» Setting the Tappets – A Comparison Between a Push Rod and Twin Cam MGA

A Saturday arvo job? – How many of us have thought we would check our noisy tappets one afternoon? A two stubbie job we have been putting off for ages – no worries. If you have a push-rod engine you would be right. All you need is a socket and tear-arse (factory slang for brace), for the rocker box bolts, ring spanner and screwdriver to do the adjustments, and a feeler gauge; maybe a new rocker box gasket if necessary. Once the job was finished you would probably have a general poke around making sure nothing had loosened; another beer maybe?

What if you had a twin cam? The first difference is you wouldn’t do it on the spur of the moment. You would firstly need to ensure you weren’t going to need the car for at least a week, secondly, have supplies of band-aid available, thirdly, buy at least one slab of beer, and finally check all the tools and engine consumables were in stock. The intimidating list of mandatory tools, bits and pieces comprises:

  • Jack and axle stands.
  • Bucket to drain radiator.
  • Various sockets, screwdrivers, pliers, ring and open ended spanners.
  • Feeler gauge.
  • Camshaft locking tool.
  • Micrometer to measure tappet adjustment shims.
  • At least twenty spare shims of various sizes.
  • Valve grinding suction tool to remove tappets.
  • Surface plate and at least 180 abrasive paper to finalise shim sizes if necessary.
  • Dial gauge and magnetic stand.
  • Home made camshaft gear adjusting tool
  • Gasket paper and cork to make new gaskets, and sealant.
  • Tie wire.
  • Ignition timing light.
  • Torque wrench.
  • Workshop manual.
  • Lots of patience.
  • An understanding wife.

Ready for Timing - in SituThe twin cam engine valve clearances, and valve and ignition timing have to be set up very accurately if it is to run smoothly. Because the valve clearances affect valve timing these must be set to within a .001in (one thou) tolerance between all valves. Adjustments are made by inserting shims of different thicknesses between the valve stem ends and inverted bucket tappets. This involves removing the camshafts and timing chain, careful measurement of existing and replacement shims, data recording, calculations and replacement of the components in the correct position. Woe betide you if you mix things up!

Theoretically the adjustments should all be right first time, but they never are. I have found I generally need to go through this process at least three to four times before I am satisfied, trying to achieve my aim for all clearances to be within .0005in (half a thou). Occasionally I need to resize a shim using the abrasive paper, removing no more than .0005in (half a thou). If things go well I can complete the tappets in a day but if not then I leave it for twenty four hours. Have you also noticed that troublesome jobs become easier after a twenty four hour break?

Once the valve clearances have been completed the fun really starts. When the clearances were changed and the camshafts removed the valve timing was lost. This has to be reset just as accurately as the tappets, and is extremely fiddly. Access to do the adjustment is limited when the engine is in situ, as can be seen from the two photos. One shows the engine in the car, the other on the floor with the timing cover removed; with the latter it is comparatively easy.

Inlet Valve Timing - Engine OutThe valve timing is set by measuring inlet and exhaust valve lift on No 1 cylinder when No 4 is at TDC, using a dial gauge. Any required changes are made by rotating the camshaft gears on a vernier adjustment. Once again I have found that this has to be done a number of times; checking each adjustment requires that everything be reassembled and the engine given a few rotations to settle everything (hurray for the starting handle). Although there is a .011in (eleven thou) tolerance I attempt to get them both the same, which is not easy, but it is worth trying to reduce it to the minimum as it pays off with a smoother running engine.

When all this has finally been completed and reassembled the last job is to check the ignition timing with a strobe. As the original timing marks are partially obscured I have made my own about 90 degrees early.

The final check of the hours spent, cuts, and cursing is a test run, hoping the time taken was worth it. You will know before you are in third gear if all is well, if not then start again.

Two important things to watch out for are: 1. Dropping something into the timing cover. If you do, that means removing the cover and with the engine installed that is another lengthy job. 2. Accidentally transposing the camshafts, which is easily done. Although they both have the same cast in part number they are in fact different, the difference being the position of clamping slots in the flanges. This is not mentioned in the manual. If you have made this mistake you cannot time the valves conventionally. If you have already set the tappets when you discover your error, and don’t want to change them round and spend another day re-setting the valve clearances there is a way of timing the engine by working backwards. I know about these two problems because I have made them both in the past!

Inlet Valve Timing - Engine in Situ

None of the above is particularly difficult but it cannot be done in a hurry, requires the proper tools and much care and concentration. Poor accessibility does not help and it pays to remove a number of other components first including the top hose.

I suspect that some of the original poor reputation the car acquired was because of the above, and possibly untrained mechanics.

Robin Godfrey

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