2 Stroke Cylinder Mapping - Part 4





Primer - Part 1 - Part 2 - Part 3 - Part 5 - Part 6

 Finding Top Dead Center (TDC)

With the help of a few tools, the engine porting can be discovered. There are software programs which aid in engine development but cylinder mapping still has to be done first. Without accurate input, port changes will not be correct.


Some basic tools - rulers, depth gauge, inside caliper, micrometer, X-acto knife, pen, pencil, compass, feeler gauge, black felt tip marker and a small hole template.


This is an indicator set which locates TDC by using a plunger to touch the piston crown. It threads into the spark plug hole. A piston stop is slightly more accurate for it though.


The basic piston stop. Using this tool you will be able to locate true Top Dead Center (TDC) - even with a flat top piston. It is used with a degree wheel. I had to draw this because I can't find mine - I know it's here somewhere.


I looked all over my garage for this thing - it was on my Yamaha GP 760. I started mapping it recently in order to calculate the port sizes before cutting the ports. This engine has been dialed in already. The bent cotter pin is indicating TDC.

Finding TDC Using A Piston Stop

Locating TDC is necessary to properly degree in an engine. With the piston installed in the cylinder - and the head off you can do it easily. The piston stop is a very important tool and it is very easy to make. Measure the distance between centers of two opposing cylinder head studs and make a note of this distance. Start with a piece of 3/8" thick aluminum about 1 inch wide and long enough to cross your cylinder. Mark the aluminum for drilling the holes (your measurement). Use a center punch to help locate the holes while drilling. Drill the holes large enough to fit over your cylinder studs. If it doesn't fit you may want to dress it with a small round file to open up the holes to make it fit.

If your motor has a domed piston and its dome goes above the deck at TDC - you're all done. If you have a flat top piston you'll need to install a bolt in the center of the piece, directly over the center of the piston - as close as you can get it. Drill a hole and tap it for a 1" long x 1/4" thickness fine thread bolt. Put a nut on the bolt then screw it in the aluminum.

In order to locate TDC you'll now need a degree wheel installed on the end of your crankshaft. Many degree wheels come with washer sets so they can be used on a variety of engines. It is not necessary but it is easier if you remove the flywheel during this procedure. The flywheel magnets will want to be pulling - so it's best to remove it. Once the degree wheel is installed leave it a little bit loose so you can move it when you need to. You'll also need to provide a pointer to indicate your wheel. The pointer is easy to make from a nail or similar piece of metal. Twist it so it can be tightened under an engine side cover bolt. Make sure it is fairly rigid. It doesn't matter where it points - yet.

Turn the engine over by hand until the piston is at TDC - eyeball close is good enough. Rotate the degree wheel and set it to TDC. Now put the piston about half way down the cylinder bore. Use a couple of cylinder head nuts to tighten the piston stop to the engine - it has to be snug. Turn the crankshaft clockwise until it touches the piston stop - do not force it - this is a light touch procedure. Make a note of the reading on the degree wheel. Now rotate the crankshaft the other direction until it touches the piston stop again. Note the reading on the degree wheel. True TDC is directly in the middle of the two readings. Adjust the degree wheel so that when you rotate the crankshaft in either direction it will stop at the same reading though one will be BTDC and the other will be ATDC.

If you set the degree wheel so the piston stops at (for instance) 20 degrees BTDC when you turn it clockwise and 20 degrees ATDC when you turn it counter clockwise you have located true TDC. It may take a couple of tries to pin point it. When it's where it should be tighten the degree wheel so it won't move. Do the procedure again. You can make small changes by bending the pointer.

Finding TDC Using A Dial Indicator

Similar to above except that after the indicator is screwed into the spark plug hole the piston is brought up until it touches the plunger. Stop it when the indicator needle has traveled 1 mm. Make a note of the reading on the degree wheel. Turn the crankshaft the other direction. Stop when it reads exactly the same as it did when turned the other direction (1 mm travel). Make a note of the reading on the degree wheel. Again, true TDC is directly between the two readings. Adjust the degree wheel accordingly. It is also possible to just let the indicator travel until it indicates the piston will not travel any further. However, there is a small margin or error in this method.

That Degree Wheel

Since the degree wheel is now set up, why not make a note of the port timing. Rotate the engine over until the edge of the piston is just at the bevel to the exhaust port. You know you're very close to where it actually opens. Within a degree or two you'll see the actual port open. This is the mark to write down. Do the same for the transfer ports and boost ports. All this information is useful when trying to determine the state of tune of an engine.


This piston is at TDC. Note that it's not flush with the deck - the top of the cylinder liner. We need to know its distance below the deck. We can use a feeler gauge to find out what it is.


Carefully match the thickness of the feeler gauge to the space that's there. Feel it with your finger. If there's a lip, change it until it's smooth. In this case the piston is .024" (.61 mm) below the deck.

What The Deck

This motor has a -.61 mm deck. That is to say, the piston when at TDC is not flush with the edge of the cylinder liner - it is .61 mm below it. If there is a deck height difference to consider, it should be accounted for when transferring the images to paper. When making a drawing by taking the measurements from the top of the ports and proceeding from there - the drawing must be adjusted to compensate for the difference in the deck difference. Also, engines with base plate spacers will need to have the spacer included when calculating the port timing this way. It is necessary to add the thickness of the extra base gasket as well.

It is necessary to know the deck height of a cylinder that is having its compression changed or when it is getting the squish band set properly. If I wanted to set the squish at 1 mm, I would have .039 mm to work with. I could use a copper head gasket of that thickness (about .015") to achieve it.

See Part 5 What To Do With The Data.

Primer - Part 1 - Part 2 - Part 3 - Part 5 - Part 6






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