VikingTrad3r

hi all. did a compression test today as part of lengthy diagnosis of a crap idle.


anway, i have my plugs out for the next few hours.

i would like to find true top dead center centre tdc for my number 1 cylinder.

should i just crank engine till i feel compression on my finger which is plugging the hole, then stick a soft tipped item in the hole (thinking wood dowel i have here) and as soon as it starts to go back in, i've just passed tdc?

i'd like to verify that my timing mark for #1 cylinder is in the correct position!

cheers. VT.

don hall

May help (?):

ex-x-fire

You could loosen the rockers on #4 & add compressed air to that cylinder. It'll go to bottom dead center there by putting #1 at TDC.

Hot Rod Roy

That Summit procedure may find #1 TDC, but don't presume that's the #1 cylinder firing position! That procedure is just as likely to put you at the top of the exhaust stroke, rather than the top of the compression stroke! If so, you'll be 180 degrees out, and the engine won't start!

Tom400CFI

That is true, but not important if all you're trying to do is confirm or calibrate your TDC mark...you can get to TDC compression anytime later.


The Summit method works, but your can also do it the "poor mans way" by taking a coat hangar (or equivalent) bend into a sharp "J" so the "J" end fits through the plug hole. Slip through the hole, then "hook" it up under/inside the combustion chamber. Hold or secure it there, then rotate, mark, rotate backward, mark, measure and mark just like in the vid. Cheap, quick, easy.

VikingTrad3r

i like this. thanks.

Hot Rod Roy

If you're trying to calibrate your harmonic damper for an accurate TDC, you'd better be extremely accurate with your technique! In the video, the guy uses a felt tip marker, then centers his pointer by eye to the APPROXIMATE center of the fat ink marks. I doubt his procedure would be even as good as +/- 2 degrees of true TDC. Setting your timing to +/- 2 degrees isn't very good!

Hopefully, all he was trying to do was to demonstrate that the timing marks on his damper hadn't shifted due to a damaged damper! (He didn't say that.) The marks on the harmonic damper should be much more accurate than his technique, unless its rubber bond is broken. His big bucks engine had an expensive Fluidampr, so he certainly didn't need to do this for his engine! I think that to promote his procedure as a good way to identify the accurate TDC is just sloppy!

VikingTrad3r

i have been told to just adjust timing by 1 degree and go up to 14 advanced base timing and just choose what works best.l and if before 14 you ping back off.

is this outer space or is this valid?

HlhnEast

You can confirm TDC compression by watching the #1 rockers. Both will be down and not moving.

Tom400CFI

I'd guess that he was showing how it's done....and doing it quick/dirty in the interest of keeping the video short.

You're right that greater care in accuracy, marking and measuring is called for, than that demonstrated in the video.



EDIT: I just watched the vid again, and you're right; even the dialogue was vague. He didn't specifically say to measure between the two. And the sharpie was hokie.

VikingTrad3r

i do not understand why u cant just hold your finger over the #1 spark plug hole till you feel compression, this confirms upstroke no? then once u know you are on the way upstroke, take a piece of wood dowel and put it in the spark plug hole and when the dowel is at maximum "height" that is too dead center.

i get that im wrong but what am i missing?

Tom400CFI

For timing, you're right, hold a finger over the hole, turn it until you get pressure, then you know your on compression stroke. You can use a device (like a wooden dowel) to approximate TDC, but there are several degrees of crank rotation across TDC where the piston doesn't move a meaningful amount -you won't be able to determin precise TDC that way.


If all you're doing is timing the motor, find compression like you said, bring it up top, then use your timing marks to TDC it.

If you're trying to confirm that the timing marks are accurate or create accurate TDC timing marks, you'll need to use a technique such as the one in the Summit vid (albeit, with more precision).

mtwoolford

that's valid; just make sure that when you verify TDC you're turning the engine in it's normal rotation; if you verify it turning it in the opposite direction, the inevitable slop in the timing chain (could be a little, could be a lot) will throw you off.

desertmike1

The only true means in order to verify the Harmonic Balancer TDC/Timing mark's, is to; use a "Piston stop" There are plenty od VID's on this subject, I just found one that's not too bad..

mtwoolford

how can you trust anything a Ford guy says? just to avoid confusion, yes, that's #1 cylinder on a Ford, but its #2 piston on a Chevy.

Hot Rod Roy

The guy in the Summit video had apparently just assembled his engine, and was trying to calibrate his moveable ignition timing pointer. That's not what VikingTrad3r was asking (and that's not what the Summit guy said he was doing). To accurately adjust the pointer, he should have done this before he installed his LH head! Pistons 3 & 5 will be EXACTLY equal distance from the top of the block at #1 TDC (piston #1 up, of course).

VikingTrad3r, your procedure is just fine for verifying that your harmonic damper is good! If the rubber bond is broken, the 0 degree mark on your damper will be shifted.

Hot Rod Roy

Sorry, but the timing chain has nothing to do with TDC checks. You could have the camshaft removed from the engine, and still confirm your TDC marks on your harmonic damper.

TimV SR

Can someone tell me if the pointer on my '85 L98 is adjustable?

I was having trouble timing it a couple of years ago and found that the zero mark was off by about an inch. I poked a nylon wire tie into the #1 spark plug hole to determine TDC.

VikingTrad3r

On my 85 its not adjustable. the jagged tooth tab is like welded onto the block.

Tom400CFI

It's not adjustable on any stock Chevy. If it was off by 1", then it's likely that your DAMPER (it's not a "balancer") had slipped. Or your TDC locating technique was faulty.