Determining TDC (top dead center)

[earlwb]

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....

[earlwb]

Here is a link to a website with a good explanation of setting up a camshaft and setting the timing for it.
Here is the link:
www.buggydepot.com/buggy-tech-center/150/gy6/yerf-dog-150cc/tomberlin-crossfire/twister-hammerhead/150cc-information/read-tech-article.php?aID=11
here are the instructions:
Camshaft Install Instructions

Overview:
A. Before installation
B. Access Camshaft assembly.
-Finding TDC
C. Remove Camshaft
D. Reassembly
-Cam Chain Tensioner
-Adjusting the Valves
E. Final Assembly and Inspection


A. Before installation
1) Wash the entire buggy. This will keep dirt from falling where you don’t want it while the engine is exposed.
2) Remove the passenger seat from vehicle.
3) Make sure the engine is cold.

B. Access Camshaft assembly
1) Loosen the intake tube at the carburetor and remove the airbox and tube assembly.
2) Remove black plastic fan shroud from passenger side of motor. (Seven 8mm screws/bolts)
3) Remove carburetor and air intake boot from motor as a unit and place on a clean surface. Force may need to be applied to remove intake boot from engine if it binds. (Two 10mm nuts)
Note: Fuel line will need to be clamped to avoid dumping fuel.
Note: Insert a small clean rag into the intake hole to keep dirt from falling into the engine.
4) Remove plastic shroud from top of motor to gain access to cam chain tensioner. The lower shroud does not need to be removed.

Finding Top Dead Center
What is TDC? Top Dead Center refers to the piston being at the very top of its travel in the cylinder. On a four stroke, TDC will occur at the top of the compression stroke and the top of the exhaust stroke.
5) Remove sparkplug.
6) Move Flywheel until (T) mark is aligned with the arrow mark on the engine case.
7) Loosen four valve cover bolts and remove the valve cover.
8) Now that you are inside the valve cover, rotate the flywheel until the cam is at the TDC position with hash marks aligned straight across the valve cover surface and the large TDC hole at the top of the cam gear. (Large TDC hole should be pointing directly towards seats)
Note: Document TDC on the flywheel using permanent ink; reference your mark to a point on the engine case.

C. Remove Camshaft
1) Remove the cam chain tensioner by removing the 2 bolts that secure it to the cylinder. Unless you have a spare, you will need to reuse the gasket. Use a razor blade to properly separate the gasket from the tensioner.
2) Remove the 4 head bolt nuts from the rocker tower and remove it.
3) Remove cam chain by tipping the cam gear downwards.
Caution: Be sure not to drop the cam chain down into the case. The case may need to be split to refit it.
4) Secure the cam chain with a cord or rope tied to a part of the chassis so it can be accessed on reassembly.

D. Reassembly
1. Insert the new cam by again tipping the cam gear down and attaching the cam chain making sure the alignment is set to the mark you made on the flywheel and the hash marks on the cam, with the large cam hole pointing at the seats.
2. Install the cam tower back onto the cylinder studs with the EX towards the exhaust valve (downward).
3. Install the head stud nuts and torque to 1st 16 lbs then on to 25 lbs using a cross pattern.
Note: The Yerf Dog manual states 16lbs on these nuts, however they come from the factory tourqed to 25lbs.
4. Recheck the cam alignment chain to ensure the cam is in the correct position to the mark you made on the flywheel.

Cam Chain Tensioner.
5. Release the spring tension on the cam chain adjuster by removing the screw cap and turning the adjuster inside using a small flat blade screwdriver clockwise until it stops. This will remove all tension from the adjuster.
6. Insert the adjuster and tighten the 2 mount screws while holding the 0 tension on the adjuster.(Make sure gasket is in place.)
7. Turn the adjuster back to full tension (rotate counter-clockwise, will rotate on its own) and reinstall the cap screw.
Caution: Failure to release the tension on the Cam Chain Adjuster during reassembly will damage the cam chain beyond use. (The cases must be split to replace the cam chain.)

Adjusting the Valves.
1. Adjust the valve tappets using a feeler gauge to a clearance of :
Intake- .002- .004 “ Exhaust .003-.006”
Note: Printer paper happens to be 0.003”
Note: Valves must be adjusted on a COLD engine.
Note: Adjust valves with engine at at TDC.
2.Loosen the 9mm valve tappet lock nut and turn the tappet out several turns
3.Sandwich your feeler gauge between the bottom of the tappet (adjuster screw) and the top of the valve
Note: Feeler gauges are thin pieces of metal which are a predetermined thickness.
4.Tighten the tappet while moving the feeler gauge back and forth until you can feel a slight drag on the feeler.
5.Tighten the lock nut with the feeler in place.
6.Ensure the gap is still within proper range by sliding the feeler back and forth again.
7.Repeat step 2 - 6 for remaining valve.

E. Final Assembly and Inspection.
1. Rotate the assy several times by turning the flywheel then recheck the TDC-Mark to Cam alignment.
2. Reinstall all remaining assemblies in the reverse direction of disassembly.
3. Check operation of motor. ( Does it start and idle?)
4. Check jetting and set as required by intake, exhaust and cam use.

[leo]

the method i use for auto engines is to remove the plug from the cylinder in question. straighten a coat hanger, or use any old stiff piece of wire, insert coat hanger in plug hole and bump the starter.
of course you can also use a helper to manually turn the crank with a ratchet on the damper pulley.
i'm not sure how the above would work for a scooter engine.

[earlwb]

Yup I have done that before too.
You carefully push on the kickstarter pedal with your hand. Or use a socket wrench on the variator pulley nut.

[earlwb]

I found the pics that one of our other illustrious forum members (if he speaks up I can give him credit) had done relating to TDC and timing and valve adjusting. Note as the Chinese change the castings, the marks may change, so it is important to manually find TDC the first time to verifiy your timing marks, and if needed, make your own reference marks. Then start taking things apart or fixing things as desired.
On a couple of people's scooter engines the timing marks were off by about 1/4" or so.

[earlwb]

Some people might find this URL interesting about cam timing chains and stuff.
www.dansmc.com/camchain.htm

By the same token, if you want to get into more details about TDC, cam timing and duration on the cam lobes, you need to use a degree wheel or a digital degree tool like this one:
www.jegs.com/webapp/wcs/stores/servlet/product_10001_10002_962683_-1?utm_source=email011608&utm_medium=email&utm_campaign=05

Here is a downloadable degree wheel pattern here:
www.tavia.com/free_degree_wheel.html
You download and print it out, and then glue it onto a cardboard form. You securely affix it to the crankshaft. Some people might try gluing it to a stiff plastic sheet or even sheet metal too.

You mount the degree wheel onto your crankshaft, with zero being aligned with the TDC point on the crankshaft. Then as you rotate the crankshaft you can see about where things work as to when a cam open and closes. Measuring how many degrees a cam lobe opens and closes determines the lobe duration for that cam design.

Here is another nice degree wheel here too:
images.google.com/imgres?imgurl=http://www.lucasinjection.com/Degree_wheel_100.jpg&imgrefurl=http://www.lucasinjection.com/FUEL%2520CAMS.htm&h=588&w=585&sz=157&tbnid=8k2seRP7ljXa-M:&tbnh=135&tbnw=134&prev=/images%3Fq%3Ddegree%2Bwheel%26um%3D1&start=2&sa=X&oi=images&ct=image&cd=2

Here is another degree wheel here too:
www.savetheledges.org/test/AVS/images/tech/CrankDegreeWheel.png
from this website here:
www.savetheledges.org/test/AVS/cam.html

Still another more simple one:
www.printfree.com/Office_forms/DegreeWheel.htm

Also you can buy nice degree wheels from various sources. You can see some here in this auto parts catalogue here:
www.compcams.com/technical/Catalogs/106-07/330-343.pdf

This site is another cool site for getting a nice high res degree wheel from too:
www.machinerycleanery.com/index.htm
They also show it being used on various engines.

One of these days, someone needs to sit down with a GY6 engine and several of these different high performance camshafts and a degree wheel and make duration measurements of the cam lobes to see what the different camshafts are really doing.

[motomech]

leo said:

the method i use for auto engines is to remove the plug from the cylinder in question. straighten a coat hanger, or use any old stiff piece of wire, insert coat hanger in plug hole and bump the starter.
of course you can also use a helper to manually turn the crank with a ratchet on the damper pulley.
i'm not sure how the above would work for a scooter engine.

I use a pencil, if the tip breaks off, well, a little ruffage is good for these things.
Working on an old L-head sailboat motor right now. Had to bend a coat hanger on that.
Motomech in Costa Rica