QuestionQUESTION: (My brother wrote to you yesterday regarding crankshaft bearings and we are really impressed with the reply).
I own a 1975 British 1275GT Mini and is in the process of rebuilding the engine. I believe the camshaft has been replaced by the previous owner with a fast-road version, and the flywheel has been replaced with a lighter one from a later model. Is it possible that flywheels are marked differently for different production years, as I could not get the TDC marking spot-on with the original pointer on my flywheel cover? I used a dial gauge to determine the TDC.
I want to install a duplex timing chain with an adjustable camshaft gear. These parts will come in a kit with all the necessary parts for the conversion, including the countersunk screws for the front plate. I want to determine the maximum full lift of the camshaft in order time the camshaft correctly. Can you please assist me on how to do this using the dial gauge?
ANSWER: Dialling in a camshaft
To do the job properly you need a dial gauge and an engine protractor.
With the head off and the timing chain cover on, set the gauge on top of the piston, attach the protractor to the crankshaft pulley, and confirm the true TDC position on the crankshaft pulley by turning the crankshaft clockwise and noting the minimum and maximum protractor readings where #1 piston is at the top of its stroke. There is usually about 10 degrees of maximum lift at the top, and you are trying to find the middle of the maximum lift. Keep turning the engine over clockwise, so that you take up any slack in the timing chain, and stop at the middle of maximum lift. Look at the TDC markings. You may be surprised how far true TDC differs from the markings on the pulley. Mark the crankshaft pulley with the true TDC position. I use a V-shaped file for this operation.
Having found the true TDC, rotate the protractor on the pulley, setting it to zero. Now fit the head to the block. I sometimes set up a piece of bent wire to mark TDC with the timing chain cover removed, so that I don’t have to keep removing and re-fitting the cover.
With your camshaft there should be a data sheet telling you when full lift occurs. My old data sheet for the Piper 285 says 106 degrees after TDC, but your camshaft may be different.
Install the dial gauge so that it touches the top of the inlet valve for #1 cylinder. (That will be the second valve from the front).
Turn the engine clockwise, and this time note the minimum and maximum angles on the protractor where the valve is at maximum lift. Calculate the middle point, which should be 106 degrees (or whatever your data sheet says). If it isn’t set correctly, you will have to adjust the vernier wheel until you get the correct reading.
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QUESTION: TQ for the reply.
(In the case of the 1275GT Mini, the TDC and other degree markings are punched on the flywheel and the pointer is casted as an arrow head on the inspection hole in the clutch housing, as the engine is a transversed East-West arrangement).
So, am I to understand that the accuracy of the markings on the flywheel is actually doubtful and we must actually determine the true TDC position for setting the ignition timing accurately.
Incidentally, the Megadyne Kent Cams also says full lift occurs at 106 degrees after TDC.
I am a little bit confused on the 'the minimum and maximum angles on the protractor where the valve is at maximum lift'. Can you please digress on this a little bit more. Many thanks.
AnswerMy reply is based on my knowledge of the MG Midget, which has a conventional front cover. The Mini is slightly different, although they both have basically the same 1275cc A-series engine. I am sure you can work out how to modify my instructions to suit a Mini engine.
The markings on the flywheel can never be relied upon. You must check them, and you may need to make different markings. If the bellhousing is in place, you can mark the flywheel with a centre punch and then increase the size of the mark with a small drill and fill the hole with white paint.
Checking the TDC mark is important, because otherwise you will have a bad reference for the timing of the ignition as well as the timing of the camshaft.
The top of the cam is flat, so it reaches its highest point at (e.g.) 100 degrees after TDC and stays there until (e.g.) 112 degrees. Do not turn the engine the `wrong' way, because the figures will be different due to slack in the timing chain.
