setting timing on 250?

[cooperplace]

has anyone got any recommendations for a brand of TDC dial guage and plug adaptor? Or is there another way of knowing when it's at TDC?

thanks

[StewartD]

I thought this had been covered before, but a quick search didn't bring anything up.

You don't need a dial indicator if you make up a cheap 'piston stop' tool. All you do is gut a spark plug and weld a bit of steel bar into the hole left by the ceramic core. The bar should be as big a diameter as the said hole, and protrude about 25mm from where the earth electrode used to be. The exposed end of it should be radiused (say 3 − 4mm radius) so it does not damage the piston.

The left side primary drive case plug is taken off and a timing degree disc is fitted to the end of the crankshaft. The disc is normally held on a small shaft or arbour, that can be screwed tightly into the end of the crankshaft, with a means of rotating the disc without accidentally unscrewing the arbour or rotating the crank. A rigid pointer should be fixed to some part of the engine so you can reliably read the crank's rotation off the degree disc.

Next remove the spark plug and turn the motor so it is away from top dead centre. Insert the piston stop tool and tighten firmly. Rotate the motor slowly until the piston contacts the piston stop tool. Note the angle reading of the degree disc, and/or mark the degree disc.

Now slowly turn the motor in the opposite direction so that the piston again contacts the piston stopper. Note the degree disc reading. if you were able to now continue turning the motor in the same direction, top dead centre would be Halfway from where you have stopped and the spot on the degree disc where you first marked it. Note the number of degrees to this halfway point from where the pointer is now.

Remove the piston stop tool. Now turn the motor so the halfway point is at the pointer. The motor is now at top dead centre. The disc should now be rotated on its arbour, without turning the motor, so that its 'zero degrees top dead centre' mark is at the pointer.

You are now ready to statically time your engine.

This method of finding TDC is accurate as the movement of the piston is much higher per degree of crank rotation at the positions of crank further from top dead centre than at TDC.

Cheers,

Stewart D

[Moto Chuck]

Yep, This is the way I learned to do it. When you learn your particular stop tool on your favorite engines you can just turn the timing disc to plus # degrees and then check it by spinning the engine back to minus # degrees to where the piston once again encounters the stop. As I remember the tool produces a 17 degree BTDC one way and 17 ATDC when turned back on my 250 MK3. Of course my 350 Sebring has a different setting that I don't remember.

[cooperplace]

sounds great.

Apologies by the way, because I accidentally posted this thread twice.

What size thread is used to screw into the crankshaft?

Thanks

[StewartD]

Hi Cooperplace,

The thread is standard M8 x 1.28 pitch. Attached are photos of my timing disc which you might find helpful.

DSCF4762.jpg

DSCF4761.jpg

The boss is much larger than is needed but I had the this bit of steel so I used it. The outer holes are unused for its new use. The inner two holes are where the M5 cheese head screws go. These are to clamp the timing disc at any position. Note that the hole in the plastic timing disc is larger than the spacing of these screws and that there is a very thin spigot for the disc to locate on. The small spacer on the M8 bolt is just to keep the bolt head and washers clear of the M5 screws.



Cheers,

Stewart D

[cooperplace]

I



Next remove the spark plug and turn the motor so it is away from top dead centre. Insert the piston stop tool and tighten firmly. Rotate the motor slowly until the piston contacts the piston stop tool.

so what worries me about this is that the motor is being turned against the compression and friction, with a pointed, albeit radiused, probe sticking down into the combustion chamber. Being the nitwit that I am, might I punch a hole in the top of the piston?

[geomoto]

Good point on the rotating of the crank/piston against compression. My "piston stop" is a "bullett plug" I believed purchased from Motion Pro. I thought it had a small "pinhole" to release compression but it does not seem to. Should there not be a "bleed hole" to relieve the compression? My Duc motor is in a race bike that has been "double plugged" so I just remove both plugs.
Also, I find that after you have used this piston stop method a couple of times you can skip it and just look inside the spark plug hole to determine TDC. You are not far of with this.