Post by earlwb on Jan 7, 2008 11:26:21 GMT -5
I thought I would break this out as a separate thread on how to determine where TDC (top dead center) is on a engine.
Basically TDC means that the piston is at its highest point in the cylinder when you turn the crankshaft.
This normally isn't a big deal, unless you are rebuilding your engine, putting on new high-perf parts, like a crankshaft or something.
One needs to determine where TDC (top dead center) is on their scooter engine. One never knows if the marks on the flywheel and crankcase are indeed accurate, until they physically check and confirm them. A couple of people have had timing marks way off on their engines.
So getting the piston to TDC while turning the crankshaft in the correct direction is the proper method. Then once you have it at TDC, check the flywheel and see if any of the timing marks line up OK. If not, you need to make your own mark.
One old method we used to use was to take a old spark plug, and break off the ceramic part and drill a hole down the center, then use a lightly oiled long screw or bolt in the center hole.
You could even use JB-Weld with a lightly greased bolt in place to fill in the spark plug shell too.
Then you would carefully rotate the crank, and watch the bolt to see when you reached TDC. They used to sell the special tools for this, but it was pretty easy to make your own.
Some machinist guys used a dial runout guage on the end of a spark plug shell too.
You can file little marks in the bolt to mark where TDC is for future reference.
But nowadays, the engines seem to have the spark plug at odd angles, so I am not sure how well it would work, plug you need to make it longer so you can screw it in and out easier.
This guy has a cool idea on using a homemade manometer to measure TDC. This looks like a really promising method to use.
If you have the head off already....
In this article about a Vespa:
www.scooterhelp.com/tuning/vespa.polini.130.install2.html
About 3/4's down is a picture of a runout dial gauge fixture setup to measure TDC. You can get inexpensive runout dial indicators from many sources such as Harbor Frieght Tools, etc.
It is easy to fabricate the unit for use on a GY6 engine, etc.
Usually eyeballing it with the head off is good enough for most of us, but when racing or doing high-perf mods, getting a little anal doesn't hurt.
In this article, they go into how to make a homemade TDC gauge as well as how different ones are used too:
www.medial.com/suzuki/ignition/tools.html
Of course then that leads into getting the camshaft timing all setup too....
Basically TDC means that the piston is at its highest point in the cylinder when you turn the crankshaft.
This normally isn't a big deal, unless you are rebuilding your engine, putting on new high-perf parts, like a crankshaft or something.
One needs to determine where TDC (top dead center) is on their scooter engine. One never knows if the marks on the flywheel and crankcase are indeed accurate, until they physically check and confirm them. A couple of people have had timing marks way off on their engines.
So getting the piston to TDC while turning the crankshaft in the correct direction is the proper method. Then once you have it at TDC, check the flywheel and see if any of the timing marks line up OK. If not, you need to make your own mark.
One old method we used to use was to take a old spark plug, and break off the ceramic part and drill a hole down the center, then use a lightly oiled long screw or bolt in the center hole.
You could even use JB-Weld with a lightly greased bolt in place to fill in the spark plug shell too.
Then you would carefully rotate the crank, and watch the bolt to see when you reached TDC. They used to sell the special tools for this, but it was pretty easy to make your own.
Some machinist guys used a dial runout guage on the end of a spark plug shell too.
You can file little marks in the bolt to mark where TDC is for future reference.
But nowadays, the engines seem to have the spark plug at odd angles, so I am not sure how well it would work, plug you need to make it longer so you can screw it in and out easier.
This guy has a cool idea on using a homemade manometer to measure TDC. This looks like a really promising method to use.
If you have the head off already....
In this article about a Vespa:
www.scooterhelp.com/tuning/vespa.polini.130.install2.html
About 3/4's down is a picture of a runout dial gauge fixture setup to measure TDC. You can get inexpensive runout dial indicators from many sources such as Harbor Frieght Tools, etc.
It is easy to fabricate the unit for use on a GY6 engine, etc.
Usually eyeballing it with the head off is good enough for most of us, but when racing or doing high-perf mods, getting a little anal doesn't hurt.
In this article, they go into how to make a homemade TDC gauge as well as how different ones are used too:
www.medial.com/suzuki/ignition/tools.html
Of course then that leads into getting the camshaft timing all setup too....