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MG MGB Technical - running in

hi everyone
just fitted rebuilt engine,how long is the running in period recommended and the revs not to exceed.
thanks in advance
bob 1970 gt
Bob Taylor

I don't think its a case of not exceeding certain revs, its more not letting the engine work too hard. That means intelligent use of the gearbox to prevent both high revving and low rev slogging. Have you done the initial start up yet? Modern thinking is that at the first start you should run the engine for 20 minutes at variable revs between 1500 and 2500 rpm, and not let it idle or turn it off too soon. This is to bed in the camshaft and followers.

I would use the car fairly normally for 500 miles or so, then change the oil and filter and then build up to full performance over the next few hundred miles.
Mike Howlett

This is in the archives, when I last rebuilt mine we went to Cornwall and after a few hundred miles babying ther car as above and varying the speed and gradually increasing the revs I eventually gave it a glaze busting run up to 4500 rpm in o/d top on a dual carriageway down there and have not thought about it since. The car still has the oil pressure it showed when started after the rebuild and it uses hardly any oil. The everyday car is an ex rental Fiat I got for change at the auctions 8 years ago. It uses no oil between changes as the rings were forced against the bores when it was new.
Stan Best

Bob-
Prior to starting a newly-rebuilt engine it is essential to prime the oil pump and the oil circulating passages. Although some commercial garages do not follow this procedure as it is time-consuming, failure to do this will risk the result of all of your handiwork being destroyed due to a lack of oil flow and oil pressure. Install a magnetic oil sump plug (Moss Motors Part # 328-282), and then you can fill the sump with the most inexpensive non-detergent 20W/50 oil that you can find, although Castrol HD 30 is an excellent choice for running-in a newly rebuilt engine. Tilt the engine by jacking up the front of the car, remove the threaded plug from the forward end of the rocker shaft, and then pour oil into the rocker shaft in order to lubricate the bushings of rocker arms and allow time for the oil to run down through the oil passage in both the rear rocker shaft pedestal and cylinder head casting to the bushing at the rear end of the camshaft (Now you know why the engineers decided to have a threaded plug instead of a press-fitted plug as at the other end of the rocker shaft). Replace the threaded plug into the forward end of the rocker shaft, then level the car. Next, oil the valve stems, and then pour a tablespoon of oil down the pushrod passages in the cylinder head in order to lubricate the tappets and another tablespoon of oil into each spark plug hole in order to lubricate both the pistons and the rings. Pour oil down the standpipe of the oil filter stand in order to fill the high-pressure oil gallery and supply oil to the main bearings, and then install the oil filter. Finally, if your engine is not equipped with an oil cooler, disconnect the large external oil feed line that goes to the back corner of the engine block at the oil filter stand and pour oil into it in order to supply oil to the oil pump. If the engine is equipped with an oil cooler, before installing the oil filter, disconnect the large external oil feed line that goes to the back corner of the engine block from the oil cooler and, holding it above the height of the cylinder head, pour oil into it in order to supply oil to the oil pump, then reattach it to the oil cooler. Be sure to complete the filling of the oil cooler by pouring oil into the aperture for the oil cooler return line in the oil filter stand. Next, pour oil down the standpipe of the oil filter stand in order to supply oil to the high pressure gallery for the main bearings of the crankshaft. Next, install the oil filter, and then rotate the crankshaft clockwise 360 degrees in order to draw the oil into the oil pump and also in order to wipe the oil onto the walls of the cylinders. Once the pump is primed, disconnect the power supply to the fuel pump, and then use the electric starter to turn the engine until your oil pressure gauge gives a steady reading. The oil cannot drain out of the pump once it is primed because the rotor of the oil pump draws oil from the sump up into the top of the pump, and then pumps it out of the top on the other side of the rotor.

For all new or rebuilt engines with domed tappets, proper assembly lubricants and a ZDDP oil additive (Moss Motors Part# 220-815) should always be used during the break-in phase. This additive provides extra protection at the point of contact, helping the domed face of the tappet to properly mate with the lobe of the camshaft. Once the break-in phase is over, the use of this additive should be discontinued. High-performance modified engines benefit from oils with superior film strength and anti-wear properties. The interface of the domed tappet and the lobe of the camshaft is the one area of an engine that has an extreme contact load. That load increases significantly when high-pressure valve springs or higher-lift camshaft lobe profiles are employed. The use of properly formulated engine oils for this application will help reduce wear and extend the service life of the domed tappets and the camshaft lobes. There are many more ways to achieve good anti-wear performance than simply adding zinc and phosphorus compounds alone. Zinc and phosphorus are widely used because they are the most cost effective solutions to achieve anti-wear properties.

At this point it is critical that, in order to avoid ruining the camshaft and its tappets, they be properly bedded in. If you have chosen to use a hot camshaft, you can minimize the risk of high spring pressures ruining the tappet / lobe interface during their critical break-in period by using soft valve springs during the bedding-in process. Afterwards, using a fitting with a quick-disconnect screwed into the spark plug holes, attach a hose (flexible pipe) from an air compressor tank to it. With the piston at Top Dead Center, the air pressure will hold the valve closed while the soft springs are replaced with their appropriate service-use items. If you use this method to hold the valves in place while replacing the valves springs, afterwards the pistons and rings should be protected by pouring a tablespoon of motor oil down the spark plug hole and manually cycling the engine 360 degrees in order to replace the oil that the compressed air displaced. the pistons and rings should be protected by pouring a tablespoon of motor oil down the spark plug hole and manually cycling the engine 360 degrees in order to replace the oil that the compressed air displaced.

Upon first turning over the engine, observe to see if all of the pushrods are rotating. If one or more of the pushrods are not rotating, then the tappets in which they are seated are not spinning in their bores and must be freed or both the tappet and the lobes of the camshaft will be quickly ruined. This may be corrected by switching tappets into alternate bores. Now you may install the rocker arm cover and its gasket, reconnect the electrical power to the fuel pump, and then start the engine.

Hold the idle of the engine at 2,500 RPM for twenty minutes, occasionally varying engine speed gently between 2,000 RPM and 2,700 RPM. It is important for new springs to take a heat-set. Never abuse or run the engine at high engine speeds when the springs are new. Upon initial start-up, limit the engine speed until the engine temperature has reached operating levels. Shut off the engine and allow the springs to cool to room temperature. This usually will eliminate early breakage and prolong spring life. After the valve springs have been broken-in, it is common for them to lose a slight amount of pressure. Once this initial pressure loss occurs, the spring pressure should remain constant unless the engine is abused and the springs become overstressed. Should this occur, the springs must either be replaced or shimmed in order to attain the correct pressure.

After this initial process is completed, change the oil and the engine will be ready to be broken in on the road. Drive for 100 miles, change both the oil and the oil filter, and then retorque the cylinder head stud compression nuts. When retorquing, use your torque wrench to loosen the cylinder head compression nuts, noting how much torque is required to break the stiction on each. It is only necessary to loosen each of them to the point that the torque reading is less than the required amount of torque for securing the cylinder head. Once this is accomplished, be sure to inspect the valve lash clearances and adjust them as required. Torque them again at 500 miles in order to complete the bedding-in of the new camshaft and lifters, let it cool, and then retorque the cylinder head stud compression nuts using the proper sequence pattern. You will find some cylinder head stud compression nuts almost tight; some can take almost a quarter turn. Because retorquing the cylinder head will reduce the valve clearances, after every retorquing of the cylinder head you will need to reset the valve clearances. Run the car for another 100 miles again. You will find that this time the cylinder head stud compression nuts have not lost quite as much torque. Run an additional 500 miles and again retorque the cylinder head stud compression nuts. During this period do not exceed 4,000 RPM or 45 MPH, operate the engine at full throttle, or allow the engine to labor in any gear. Until the next 1,000 miles total has been completed, limit engine speeds to around 4,500 RPM when shifting gears. Cruising on the highway should be limited to no more than 3,500 RPM. Keep varying the throttle opening and engine speed. The secret is to constantly vary the speed and load without creating excess heat through full throttle laboring and high engine speed operation. After 1,000 miles of following this procedure, change the oil and oil filter and refill the sump with a quality oil such as Castrol 20W/50. After another 1,000 miles the engine will be properly broken in and ready for service.
Steve S.

This thread was discussed between 07/03/2009 and 08/03/2009

MG MGB Technical index

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