BTDC = what?

[Invid]

The engine should be positioned at approximately 75 BTDC on cylinder no.1 compression.

Sorry some very, very Noob questions:

Can some one explain how to do this?

Is BTDC when the timing mark on the crank pulley lines up with the 0 deg timing mark and the dizzy is pointing at no.1?

[MR22ZZ]

before top dead centre.

any yep top dead centre is when the #1 piston is at it's maximum height in the cylinder, so as you say, where the marks align.

[ed]

Is BTDC when the timing mark on the crank pulley lines up with the 0 deg timing mark and the dizzy is pointing at no.1?

what youve described above is TDC (top dead centre), ie when the #1 piston is at the very top of the bore and all the marks line up.

BTDC describes when the timing mark on the crank pulley is near, but not quite yet lined up with the 0deg timing mark ie the piston is still on its way up the bore. how ever many degrees its is before the 0deg timing mark, is how many deg BTDC the crank is from putting the piston right at the top of the biore. its used for describing cam timing events, ignition timing etc etc

[Invid]

Found this in the search.
http://www.toymods.net/forums/showth...highlight=BTDC

So to find 75 BTDC
I should set the engine to TDC, the rotate the pulley back till the timing markpoints at 75 deg (approx)?

[takai]

To preempt the next question ATDC is After Top Dead Center. Basically the reverse of what Ed has described.

[LeeRoy]

Found this in the search.
http://www.toymods.net/forums/showth...highlight=BTDC

So to find 75 BTDC
I should set the engine to TDC, the rotate the pulley back till the timing markpoints at 75 deg (approx)?

Only turn the engine in its normal direction. So turn it 285* forward, not 75* back.

PS. The distributor will the rotor button relative to the cap/lead for cylinder 1 line up when the engine is at around 10* BTDC. At TDC the rotor button will have passed the cap/lead position for cylinder 1. This is because ignition happens prior to the cylinder reaching top dead center (TDC)

[RA35GT]

Leeroy speakz the truth, only spin the engine in the nromal direction.

But if you are at TDC with #1 on compression, you need to spin it 645 deg to make Cyl1 be on 75deg BTDC compression again. (For four stroke)

[Invid]

Thanks heaps for all the info, that has helped a lot.

Out of curiousity, why only rotate forward? Is there some detrimental effect on the engine by rotating the crank backwards?

[RA35GT]

I'm sure there are other reasons, but these are the ones which I recall.

Most of the belts/chains/tensioners are designed to operate with one side tight and the other loose, if you go back this reverses and may cause things to dislodge or skip teeeth when you go back the normal direction.

[mick08]

If done repeatedly it can loosen crank pulley bolt as well and as most belt tensioners are spring loaded oneway it can put ya fan belt out of line or slip it off a pulley if it loosens due to turning the belt the wrong way,pistons won't hit valves or anything silly unless the timing belt jumps a tooth as stated above but not a common occurance.

[LeeRoy]

The oil pump only works in one direction too.

Have a look on how stuff works dot com as there is a pretty good running engine .gif illustration that shows all the cycles (injection of fuel, ignition, the burn stroke, etc)