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TDC on a Rover 4.6 V8


garrycol

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You don't want to check both directions... The engine only rotates forwards and TDC with backlash taken up when rotating forwards is "the" TDC you want to know about. Knowing TDC with reverse backlash is useless information. Dial gauge on the piston while rotating forwards.

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But.... By rotating the crank forwards, i.e. not in a way it would expecting (no 'bang' making it go down) is false anyways!

So technically you have the reverse backlash in it when you check it with DTI.

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As red90 says, the crank position will be slightly different for same piston position depending on which way you turn the engine.

The two positions should be taken with the engine turned in the same direction as when its running.

I suppose i could attempt to calculate it but i really dont think its significant unless you are F1 team aiming to squueze every ounce of power out the engine.

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I am now out of this discussion as I am happy with the original suggested approach and that is what I am doing. In my case this is a new rebuilt engine with new bearings all round etc so if there is any slop it will not be measurable - but if there is I agree it is cancelled out with each of the measurements.

Thanks to everyone for your input.

Now if you want to continue on discussing/arguing about whether any slop counts or doesnt count go right ahead.

Cheers

Garry

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The use of a piston stop is not uncommon, my understanding is that it allows a hard stop on the crank/piston movement that is away from the area of TDC where there is maximum rotation for minimum piston movement which is the area where there is most chance of error. I have seen quite a few instructions written using a hard stop and rotatiing between them in both directions. OK I am an amateur and experts use a dial gauge etc. but for a trigger wheel set up this is by far good enough. My trigger wheel is still set up using the TDC mark and pointer so completely inaccurate but is much better than using the dizzy ignition!

Of course my best option is a TDC location pin in to the flywheel like Triumph twin motorcycles but lets face it most early car engines are only timed there and there about's but motorcycles were intended for performance so tuning was more important.

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