[colhogg]

Hi,

Been thinking about the up and coming re-build of my engine and was wondering how to tell if the piston is at TDC of it's compression stroke as opposed to its exhaust stroke.
The head has been off and the engine had dropped a valve so it wasn't possible to set before strip down.
Am I being a retard? :excuseme:
Is it a case of turning the engine over by hand (socket on the crank) with the ignition on and watching for a spark at the plug?

[zrxsteve]

just line up the tdc mark. The valves determine what stroke it is on.If you already have head installed then line up the tdc mark and see that the valves are closed. Cam lobes pointing away from valves

[colhogg]

Yeah, but surely there are two TDC's for each complete cycle of the engine (four strokes) and therefor possible to set the timing 180 degrees out.

[Bugstain]

Yeah, but surely there are two TDC's for each complete cycle of the engine (four strokes) and therefor possible to set the timing 180 degrees out.

Except that at TDC on the exhaust stroke, the exhaust valve would have just finished closing (watch as you turn the engine to TDC), and the inlet would be opening, or getting ready to open.

On the compression TDC, both valves will be closed long before you get there, and will remain closed long after you pass TDC

The easy thing is to watch the cam lobes for the last 90 degrees or so of crankshaft rotation as you crank the engine toward TDC - if there is no valve activity, you're approaching the compression TDC. If either of the valves is opening or closing, you're on the wrong stroke.

[bronco78]

TDC compression stroke, both all valves are closed, cams are at 10 and 2... if the cams are out......it's dead simple. Set the flywheel on the TDC mark, install the cams at 10 and 2... Your done.

[colhogg]

Except that at TDC on the exhaust stroke, the exhaust valve would have just finished closing (watch as you turn the engine to TDC), and the inlet would be opening, or getting ready to open.

On the compression TDC, both valves will be closed long before you get there, and will remain closed long after you pass TDC

The easy thing is to watch the cam lobes for the last 90 degrees or so of crankshaft rotation as you crank the engine toward TDC - if there is no valve activity, you're approaching the compression TDC. If either of the valves is opening or closing, you're on the wrong stroke.

I realise this, but the head has been off, so I could put the cams back on at ten and two, i.e TDC of compression, and the bottom of the engine, could be on it's exhaust stroke.

The cdi doesn't know someone has been moving things around.

Am I missing something here or what, surely there is only one spark per two revolutions of the engine - hence 180 degrees out.

[Speckypete]

I believe it sparks on the compression and exhaust stroke anyway, so there are no worries...

[bronco78]

I realise this, but the head has been off, so I could put the cams back on at ten and two, i.e TDC of compression, and the bottom of the engine, could be on it's exhaust stroke.

The cdi doesn't know someone has been moving things around.

Am I missing something here or what, surely there is only one spark per two revolutions of the engine - hence 180 degrees out.
Your over thinking it.........if the crank (flywheel mark) is at TDC.. Just toss the cams in, move the sprockets (and cams) back in forth until the lobes are at 10&2,...If using stock cams, you can align the arrows on the flat surface of the head and count 15 chain pins to help you be sure they are in the right spot.

[drzrobbcanuck]

:applause: I believe it sparks on the compression and exhaust stroke anyway, so there are no worries... :applause:

yes. So it doesnt matter if you reinstall things as they were before you took it apart. You determine whether it is on compression stroke or exhaust stroke by how you install the cams. If you install the cams with the lobes at 10 and 2 o'clock as instructed above, you will be installing the cams in the ideal position to check your timing and valve lash. You can install them 180 degrees out if you choose but will be torquing the valves open as you install the cams. The spark plug fires every time the piston hits about 10 degrees before tdc

[Bugstain]

I realise this, but the head has been off, so I could put the cams back on at ten and two, i.e TDC of compression, and the bottom of the engine, could be on it's exhaust stroke.

The cdi doesn't know someone has been moving things around.

Am I missing something here or what, surely there is only one spark per two revolutions of the engine - hence 180 degrees out.

No, you couldn't do this unless you ignored the timing mark on the crankshaft. If the valves upstairs are properly oriented, and the timing mark in the crank is in the right place, all is right with the world. The CDI unit triggers off of the crank position, so it doesn't care.

[Noble]

"Am I missing something here or what, surely there is only one spark per two revolutions of the engine - hence 180 degrees out."
Actually it would be 360 out but there is no 360 out because it does spark every 360. In fact most cars and bikes these days have a waste spark on the exhaust stroke. If you have no distributor you probably have a waste spark on the EX. Just think - those 100,000 mile spark plugs would last 200,000 if they only sparked when needed.

[colhogg]

Thanks Guys, not usually like me to over complicate things!!