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Old 02-18-2011, 06:34 AM
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Cam/Crank timing

I am new to this forum and need some help from some experienced guys. I am a novice engine builder and have only built a few.

To start i have a sbc 406 that i am rebuilding. All new parts except the block itself. This is a moderate performance engine. My question is regarding the timing marks on my damper wheel. I lined up both dots on my timing gears. Cam=6oclock Crank=12oclock. I then installed the damper wheel, timing tab for a 8" wheel. When my #1 piston is at TDC it reads -4 on my timing tab. It is 4 degrees after the 0 mark. I found TDC on #1 by ensure both valves were closed and put my thumb over the spark plug hole until i felt the compression stroke. My cam comes 4 degrees advance from the factory. Not sure why it is NOT lining up on the 0 mark. Could my timing tab be off? Why would it be 4 degrees atdc? Should i double check with a degree wheel to make sure i am not an idiot?

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Old 02-18-2011, 07:17 AM
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timing..

first if u know for sure the timing chain marks are together then that IS tdc.. move the tab to read that. they can be bent and read off...
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Old 02-18-2011, 07:20 AM
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Quote:
Originally Posted by delawarebill
first if u know for sure the timing chain marks are together then that IS tdc.. move the tab to read that. they can be bent and read off...
I have a brand new timing tab. Is that still common for them to be off? The tab is at the 2 oclock position. My timing tab is not adjustable. How would i adjust it? Thanks for the response.
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Old 02-18-2011, 07:24 AM
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Use a dial indicator to find true TDC. Cam gear, crank, or both could, be a little off which may stack together to be + or- 4*. Use of a dial indicator and piston stop will remove all doubt.
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Old 02-18-2011, 08:53 AM
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Quote:
Originally Posted by Greg T
Use a dial indicator to find true TDC. Cam gear, crank, or both could, be a little off which may stack together to be + or- 4*. Use of a dial indicator and piston stop will remove all doubt.
I am familiar with using dial indicators bc I work at a tool and die shop. Will u please explain how and where I put the indicator to check? Thanks again for the help
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Old 02-18-2011, 10:33 AM
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The following is from another post I made on this:

Quote:
There are several different dampers. Both the diameter, thickness and the placement of the line on the outer ring of the damper can differ. So you need to know what you have, to know what tab to use.

The three most common locations for the timing mark on the damper:

The exact years of the type of damper timing marks overlap one another, depending on the exact application.

The pre-'69 damper has the TDC line on the outer ring at the 2:30 o’clock position- or 2º before- (i.e. to the LEFT of the keyway, looking at the front of the engine) the keyway centerline. The keyway is seen in the ID of the damper.

The damper used from '69 to about ‘84 has the TDC line at the 2 o’clock position- or 10º before the keyway. You'll find that aftermarket dampers are the 10º type, as are the bolt-on tabs sold in the aftermarket, unless they're adjustable.

A third timing mark was used from about 1978 to about 1995. It is at the 12 o’clock position- or 40º before the keyway. This damper uses a timing cover that has the tab welded on at about the 12 o'clock position. Professional Products lists the years for this type damper line as being ‘84-’95 and is a 6-3/4” diameter damper.

If you're unsure, or just want to check the accuracy of the tab/mark for TDC, DETERMINE TDC and MAKE A TIMING TAPE will help you to see for yourself.


The 400 should have the 10º TDC mark position from the keyway. There is just the one 8" OEM balancer for the 400, AFAIK.

You may have a pre-'69 damper on the engine. Problem w/that is this damper was before the 400 came out in '70- so it would be a neutral balanced damper, not scalloped like shown above for an external balanced 400 SBC engine.

If the tab shows 4° ATDC- and the damper is correct for a 400 SBC- the easiest solution is to just remark the damper w/a correct line for TDC and ignore the original line.
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Old 02-18-2011, 10:40 AM
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Quote:
Originally Posted by cobalt327
The following is from another post I made on this:




The 400 should have the 10º TDC mark position from the keyway. There is just the one 8" OEM balancer for the 400, AFAIK.

You may have a pre-'69 damper on the engine. Problem w/that is this damper was before the 400 came out in '70- so it would be a neutral balanced damper, not scalloped like shown above for an external balanced 400 SBC engine.

If the tab shows 4° ATDC- and the damper is correct for a 400 SBC- the easiest solution is to just remark the damper w/a correct line for TDC and ignore the original line.
I do have the xternally balanced wheel and timing tab. Thanks for all the good info. I will double check all this. My damper wheel is aftermarket.
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Old 02-18-2011, 12:14 PM
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Quote:
Originally Posted by chevyjeep
I am new to this forum and need some help from some experienced guys. I am a novice engine builder and have only built a few.

To start i have a sbc 406 that i am rebuilding. All new parts except the block itself. This is a moderate performance engine. My question is regarding the timing marks on my damper wheel. I lined up both dots on my timing gears. Cam=6oclock Crank=12oclock. I then installed the damper wheel, timing tab for a 8" wheel. When my #1 piston is at TDC it reads -4 on my timing tab. It is 4 degrees after the 0 mark. I found TDC on #1 by ensure both valves were closed and put my thumb over the spark plug hole until i felt the compression stroke. My cam comes 4 degrees advance from the factory. Not sure why it is NOT lining up on the 0 mark. Could my timing tab be off? Why would it be 4 degrees atdc? Should i double check with a degree wheel to make sure i am not an idiot?
You should start with a degree wheel, that takes all the guess work out of where the cam is in relation to the crank. Once that is found then you proceed to the damper to check if it and the timing tab are accurate. You cannot use your finger in a sprk plug hole to determine TDC. The crank swings through a lot of degrees where the piston hardly moves at this and the BDC points. To find TDC requires a positive stop, a degree wheel and the process of calculating and then rotating the crank to true TDC. Once a pointer is aligned to show true TDC, you can proceed to measure the cam events and the accuracy of the damper and pointer.

Bogie
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Old 02-18-2011, 04:00 PM
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The "finger in the hole" method is not accurate enough to find true TDC for verification of the balancer and timing tab. With the head on you will need a screw-in TDC stop that screws into the spark plug hole and a degree wheel.
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Old 02-28-2011, 04:39 PM
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Degree wheel not really needed.

Quote:
Originally Posted by ericnova72
The "finger in the hole" method is not accurate enough to find true TDC for verification of the balancer and timing tab. With the head on you will need a screw-in TDC stop that screws into the spark plug hole and a degree wheel.
If you use a screw-in TDC stop, you rotate the motor until the piston hits the stop. Then put a mark on your damper that lines up with 0 degrees on your timing tab. Then rotate the engine the opposite way until it stops. Mark the damper again. Half way between these two marks is TDC.

The method originally used to try to find TDC (finger on spark plug hole) could easily be ten degrees or more off.
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Old 02-28-2011, 05:03 PM
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The same results can be had using a homemade timing tape, for that matter. Below are instructions for the timing tape, a degree wheel and finding TDC.

MAKE A TIMING TAPE. Useful for determining total timing, etc.

DOWNLOAD A DEGREE WHEEL.

DETERMINE TDC. This will also allow you to be sure the timing tab and damper are correct for TDC
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