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Old 06-12-2003, 04:49 PM
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Question SBC Timing indexing a unmarked 4.25 blower hub and custom tab

Question , on a small block v8 chevy is the timing dots on the timing gear 12 and 12 at tdc #1 on the fire stroke?

With #1 cylinder both valves closed and on the compression stroke , then at TDC fire , my pete jackson dots line up at 12 and 12.

The reason Im asking this normally unimportant question , is because I am running a 4.25 blower hub and am having to configure my own timing marks/timing tab.

When the cam was installed the engine gear was at 12 o' clock and then the cam gear was at 6' oclock(the obvious) , but Since Im having to index the timing mark,tab myself on a custom 4.25 hub The only way to get that indexed is to turn the motor over to TDC #1 and watch the cam to ensure the #1 cylinder is on the compression / fire stroke. And again at #1 fire the dot orientation is 12 and 12, is this correct. If not what would I be misunderstanding>?

If the marks are alingned at 12 and 6 on #1 tdc fire , the cam is showing 180 out. SO AM I CORRECT?

Thanks as always.

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Old 06-12-2003, 07:35 PM
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The best way I've found to find TDC is with a piston dead stop. This can be bought or made from an old spark plug by removing center electrode and ceramic and welding in a short extension to interfere with piston travel. Remove #1 spark plug, bring the engine to the start of a compression stroke, install dead stop in spark plug hole. slowly rotate engine with a wrench on crank (NOT THE STARTER) toward TDC. When the rotation stops, Mark this point on the hub or balancer with a wire attached to the block. Then rotate the engine in the opposite direction until it stops. Half way between this stopping point and the wire is exact TDC.
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Old 06-12-2003, 08:25 PM
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Ok so your talking the method used to find true tdc like when degreeing a cam. Got that.

What I want to know is if at tdc #1 on the fire stroke if the dots on a chain/gear are 12 o clock crank and 12 o clock cam gear because thats what Im getting.

Last edited by 614streets; 06-12-2003 at 08:42 PM.
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Old 06-13-2003, 10:50 AM
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If it's definately on the compression stroke and the distributor points at #1, both rockers are loose on 1, and the motor ran timed this way. I would look at which Pete Jackson set you have , as I remember they had a first and second design. One timed with the crank at 12 oclock and the cam at 6. The other timed off 2 dimple marks one lined up with the crank at about 2 oclock and one with the cam gear at about 4 oclock, on I believe the dr side idler. If rotated it may end up in the position you decribe, I've never checked it.

All Chevy small blocks with a chain TDC time with the cam at 6 and crank at 12.

Last edited by sc3148; 06-13-2003 at 11:01 AM.
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Old 06-13-2003, 11:15 AM
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Your timing marks are correct for a running engine.

when building a engine you install the crank mark at 12 o'clock. and the cam at 6 o'clock. But, then you turn the motor two full revolutions. This puts your marks at 12 and 12. The timing is correct. If you index from this point you should be right on the money

michael
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Old 06-13-2003, 03:11 PM
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Gumby if you turn it 2 revs its back in the same place! On my small blocks the cam turns half the speed of the crank, 1st full rev it would be top of exhaust stroke, 2nd back at TDC, feel sorry for you if your's doesn't do this. Sounds like its on top of exhaust stroke not TDC.
614 check number 1 on dist, that should tell. Does it run like this? if it does its right. then indicate like SC said.
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Old 06-13-2003, 03:31 PM
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614 you might want to go back and check your manuals. any rebuild manual will tell you to install the dots toward each other. Then turn the crank gear two full turns. This is how I have always done it. and it works perfect.
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Old 06-13-2003, 03:33 PM
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Sorry, thet last post from me should have been to fortyfordcoupe
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Old 06-13-2003, 03:57 PM
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Better go back to your book Gumby. If you turn it 2 revs its at the same place. The motor will go completely through the firing order and back to #1 TDC in 2 revs. This will not help with either of his questions. They generally ask you to rotate it 2 revs to align everything before finishing a fresh motor.
Let's quit arguing and try to answer 614's question.

Last edited by fortyfordcoupe; 06-14-2003 at 07:13 AM.
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Old 06-14-2003, 04:26 PM
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Hey guys thanks for the posts.

I had suspected I was correct all along , and last night I verified it on another engine.

AT TDC #1 cylinder , neither valves moving the dots always line up at 12 /12.

And when Installing the cam you line it up at 12 crank , and 6 cam , tdc.

The thing that was throwing me off was the fact that for some reason I thought that at 12 crank and 6 cam , the engine was phased to fire on tdc , but this is not true.

The marks go 12 and 12 when the engine is at tdc and neither valve is moving , fire stroke.

Oddly the 12 crank and 6 cam position used to install and index the cam to crankshaft , does not represent tdc fire , it is infact 180 at 12/6.

The reason Im sure I was confused , and probably confused others is that normally , with all engines , you just line up the dots at 12 crank and 6 cam(Which is what I did) , slap on the cover , install the damper , turn over the motor and match up the damper mark on the timing tab and then you can just watch the valves to know when your 180 or tdc fire stroke.

The confusing part for me was during this indexing of the blower hub , I watched the cam and crank dots line up at 12 and 12 when the cam lifters on the #1 cylinder were on the base circle and the piston at #1 tdc , fire. I and most likely anyone who has never had to index their own timing tab/marks on a 4.25 blower hub , would never suspect that the dots go to 12 and 12 when the engine is actually on tdc fir for #1.

And thats what I just got chocked up on , I mean who has to even see that it lines up 12 and 12 on #1 fire , when you have a stock damper and the corresponding timing tab?
The anwser is you dont , and me for me , seeing this , and not being able to point to a article or a paragraph in any of my books and say ah thats the way it is , had my brain in scramble mode.

So what I learned is that when you install the timing chain , you line it up at 12 crank and 6 cam , like we were always taught/read , but the part that you might not ever comprehend unless you have to see it , is that the #1 fires with the dots at 12 and 12.

I can personally say I would have never know this to be true if I hadnt seen it with my own eyes!

Thanks again for the responses - Will
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