Timing Tab & TDC

[Thomas S.]

Going through a "clean up" exercise with my timing (63 - 327/340). I already know about the markings on the tab being incorrect for that engine model. The tab has never looked correct to me and I wanted some opinions to confirm my belief that it was bent at some point. I am assuming that the edge of the timing tab should be parallel to the TDC groove on the Damper. Not talking about Initial timing just an alignment question.

My second question is about finding TDC. Iv'e read a lot of posts on the site and also on the web. I understand there are a few way to determine TDC. So, since the tab on my engine appears to be bent, I don't want to simple straighten it out and assume when the damper mark is on zero, that I am at TDC.

I don't want to pull the valve cover. I know how to determine that I am on the compression stroke for cylinder 1. When using a TDC threaded tool in the spark plug hole, isn't there a chance of damaging the top of the piston? Those tools seem to be designed for a plug that would be closer to a 90-degreed angle relative to the piston top.

The crank will be turned by hand obviously, but not having done this before, will the contact of the piston top and the tool be obvious enough to feel when using a breaker bar?

[Chuck G.]

Re: Timing Tab & TDC

The timing tab looks OK to me for a 63 340 car. Looks like it's bent up a bit, not bent forward or back. I would imagine that if you pushed it downward with your thumb, you could align it more "parallel" to the harmonic balancer.

Although I cannot see the markings on the tab for "O", "A", etc. it looks like you have a white mark where the "O" should be on the tab and a white mark on the TDC line on the balancer.

I am NO engine rebuilder... I'm a backyard mechanic, but in my lifetime, I've determined TDC using a piston stop in #1 cylinder using a breaker bar to turn the crank. It's easier if you remove all of the spark plugs.

Turn the engine clockwise till you hit the stop. Mark the balancer using the "O" mark on the tab. Turn it counterclockwise till you hit the stop and mark the balancer again.

TDC will be the halfway point between your two marks.

That'll give you an idea of how accurate the mark on your balancer is. The outer ring of the balancer has been known to "slip" giving inaccurate readings. There are also other 8" balancers from other motors that fit...but have the TDC mark in the wrong location for your 327/340 car.

[Thomas S.]

Re: Timing Tab & TDC

I was concerned that I wouldn't be able to feet the piston stop without damaging the top. It turns out the groove on the damper was in the correct place so all I had to do was to bend that timing tab back square to the timing cover and I was good to go. I use a dial-back timing light for timing, but still need to have a zero reference.

[Chuck G.]

Re: Timing Tab & TDC

I was concerned that I wouldn't be able to feet the piston stop without damaging the top. It turns out the groove on the damper was in the correct place so all I had to do was to bend that timing tab back square to the timing cover and I was good to go. I use a dial-back timing light for timing, but still need to have a zero reference.

Unless you have the strength of Godzilla, I doubt that you could damage the piston top.

Glad you have it figured out.

[Dale M.]

Re: Timing Tab & TDC

Just for your information, there is a "top dead center whistle tool". I have not used one, but as you turn your engine with #1 coming up on compression stroke, the tool will make a sound (whistle) and stops as soon as you come to TDC. Look them up on e-bay or somewhere. they are supposed to be accurate, but any responses are welcome.
Good Luck,
I have used the piston stops, and as long as you aer careful you should not damaged your piston. I always thought of something made out of plactic to stop the engine would be great. You must be careful to rotate both ways and get the middle setting. A degree wheel is best.

[Richard G.]

Re: Timing Tab & TDC

I believe any whistle tool would be designed only to identify the compress stroke from the exhaust stroke.
It is unlikely, well impossible, to get an accurate enough sound to identify the TDC of an engine from one.
The piston stop method works well. It will give consistent and repeatable results.

[Dale M.]

Re: Timing Tab & TDC

Richard, I agree with being accurate, I have not experienced but was told they work very well. Likely with just street cars, it's sufficient, but have no experience or ability to evaluate. It would be really good to try both methods and see how accurate it is.
Maybe I will order one and test it. I have been putting several EFI units on these older street cars, and you are never sure about timing marks with some of the custom timing covers.

[David M.]

Re: Timing Tab & TDC

I just posted this on ClassicOldsmobile dot com

With #1 spark plug removed, stick your finger in #1 spark plug hole while someone (or you using a remote start switch) bumps the engine over until it spits your finger out. Now #1 piston is at or about TDC.

Now, look at where the mark is on the crank damper. Should be somewhere where you can see it. Now move the engine(damper, by hand) to line up the mark on the damper with 0* on the timing tab. You are now at top dead center...TDC.

With the distributor cap off the rotor should be pointing towards #1 cylinder and the #1 tower on the cap. If it doesn't its off a tooth or two.

Put it all back together and fire it up. With a timing light, move the distributor (with the engine running) until the mark on the damper lines up with 10 or 12* BTDC on the tab(with the vacuum advance plugged off, no vac leak). (10-12 may not be correct for a 327, it is for a BB Olds.)

Once the engine is running and timing is adjusted hook the distributor vacuum advance canister back up to the carb base or manifold vac source and adjust the carb air fuel screws.

A vacuum gauge hooked to a manifold vac source will greatly assist you dialing in the A/F mixture. You want to obtain the highest vac reading possible alternating back n forth with both the timing and A/F adjustments.

Let us know if this helped

[Troy P.]

Re: Timing Tab & TDC

It would spit your finger out on the exhaust stroke as well. In that case the rotor would be pointing at which tower on the cap?

Also lining up the tab and damper lines could give you a false TDC if the damper has slipped or your engine has the wrong tab/timing cover. But your instruction should work under normal circumstances.

Since we are dealing with such old cars that could have abnormal issues the piston stop method of determining TDC is the best way to go first time around until you determine everything is/isn't in order.