Cambelt change - finding TDC 2,0JTS - Alfa Romeo Forum
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156 JTS

Cambelt change - finding TDC 2,0JTS

Please, do anybody know how prezise TDC must be found when changing the cambelt? TDC is found with the help of a dial gauge in cylinder 1. But due to very small movement of the piston close to TDC, an inaccuracy of this measurement of eg. 0,01mm results in an inaccuracy of the TDC determination of +/-1,5 degree.

How accurate should the TDC be found? And how do you find TDC by means of the dial gauge (mean value of measurement before and aft TDC, when the dial gauge starts not to move any further...)?

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The dial gauges are pretty accurate. Can't remember the divisions on mine, but they're fractions of a millimetre.

I usually move the engine forwards and backwards around tdc to work out the maximum point, which is what you're asking I think.
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On the auxillary belt pulley there is a notch. This lines up with a mark on the lower belt cover when the engine is at TDC

The mark on the belt cover is around the 10 o'clock position
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+/- 1.5 degrees is nothing to worry about, I've found that valve timing on a stock Ducati engine for example can be up to 10 deg out due to manufacturing tolerances. Its one of the things that gets corrected when the engine is blueprinted.
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In order to find true TDC you need to use the DTI gauge and rotate the crank clockwise until it reaches the highest point, there you need to make a mark on the crank pulley and teh block, rotate even further so that you go past the highest point and then rotate the engine anti clock wise in order to find the highest point again. When you do then make another mark onn the pulley and the block. Divide the distance of the 2 marks on the pulley by half and this will be your real TDC. If there is no distance between them then you have a pretty precise hand.
1.5 degree is not too bad i guess but i would try and have the engine set up bang on so that when the belt streches with time the timing doesn't change too much.
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You are absolutley right.. just sticking a clock in the plug hole and looking for maximum deflection is not a very accurate way to determine TDC. Due to the piston "dwell" at around TDC. (It's caused by the piston slowing down as it approaches TDC and shows up on the clock as an apparent dwell around TDC). As new alfisti says, the accurate way to do it is to determine an amount of deflection on the clock before top dead centre, fit a timing guage or protractor and pointer to the end of the crankshaft, note the reading in degrees.. rotate the crankshaft over top dead centre to obtain the same deflection on the clock.. note the reading in degrees and true TDC is bang between the two degree measurements..

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