on the front underside of my wifes morris traveller,I sat in the cars passenger seat with a bucket of water,then went up with the car ramp,with the welder under the car ,when things got a bit hot and smoky i sprinkled water on the hot bodywork, I always remember one thing I saw on that saturday afternoon,high up on the ramp i could see there was a mark 3 Cortina right over the back of the garage, a real cut and shut ,front an back were two different colours some doors and other panels were in further different colours.I had made up the sheet metal panel I wanted welded in place from good clean sheet metal formed up in a metal bender ,the welders comment " we dont see good material nicely bent like that,we usually cut up an oil drum and beat it flat to get some steel.

If fitting skin sills over the top of the originals (NOT good practice!) beware of foam filled sills and petrol pipes running through or alongside the sill.

A trigger spray set to fine mist works best.

I used to have a Maxi which had a slight petrol leak. Wasn't much, but when I went for a new exhaust and the people approached with a lit welding torch, I slowly drifted outside, just in case you understand. Fortunately, nothing did happen, but it was something of a worry.

Peter G. Shaw

Those who remember the old BASEEFA standards for equipment mounted in hazardous atmospheres (EXd ) always wonder how car petrol tank level sensors were just a resistance wire and slider , in the tank.

Probably still are.

Yet never a mention of an explosion in normal use.

Incredible.

Modern cars are even 'worse', because the electric fuel pump is installed inside the tank, and is often connected with uninsulated spade terminals. So that's many more million cars on the road with fitted even more dangerous parts that don't cause problems. Perhaps you worry too much?

I have a Fairway Taxi (the old-school London cab). Some previous owner, possibly the first, had it waxoyled in a totally futile attempt to prevent it rusting to bits.

I keep a small trigger spray full of water to hand during its yearly appointment with the oxy-acetylene torch, to snuff the inevitable waxoyl fires emanating from whatever box section is currently subject of attention (you don't need to be able to see the fire - just misting into the box section through some convenient opening towards the top does the job, a bit like putting out a chimney fire by throwing a cup or two of water on the hot grate).

Re. petrol tanks: in theory the mixture inside the tank is too rich to ignite. Furthermore, petrol is non-conductive (a dielectric I presume?), so I suppose there's no need to insulate the terminals of pumps, etc. - though the submerged pumps that Jaguar used on early Mark 10s were quite carefully earth bonded to the tank if I remember rightly.

No problem with electrics in fuel tanks! You have all been watching too many Hollywood films where a car has a crash and explodes like an Exxon fuel tanker. Petrol is not that easy to light, as demonstrated to me on my first days induction working in a car factory. The instructor flipped a lighted match into a bucket of petrol and it just fizzled out. Apparently the most dangerous place for fire outbreak in a car factory was the trim shop with all the glue vapours, second most was the glycol anti freeze store, then the grease store and finally the petrol.

On the engine test beds we often had neat petrol dripping onto red hot exhaust manifolds! The biggest fire I saw was when a V8 engine broke a conrod and the sump oil exited the engine over the exhaust pipes, that was a fire!

To ignite petrol needs an air to fuel ratio of between 10:1 to 20:1 outside of this range it is pretty safe, that is why the electrics in a fuel tank (remember the fuel pump motors are brush type and liable to spark) are considered intrinsically safe.

I too have only taken one fuel tank out - a 1972 Corvette with a history of running fine until it didn't.
Car would stop randomly after half-mile or fifty miles and take between half-hour and twenty-four hours before it would re-start.
Friends wife alerted me to the problem and so bought the car as a steal because of this.
Long story short - after blowing through all three lines from tank to engine and replacing mechanical fuel pump, took tank down wearing my diving gear to counter seven years of Saudi sand.
Drained /emptied tank and when shaken it rattled ! only two things were shaken out - the gauze pick-up filter and a plastic bottle-top insert seal
Evidently the plastic seal was flotation neutral in petrol and occasionally would come up against the unprotected pick-up pipe and adhere with the suction and only when the suction eventually decayed and the seal dropped off could the car be restarted.
I gave myself a notional pat on the back when that proved to have resolved the problem and decided I never wished to repeat the exercise, especially as it was performed on granite chippings in the street.
DaveD

Hi, there was a chap who run his own garage who was on the same advanced welding course that I was on and he had to drain and remove the petrol tank on a car, but only to get access to a part that needed welding which had to be done with oxy-acetylene, but he did have a proper pit to work in. After he removed the tank and put it a safe distance away, he proceeded to do the work, but no sooner he lit his blow torch he was engulfed in a fireball and suffered some vary serious burns and put him off work for a few weeks for recovery. Now you may think he had missed a pipe or something that had petrol in it, but it wasn't the case, in fact is was the petrol vapour lingering in the pit that flashed over. I think during the time of draining and removing the tank, he'd become used to the smell and could not smell it when he went back down into the pit, something he didn't forget about when he went back to work.

I've never taken a tank off when welding sills, but always did disconnect the battery and make sure trim etc. inside the car, couldn't catch fire. Haven't done such jobs for years and probably never will again.

Regards Nick.

Edited By Nicholas Farr on 16/07/2020 21:36:12

... To ignite petrol needs an air to fuel ratio of between 10:1 to 20:1 outside of this range it is pretty safe, ...

Too true! The fuel connector into the carb came adrift and spraying petrol over the engine, while at over 100mph, back in 1977. The only time I ever turned off the ignition key at that speed. Fuel was still boiling on the engine after stopping and baling out. The first thing I knew of a problem was when the car filled with petrol fumes. Lucky escape, which I put down to one hell of a draught and a cross-flow cylinder head keeping the fuel away from the sparks. A good job we were not smokers, either!