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8.1 - finding TDC

bobct

Advanced Contributor
How do I do that w/o any timing marks? I tried using the screwdriver method but it gets jammed, I think because the pistons don’t have a flat top. I did this on my 454’s and it definitely does not feel the same. Do the 3 holes in harmonic balancer mean anything? I also don’t see a rear timing cover but just took a cursory look.

Since the timing isn’t adjustable, nothing jumps out in the manual. I want to do a leak down test on each engine as a baseline.

thanks
Bob
 
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How do I do that w/o any timing marks? I tried using the screwdriver method but it gets jammed,
Be careful...... you can cause damage doing that.

Do the 3 holes in harmonic balancer mean anything?
Not for your purpose.

I also don’t see a rear timing cover but just took a cursory look.

I want to do a leak down test on each engine as a baseline.
Since this is all you are wanting to do, there is no real need for an accurate #1 TDC Compression Stroke location.

However, if you want to know where #1 TDC C/S is located, you will need a PPS tool.
At near TDC (as close as you can estimate) make a mark on the balancer and a corresponding reference mark elsewhere.

While NOT near TDC, the PPS tool will be threaded into the #1 spark plug port.
Bring the crankshaft around around until the #1 piston gently touches the PPS. Be gentle!
Make a new mark on the balancer.
Now rotate crankshaft in the opposite direction until the #1 piston once again gently touches the PPS.
Make a another mark on the balancer.

Shoot for a 30* +/- spread.
If not near 30* +/-, increase the depth of the threaded PPS stop portion, and repeat the process.

With these two new harmonic balancer markings equally divided, you will have a center mark.
Now remove the PPS tool.
Rotate crankshaft until the "center" mark aligns with your corresponding mark, and you will be a TDC (not necessarily on the C/S).

In order to perform your cylinder leak-down test, you will begin with #1 at TDC on the C/S.
Perform your leak-down test.

You will now rotate the crankshaft 90* of which will place the #8 cylinder at TDC C/S.
Perform your leak-down test.

Continue this 90* rotation stopping at each cylinder that is next in the firing order.




thanks
Bob
 
Rick, are you saying that “close enough” on TDC is acceptable for a leak down test? I can get it close by using the hose from my compression tester and feeling the air blowing out.

Since I’ve used the screwdriver method before on my 454, I wasn’t expecting it to jam up the way it did. I caught it pretty quickly but had to counter-rotate to free it up. I can see a scratch mark on the top of the piston in the carbon, I hope it didn’t do any other damage.

Thanks
Bob
 
Rick, are you saying that “close enough” on TDC is acceptable for a leak down test?


Correct!
As long as the piston is very near the top of the compression stroke, and as long as both intake and exhaust valves are closed, you can perform the cylinder leak-down test.
 
I have one problem with a leak down test done at TDC only.


FYI.... Had a 275 Mercury Verado that was pushing oil out of the valve cover.... Passed a leak down test with less than 5% leakage @ TDC of compression stroke. As soon as you let the piston slide down the bore approx .75" off of TDC the cylinder failed leak down and it was pushing boost into the CC.

So in short, I do a compression test with 4 engine revolutions. If the numbers are good and the engines perform as they should that is the end of my test.... If numbers are off, then I do a leak down test on the weak cylinder and let the piston slide down the bore slowly to check the whole cylinder.

Hasn't failed me yet.
 
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