Technical Torque

Questions

I have been restoring my 1953 TD for some years now and recently started my home-rebuilt engine for the first time. It started on the button and ran smoothly, but with a distinct "knock". A close ear to the block led me to the left side of the sump directly below the oil pump. With a flat hand on the side of the sump, the knock could be felt in time with crankshaft rotation and appeared to be concentrated at the oil pickup pipe connection. It sounds and feels as if a crankshaft lobe is hitting the pickup pipe inside the sump. Does anyone know if this is possible? If not, can anyone suggest any other likely cause and remedy?

The engine is installed in the car and I am therefore reluctant to remove the sump and all that entails, without feeling fairly confident that I know what the problem is. During the rebuild I dismantled the sump intemals for cleaning. Is it possible to re-assemble them incorrectly such that crankshaft/pickup pipe interference can occur? The engine is the original1250cc unit, with engine number XPAG 26699. The rebuild was extensive and included rebore, new pistons, big end and main bearings, camshaft and bearings, rocker shaft and bushes, oil pump gears and spindle.

Terry Fiddler, Farnham, Surrey

Has anyone produced a list of torque settings for the numerous bolts and nuts on the 'TF' series engine and transmission? I know that I could calculate them from data tables but a ready-made list would be nice.

Andrew Mayfield, Deeside, Flintshire

Whilst at the MG Show Day-Stoneleigh, I purchased one of these low level Luggage Racks as shown in the centre fold of Bulletin 359. Sadly, on fitting, I find that in my case I cannot remove the spare wheel as the space between the spare wheel carrier mounting point and the main cross shaft of the luggage rack is too small to allow the wheel to pass through.

Has anyone else with a disc wheel car had this problem and how does the Dutch owner of the TD shown in the pictures manage to extract his spare wheel? (see sketch below).

Basically, the inside of the wheel rim goes under the carrier mounting points with the wheel in the upright position needed to clear the main pivot point of the luggage rack, thereby trapping the wheel.

John Beavan, Bartestree,Hereford.

I have recently bought a luggage rack from Moss in Goleta, California for my TF but I haven't fitted it yet. It looks to me to be the same as the one in the picture in the April Bulletin. It was expensive. I think it needs some drilling, perhaps someone has some tips about rust prevention for the holes?

R.Keith Doherty, (keith.doherty@Virgin.net)

Why are ignition coils polarity sensitive? I can't detect any difference if it's incorrectly wired. (explanation to be no more difficult to follow than’O’level physics, please) (No 359).

The simple answer is that they aren't! The reason that they have polarity markings is to ensure that the HT cable to the spark plugs is negative polarity . Spark plugs are much more efficient with negative polarity at the insulated electrode and positive at the earthed electrode.

Sparks jump from negative to positive and will more easily leave a hot electrode than a cool one. The negative electrode of the plug is arranged to be smaller and thermally insulated and hence runs hotter than the positive electrode which conducts its heat to the water cooled cylinder head. The coil voltage needed to jump the gap would be much higher if the polarity were reversed, which might cause misfiring at higher engine revs if the coil was unable to deliver.

A secondary reason for the importance of coil polarity is because the spark carries with it a minute amount of electrode metal. When the coil is connected the correct way, it is the negative central electrode which erodes and not the earthed electrode. Erosion of the earthed electrode could result in it breaking off and damaging the cylinder.

Just in case some inquiring minds start to worry about how this works on modern cars with negative earth systems, I should point out that the voltage across the spark plug gap is in the range 5,000 to 20,000 volts so it makes little difference if the spark jumps to an electrode at 12 volts or one at 0 volts.

Cliff Knight, Newnham, Glos.

The following is the practical physics laboratory method!

If you feel brave and want to conduct an investigation, detach a plug lead and with the engine running, hold the 'lead' tip of a sharp pencil in the air gap between the plug lead and the plug top. You will see a stream of sparks I. flowing towards the top of the plug if all the connections are correct.

Remember that pencil lead (graphite) is an electrical conductor so keep hands clear of the pencil end.

(Andrew provided a complete answer with much the same information given by Cliff MT)

Keep that floor clean

Whilst in my local 'Homebase' DlY store I found a cheap and cheerful drip trayto put under my 'T' type. A plastic grow bag tray approx. 1m long, 450mm , wide and 50mm deep at £4.99. Fill with some 'kitty litter' to soak up the oil.

John Beavan, Bartestree,Hereford.

In response to the question about oil working its way up the speedo cable (No I 357) it was suggested (in No359) that a possible solution may be to reverse the ( cable end for end. Surely, by doing this you will only end up with exactly the same results as before after having gone to the trouble of removing and resweating on the small brass ferrule which holds the cable up at the instrument head, under the knurled finger nut.

To illustrate this, imagine a threaded rod, say 1/8" BSF right hand thread. Run a nut along its length then turn the rod end for end and repeat. The nut again turns the same way along the thread.

Nice idea, but I feel that R.A.Reffell (No359) offers the correct solution by using a cable wound in the opposite direction to the one for his YB.

To conclude, it is a requirement of the MOT for the speedo to be in full working order if fitted.

Tony Whitmill, Kettering, Northants.

I recently had a problem with a head stud pulling out of the block on our NA Allingham. I tried tapping out the hole to clean it up but when I put the head back the stud pulled out again.

It was obviously a case for an insert so I bought a Recoil kit at about £26. This comes with a tap to form the oversize thread, an inserting tool to wind in the insert and about 10 inserts. A drill is also required to drill out the hole to take the tap (this was 25/64" for the 3/8" BSF stud). This drilling and tapping must be absolutely vertical; this can be done by using a mini square on top of the block in two directions at right angles.

Once the hole is tapped out the insert can be screwed in, it has a tang which goes in first and the slot in the inserting tool locates with this and so winds in the insert from the bottom. If you try to insert from the top the coil simply tries to increase in diameter and jams in the hole. When the insert is in position the tool is reversed, snapping off the tang and leaving a clear thread to take the stud.

As the hole was deep I tried to insert two recoils but did not manage to stop the second one climbing over the lower one. However, the head stud pulled up tight with the single coil so I'm hoping that this will be satisfactory.

When you refit the dynamo onto its plinth on the engine nosepiece it is essential to ensure that there is enough backlash to prevent the gears being pushed too closely into mesh. This must be checked with someone pushing down on the clutch pedal; this is to ensure that the crank is pushed forward to the limit of its end float, otherwise the crank float will take up the backlash on the dynamo bevel gears. (There speaks a man who has had more MMM dynamos out than hot dinners!Ed)

Water jacket plates are made of thin material. They were designed in this way to accommodate the pressure caused by water in the block expanding during freezing. As the material is so thin the plates tend to be corroded and soon need replacing. The rubber gasket only goes round the perimeter of the plate and so the water can still attack the rest of the plates. I have made my own , rubber gaskets which are the full size of the plates and are solid so preventing the water from attacking the plates. However, the rubber is flexible and will cater for expansion during freezing.

We had been having a lot of trouble with the ND misfiring and even changing the coil, pump, contact breakers, etc., did not cure the problem. We found that on the reserve petrol pipe this misfiring didn't occur and so concluded that the problem must be with the main petrol feed pipe. I pumped air through it and even used a flexible cable to clean out the line but that only cured it for a short time.

After our trip to France for last year's 'Le Kimber' trial, petrol was leaking from a cracked soldered joint at the tank take-off, probably caused by catching the pipe on a rock or something. I took off the pipe and resoldered the joint having checked that the pipe was clear all through. As the petrol had been drained from the tank I thought it would be worthwhile to check the take off pipes in the tank itself. When the main feed was removed it was found to be very non- standard. Instead of a 2" long copper feed pipe enclosed in a closed off cylinder of fine wire mesh it looked like a nozzle from a welding torch. The open end of this had been almost closed off by a badly soldered filter mesh which probably restricted the flow by 50%. In addition, a piece of sealer or similar had got caught in the mesh and was effectively acting as a flap valve closing off the opening when the fuel was flowing, but opening when we had blown it back with an air line.

The solution didn't cost anything which made a nice change!

Philip Bayne-Powell, Normandy, Surrey.

That's all for this issue. I hope to have more interesting information for you next month including another of Neil's ' Back to Basics’articles.

Malcolm

“Messrs. G. T. Riches & Co. Ltd., London, have introduced what may be termed an air cooled spark plug.

The insulated central electrode is in the form of a tube with a ball valve at the lower end acting as a non- return valve (see Fig. 204).

Fig. 204-Ventilated Spark Plug

On the suction stroke. air is drawn down through the tube and valve, not only cooling the plug, but it blows away any soot which is so liable to accumulate round the sparking gap. This also prevents pre-ignition, as the plug cannot get hot enough owing to the aforesaid cool air passing through it. Again, this plug forms a type of automatic extra air inlet, and no matter how carefully the carburettor may be set, a decided improvement in running will be noticed immediately these plugs are used. Engine also responds immediately to every alteration of the throttle. Injections of paraffin or petrol can be inserted through the top of the plug, easing the pistons and greatly facilitating starting."