What's the best method to find TDC?

[Nacla]

I'd like to perform a cylinder leak test on my '93 6.5L.

The leak down test instructions require positioning the cylinder being tested at top dead center (TDC) on the compression stroke, to prevent the air pressure from moving the piston in the cylinder.

Question: What's the best method to find exact TDC for each cylinder?

This is probably easier than it sounds. If this topic was addressed elsewhere, I haven't found it yet.

Thanks,
Nacla

[DmaxMaverick]

The easiest way is to remove the injector, and use a length of small hose (or something similar) inserted through the flash hole in the pre-cup. Insert it only a little, as the piston will close the gap almost completely, and you only need to "see" the last bit of movement, and you don't want the piston to crush the hose. With some of the cups, you can actually see the piston crown with a flashlight, but most are angled so you can't. The piston will be at TDC for a few degrees, so put it in the middle.

Remove all of the glow plugs, which makes turning by hand much easier. Be sure to "lock" the crank once TDC is found. I use a long breaker bar and socket on the crank bolt, secured to the frame or other solid point. Even if it is at TDC, they will sometimes rotate.

I use to have a table with the crank degree locations, but it was very inaccurate from engine to engine (ended up being more work than removing the injectors).

[Nacla]

Thanks, Maverick!

Was sure someone here would know what to do to make it an easy task.

Regards,

-Nacla

[Robyn]

You can use the timing mark up on the balancer to give you #1 TDC
From there every 45 degrees (in normal rotation) will put a cylinder at TDC

Firing order is 1-8-7-2-6-5-4-3 odd on LH even on RH side

Get a cheap little degree wheel at the parts house and your set

This will put you as close as you're gonna need to be to do a leak down test

The reason its spec'd to be at TDC is that the valves are closed and the entire package is ready to deal with combustion pressures.
You can be a few degrees either way and still get the same results (checking for leaking valves/rings)

The trick is to hold the crank stable.

As Maverick mentioned, lock the crank with a break bar and a socket.

WORD OF CAUTION


Make sure the break bar is stable and the socket is on the bolt well, and keep clear of that bar while things are under pressure.

I had a bar slip off doing exactly what you are doing, and came flying out of the engine bay just a $HITTIN AND A FOGGIN.

It landed in the neighbors yard some 200 feet away :eek:
Just missing his wife's Mercedes :eek::eek::eek:

Have fun and be safe

Missy

[convert2diesel]

With the air turned OFF. Connect the tester and turn the engine. As it approaches TDC, the gauge will start to rise. Turn until you have reached max and you're good to go after locking the breaker bar. If your bar is long enough, you should still be able to rock the crank while under pressure to find the sweet spot. (note Robyn's cautions when doing this). Still have a scar where a wayward aircraft propeller tried to chop my arm off :eek:. (first rule: NEVER LET GO OF THE PROPELLER/BREAKER BAR WHEN UNDER PRESSURE.) TDC is when there is no tendency for the bar to move in
either direction...refer to rule # one.
Bill