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Finding Tdc For The Megajolt

Guest HS88

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I have a mejajolt kit to fit and before I order a new trigger wheel I want to find out how easy it is to find TDC or what tools/who I would need to help.


See below for my current wheel. I was thinking of bolting the trigger wheel to the pully through the central bolt, and also roll pins or bolts to stop any rotation.


Any other suggestions for attaching the trigger wheel welcome, as well as a good method to set it up correctly.



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Dial guage and degree wheel is prefered way

You can however get very close with a spark plug out and a rod down the plug hole no. 1 plug

Find the highest point

Mark on pully

Rotate a little forward until rod just starts to drop

Mark on pully

Rotate other way until rod starts to drop

TDC is exactly between the 2 drop points


I also used the pully bolt then when happy with the position drilled a small hole and used a spring pin to lock it

Edited by Snapperpaul
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Guest snotfart

another way is to use a "dead stop" which can either be a suitable sized bolt or a modified spark plug to stop the piston on number one just before top dead centre,carefully rotate the crank using the front pulley bolt then you can mark the pulley at the two points that the crank stops turning,TDC is centre of the two marks.machining of head or block makes no difference to TDC.

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thanks snofart - could I use the notches on my wheel?

see images below


I welded the trigger wheel on the back of the pulley, this means you do not have to disturb the VR sensor when removing the fan belt.




This is ideally what I would like to be able to do...

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Guest Alan_builder



I thought you could slap the trigger wheel on near enough and dial any error in to the electronics to correct for misalignment! Ok you need to set the thing as best you can so it will run but most engins run with no load over quite a big range, do you remember swinging the distributor? Dyno test or rolling road to get the ultimate.

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Guest Alan_builder

the existing notches cannot be relied upon until proven by one of the methods above,if they turn out to be accurate go ahead and use them.

when it is so easy to precisely determine TDC why just "slap the trigger wheel on near enough" when it can be perfect?


and how accurately can you fix the trigger wheel? How thin has the head gasket been squashed? What error is there in the markings? etc.

Just don't see spendings lots of time and trouble setting it up only to find you need another setting that you can easily adjust by a code change! And readjust by another so it is perfect not just within the limits of your measuring system.

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could the trigger wheel be set to the notches and then adjust the sensor to get exactly 90 degrees before TDC?


Not sure how to measure and ensure the sensor picks this up correctly...

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Guest snotfart

I could fix the trigger wheel to within 0.02mm of actual measured TDC if i was feeling all technical but for a "quicky" within 1 degree is easy peasy.

head gasket thickness has nothing to do with TDC.

the error in front pulley markings as standard is kind of the point of this thread.

using either a dial gauge or the dead stop method is quick and easy.

i agree completely with your point about software corrections but i am an engineer and as such i like a good datum.

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