Care &Maintenance:
Page 55
If you've noticed an error in this article please click here to report it so we can fix it.
Once more with feeling
Take time to set tappet clearance precisely and save engine downtime
FEELER GAUGES and hacksaw blades are inexpensive enough, and they may seem to be pretty unimportant. But the feeler gauge is a precision instrument, and should be treated accordingly.
Equally, if hacksaw blades are misused, they will perform indifferently, and can end up as a costly item on the workshop tool and equipment bill.
A set of folding multi-feeler gauges is to be found in nearly every tool box. They are in daily use, usually as a rough guide to clearance rather than to set the clearance precisely, in terms of thousandths of an inch or tenths of a millimetre.
A lot of care may go into the rebuilding of an engine only for the tappet clearances to be set at an approximation of the required value -and before long, the vehicle is back in the workshop requiring extra attention.
The correct use of feeler gauges should be passed on to all trainees, who should be encouraged to develop a "feel". One excellent method of setting clearances, which also helps develop "feel", is the "go" and -no-gosystem.
If the clearances should be set to 0.038mm OD 01 5in, then a blade or combination of blades should be set to 0.036mM ?0.014in1 and 0.040mm 10.016iny
The smaller combination should slide in fairly easily, while the larger one should not go in at all -except under force. Then, the feeler of the correct thickness should be inserted, and there should be some resistance -just a little.
Obviously, before any feeler gauge is used, it should be wiped clean, to make sure that there is no grit that can score the blade or the rocker, or which can alter the feel of the gauge.
Then, if more than one blade is required for a certain clearance, as few blades as possible should be used. This time, cleanliness is absolutely essential.
Not only can the grit increase the overall thickness of the blades, but it can damage them as well.
Another important point to watch is that the blades are not bent at the ends.
If the blade is used on its own, this is not too serious, but if it is used in conjunction with another one. it will tend to spring away from that blade, making the gap seem larger than it is.
When it comes to checking the valve clearances, a little thought is needed. To start with, if new rockers are fitted to an engine, their faces should be washed off first, to remove any protective coating.
Quite often, if the coating is not removed, the engine is noisy when run up, and so the mechanic ends up setting the clearances to make the engine quiet. The end result is that the valves have virtually no clearance, and before long, a valve seat is burned out.
Then, another problem is that with time, the faces of the rockers wear slightly where they contact the valve stems, and it is passible for the feeler gauge to lie on the unworn surface, so that the gap appears to be much less than it is.
The answer here is to put the feeler straight in from the side of the engine, and to try to feel any variations in the surface. Whenever the rockers are removed from the engine, they should be refaced lightly to eliminate this problem.
This job should be done with the rocker held absolutely square, and if you are in doubt about getting the surface square, you may as well not bother, because you will end up with point contact between the valve and stem. which is no good at all.
Tappet clearances should never be set with the engine running. However grand this may look, it is not a reliable method, and knocks hell out of the blades. And if there is one thing that is important about a feeler gauge it is that if it says "15on the blade, it should be 0.015in thick, not 0.014in.
Unlike most tools in the workshop, a hacksaw blade only fails once, and it has to be thrown away, and for this reason these blades are the fastest moving items in the tool or consumable stores.
The main reason why blades fail, apart from the mechanic's desire to get a boring job finished quickly, is the use of the wrong blade for the job.
As a general rule, three hacksaw teeth should be in contact with the work at all times. But the materials and shape of components vary a lot, and so different blades are needed.
A blade with fine teeth will cut thin material or tubes, and the necessary three teeth can easily be kept in contact with the tube, but it may jam in a soft material such as aluminium_ Hand hacksaw blades have a range of 14, 18 and 24 teeth per inch as a general rule, and most engineers choose the good old compromise of 18tpi.
Although some failures arise from a blade being lent to someone who uses it quite differently, the main problem is that the one type of blade is asked to tackle materials such as steel, aluminium, copper. cast iron, and even timber and rubber.
To obtain a long life from a blade, the mechanic should keep it for one range of materials only To make the most of such a system, a number of hacksaw frames will need to be in use, and these should be colour-coded.
All this requires is a bit of paint on the frame and blade, and ideally, an indication of the application or tpi on the frame as well.
As a rough guide, aluminium should be cut with a blade of 14tpi as should copper For aluminium tubing, however, and solid brass, cast iron, and solid steel, the standard 18tpi blade is ideal.
For steel tubing, iron pipes and conduit, and brass tubing. 24tpi blades are recommended.
In addition to using the correct blade, you should make sure you start the cut carefully, particularly at sharp edges, to avoid stripping teeth.
The blade should be kept square to the work at all times, and excessive pressure should be avoided; then, do not attempt to start in an old cut — it is too easy to break the blade.
Nearly all these comments apply to the standard handhacksaw, but don't forget the miniature hacksaws.
These are invaluable when you need to work in confined spaces, such as near the plumbing on the chassis, or under a wing, or in a corner of a cab.
Of course, cutting tends to be a bit slower with these small saws, so patience is needed.