Finding top dead center

[Guest]

What is the best way to find TDC on an Indian 4? I’m
trying to check the timing and the manual says to stick a piece of wire in the spark plug hole. That doesn’t seem to be very accurate but what else can a guy do other than removing the front cover or the head. What are you guys doing? Thanks.

[tfburke3]

I havent done it but you can get various small diameter endoscopes amazing cheap on ebay or amazon for smart phone, tablet etc.Some indy shops by me use them for quick cylinder bore inspection.Tom

[T. Cotten]

Low tech rules, Dustydog!

Gut a sparkplug so you can attach some thin tubing.

Then a drop of oil in it will rise and fall with the piston, and you can set your degree wheel.

....Cotten

[Guest]

Wow! Those were fast replies. I was thinking about the camera method. I think I already have some questionable spark plug type tool. But I think Cotton is right. It’s a low tech job. Thanks you guys.

[kitabel]

It can find TDC, but (unless you know that the spark plug thread is vertical) don't use it to set spark advance.

[Guest]

Cotton does the air pressure in the cylinder move the drop of oil up and down in the tube?

[Guest]

Yeah that’s the problem. The plug hole is not directly over the top of the piston.

[HDSmitty]

I wouldn't hesitate to use the method with the wire in the plug hole. If you keep a good eye on it for the change of direction (slowed down for your convenience, pull both plugs) it will certainly be accurate enough to check/verify timing. As Cotten said......simpler is better. Also, it's in the manual....so it must be right!! Smitty

[T. Cotten]

A soda straw is safer, Smitty!

...Cotten
PS: And its easier to mark with a felt tip.

[kitabel]

The difficult thing to do is... when, exactly, does the piston stop?
There is probably .0005" each worth of pin and big end clearance that will reverse to hide crank rotation: the piston won't move, but the crank will rotate, at least for 1-2 degrees B/ATDC (and possibly more if the skirt rocks over as it passes TDC).
Much safer to pick a position as far down as your probe can reach, rotate the crank forward until they touch (very delicate!). Remember: if your probe is curved, even slight rotation changes its length in the cylinder.
I have a degree wheel, but you can use a piece of coat hangar, attached securely to the engine or chassis to point at the engine sprocket. You can print a degree wheel off the internet. Bend the wire to e x a c t l y touch 1 tooth. Now rotate forward past TDC, and down even more. Back up until contact, and note how many teeth passed. 24 teeth = 15 degrees each, etc. If it falls between 2 teeth (usually), I use a steel ruler across the root to get the percentage. TDC is 1/2 of that.

The "positive stop" method is better, but requires a rigid probe, and thread directly above the piston.
For flathead racers: something to consider: a 12mm vertical plug thread in the head, right at the edge of the bore, vertically above the pin axis will make timing much easier.
If you know the exact plug thread angle, simple trigonometry will solve how many degrees of rotation = how many thousandths of piston movement, etc.

[T. Cotten]

The difficult thing to do is... when, exactly, does the piston stop?...

Lash is certainly the issue, Kitabel!

That's why the oil drop in a tube method lets you gently turn the crank back and forth, felt-tip pen at TDC, and one more very slight backward and forward wrenching to that mark lets you set your degree wheel precisely.

Degree wheels are the easy part. Just print one out to a convenient size.

....Cotten

[Guest]

Thank you everyone for all the replies

[kitabel]

Oil sounds really good, since the oil level will be a large function of the piston motion, by the ratio of the bore^2 ÷ tube^2. If the bore is 3.25" (Chief) and the tube ID is 1" the oil will rise rise & fall over 10 times as far as the piston.
What do we learn from this: don't use a small tube, or you'll get your ceiling lubricated.

[T. Cotten]

Just get it close to TDC first, Kitabel!

A tiny tube uses a tiny drop, which you can suck in from a toothpick-full by wrenching the crank backwards a few degrees.
Then forward again to take up lash and mark it at its extreme.

Yes, the oil moves quite dramatically, much more than the actual distance the piston moves.

That's what makes it accurate.

....Cotten
PS: I believe I got this tip on the VirtualIndian mailing list, from Keith Lummus, RIP.
PPS: I hope I'm wrong, and he's still alive!

[Guest]

And don’t look down the bore of the tube