x-post: Easier Way to find TDC?
 

I just bought the equipment to to leak-down tests on my 968 but wondering if anyone has a method of most easily determining top dead center TDC (with valves closed) for this test as you go from cylinder to cylinder?

Thanks,

Harvey

I know that if the rotor is pointing to that particular plug wire, that cylinder is ready to fire meaning all valves for that cylinder are closed.

hpaulb used a length of nylon rope to find it on my car when he did my timing belt...
lord knows what he did, i was too busy inside the car fiddling with the ignition switch.

I'm not sure what compression testing technique you are trying to do, so I don't exactly know how to help you. I know that tdc for piston 1 is marked on both the camshaft cover and the flywheel on the 944 if that helps. Not sure what the rope trick is, but it would be interesting to hear about it.

i'll PM him and ask him if he would explain it to us..

theres a squareish hole that is right near the referance sensor on the bellhousing. When the engine is at TDC, a line and the letters "TO" will appear in the hole. use that to get it on the right stroke, it will be danm near TDC when that TO appears there. then use a wooden dowell in the first cylinder spark plug hole to tell when the piston is all the way to the top.

You can find a better explination of that at clarks-garage.com check under timing belt and you find it.

i'm pretty sure that's what it was..
he put the length of rope down the first cylinder spark plug hole and raised the piston to the top.. when there was resistance, he pulled the rope out and knew he was at the top..

i believe.. lol

there wouldnt be resistance against the rope, there would be slack, but thats a matter of verbage. As for just making sure its at the top, if that is all you did, then you could possibly get the first cylinder on the compression stroke rather than the intake stroke, which would mean that you arent at TDC which would then lead to messing up your car. Either way, clarks garage has a good write up and getting a really close approximation on TDC with out pulling the head, which will help out. Pulling the head is not that big of a deal and if you have the timing belt off, a head gasket is like 8 bolts away and you would be a way to confirm that it is at 100% TDC... just some food for thought.

These are all good suggestions for finding TDC on piston 1. The other cylinders you could pull the distributor cap off and see when the rotor is pointing toward the appropriate spark plug wire and then use the dowel approach to find TDC for that cylinder. The part I am unsure of is what type test is he looking to run. If he has a compression gauge in the spark plug hole, you can't use the dowel trick to find tdc on that cylinder. But since the firing order is 1-3-4-2 and if piston 1 is tdc, then cylinder number 4 should be at tdc as well, just the exhaust stroke not the compression. Same holds true for cylinders 3 and 2. So, it seems to me if you put a compression gauge in cylinder 1 then use the dowel in cylinder 4 to find tdc, you should get the reading on the gauge you are looking for. If you are running a compression test for cylinder 2, then find cylinder 2 spark plug wire location on the distributor cap and stop the engine when it's pointing at that plug wire and then rotate then engine until the dowel in plug 3 is at tdc. For cylinder 3 stop the engine at cylinder three spark plug wire location and then find tdc with a dowel in cylinder 2. For four, do the same only use cylinder 1 to find tdc. You need to check the rotor location so that you know the valves are closed and you are on the compression stroke. The opposite cylinder in the firing order will be at the same location, just with the exhaust valve open. Then use the dowel trick mentioned earlier to find tdc.

Again, I'm not sure what test you are running, so I'm making a lot of assumptions here. Hopefully this was of some help.

Still want to hear about that rope trick though....I've just never heard of it and I'm interested to know how it works.

I use the dowel method but it's only good every 3 or 4th round. I like the idea of the rotor point to thr plug wire. that would be easy to do and could be done without a lot of hassle.

THanks! just what I was lookin for.

Harve

For the 944 the easiest way to TDC is to pull the #1 plug and stick a long rod or somthing in the cylinder. Preferable somthing plastic. While turning the crank, keep one hand on the rod to keep it from binding. You can feel it at top. Then look down the little hole and look at the mark. The square hole showing the timing mark on the flywheel. It's really hard to find the hole and hold a flash light. Straight down neer the fire wall and oil tube and pushing a few wires out of the way.