This is the last of our series of four articles on car maintenance.
AN OVERHAUL FOR YOUR CAR
The soundest piece of advice to give any amateur mechanic about his car is: “Leave it alone until it really needs attention, then do the job properly.”
We have left the final home-maintenance, which anyone with a reasonable aptitude for machines and tools can undertake, to include what are called top overhauls, the only type of overhaul which the home mechanic should attempt.
But before attempting it, be certain to check everything else which may be making your motor misbehave. A motor is sick when it fires unevenly, pinks excessively, overheats badly, makes really unpleasant noises, uses lots of petrol and oil, or won't start easily.
First thing is to go through the checks mentioned in earlier articles, and if everything on the outside is right, then (and only then) you can decide to decarbonize the cylinder-head and possibly grind the valves. You might as well do the latter while the head is off.
Preparation for a job like this is essential. Make sure you have enough tools, and the right tools, to take off all the accessories and the head itself. Clean the motor down so that nuts and screws do not get lost in grease. Allow yourself a lot of time. Find little boxes in which to put parts, nuts and bolts removed. Keep plenty of clean rags handy. And remember the order in which you took things off.
Here are the things you will need in the case of an overhead-valve motor:
Ring spanner of the right size for removal of the cylinder-head.
At least two shifting-spanners (small) and an assortment of screwdrivers and other suitable ring-spanners.
Valve lifter (for compressing valve springs), suction grinder and grinding-in paste.
Old blunt knife and blunt screwdriver for scaping off carbon deposits on head and crowns of pistons.
New cylinder-head gasket and valve-cover gasket.
In the case of a side-valve motor you will also
need a set of manifold gaskets and a sideplate gasket.
Then go ahead with the job according to your instruction-book; unless you are experienced don't try to do it without reference to an instruction-book; which will give you the correct methods for your particular model. Don't forget to drain the water off first.
Now here are some tips which will be handy. When decarbonising the piston crowns on a side-valve motor, stuff rags down the cylinders which are open, to prevent carbon and dirt dropping down and damaging the walls.
Do not scrape piston crowns or cylinder-head heavily, as this may cause scoring and later heavy accumulation of carbon.
If in doubt as to whether the valves need grinding, get your garage to give the motor a compression-test. It will show which cylinders are weak, and when the head is removed examination will show whether valves or a sloppy piston are responsible.
If with the piston at top dead centre you can rock it back and forth more than a fraction of an inch—say 1/32nd—it means that new rings or even a rebore are needed. Don't try to put in the rings yourself. Give the head an overhaul, and next time the motor deteriorates badly either have a full overhaul carried out or get rid of the car.
Reverting to the actual top overhaul, when it comes to replacing the head make sure there are no obstructions before putting on the new gasket (which can be sealed with a smear of yellow grease, but should not need gasket cement) and then plug the sparking-plug holes with rag before screwing the holding-down nuts on by hand. Nothing is more infuriating than to lose a nut down a plug-hole, which can occur on some makes.
Tighten the head down gradually, by working on the nuts diagonally and lengthwise, moving from the ends of the block towards the middle. If you can borrow a tension-wrench from a mechanic, so much the better. Twenty to thirty pounds tension should be sufficient to seal the gasket in the first place.
When everything is back in place, fill the radiator with water, give the motor a few turns on the handle to make sure there is nothing loose rattling around inside, enough turns on the starter to get petrol through, switch on, and she should go.
The motor should be run for about 20 minutes, allowed to cool down, and the cylinder-head nuts checked again for tension and the gasket examined for leaks.
Over the next few days it is as well to check the head once more, and have a look at the oil in the sump to make sure that water is not escaping down into the cylinders. A few pounds more tension will usually be the cure, but do not wind away indefinitely at a cylinder-head nut or it will quietly come away in your hand.
If after reading all this you would rather take your car to a mechanic, you are wise. But most of us like to have a go ourselves, and if the car is a bit of an old heap there's no harm in learning the hard way—and a lot of fun and quiet pride when the job's done. That's the lot of this series of hints on maintenance.