How the smeg do I find TDC?

[MrAE86]

I'm a bit confused, (not difficult). I'm trying to find TDC on my 2ltr air cooled. The big wheel behind the plastic domed mesh has 4 visible marks on it going clockwise the marks are: large. small, small, large. The mark on the dizzy is between 3 and 4 o'clock. When the large left hand mark on the wheel is lined up with the vertical engine mould line (about 12 o'clock) the rotor arm is at 3 o'clock. But no1 cylinder does not seem to be at TDC until I move the wheel a bit further clockwise to line the rotor arm up with the mark on the dizzy case. So can someone please put me out of my misery, have a good laugh and then explain what i'm going wrong, preferably using small words and big pictures

[getunder]

On my old motorbike I used to hold a piece of rod in the spark plug hole and move the wheel each way until I could tell the highest point as it moved up and down and then mark the rod. tin ware might make it difficult on an Aircooled. I used to have a TDC gauge which was like a spark plug with a sliding rod inside it and a calibrated outer . Might still have them in M/cycle shops.

[bigherb]

You should have a plastic timing scale mounted around the plastic mesh with TDC lines up with 0 on the scale.

[MrAE86]

Thx guys, the TDC gauge sounds interesting. Unfortunately the white ring is not there, guess it wasn't put back by a previous owner.
Don't you just love these old vans

[lloyd]

Unless you check valves on number one, you can get TDC mark wrong. 50/50 chance cus timeing marks reach TDC twice... so you could think you have number 1 TDC and have number 3 TDC.

Trace plug lead from number one and see where it connects to cap, and where rotor needs to be to fire it. Turn engine so rotor is in that area, With sparkplugs out, turn engine back and forth and you should be able to feel when the piston is at TDC as crank will move easier... Doesn't give to the exact TDC position, but within a few degrees. Or remove right side valve cover and find TDC for number 1 with both valves closed. Still not exact TDC position. Like getunder said; need to feel/see piston to get it right, and even then need to do it a few degrees below TDC on each side and then mark the difference between to get accurate TDC.

Hope that helps and doesn't confuse you even more.

[neil3965]

Work it out with a pencil? See the following.

http://www.vw-resource.com/tune-up.html#tdc

It worked for me!

[selimg]

This may seem silly but because these are flat 4s surely sticking a pen, guage or whatever into the spark plug hole won't be that accurate. I say this cause the piston moves laterally so it would just jiggle whatever is down there all over the place. Compared to say an inline 4 where the object would sit on top of the piston. Or have I missed something out?

And does the rotor arm rotate in-sync with the crank or at 1/2 speed?

[lloyd]

Hold pencil horizontal too.
dizzy/rotor and cam turn once for every 2 revolutions of crankshaft.