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Old 11-03-2009, 07:15 PM   #1
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Timing Chain?

Question? I have a 1968 firebird 350 with the yj engine code. YJ is the code for the 68 engine. It runs but I opened the timing cover and changed the oil pan gasket and found nylon timing gear parts in the oil pan. So I started to look at the timing gear marks and found that the cam gear was at 12:00 and the crank was at 12:00 The car was always hard to start.
The manual says that the cam gear sould be at 6:00 and the crank at 12:00 ( dots closest to gether ).
Now in 1971---1974 (maybe it's 1979 ---1981) 12:00 and 12:00 is right but my engine code says that my engine block is 1968 witch sould be at 6:00 and 12:00.
My real question is will an engine run if the timing marks are off 180 degeas? or maybe someone put the timing gears from a 1979 --- 1981 engine on my car?
I thank you for any help John H
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Old 11-03-2009, 08:11 PM   #2
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Re: Timing Chain?

The car's timing and your manual are both correct, and your car does not have a timing problem.

Notice that the cam gear is twice as big as the crank gear, so it rotates at half the crank speed.

Try this.... with both gears at 12:00, put a wrench on the crankshaft and rotate the crank 360 degrees. You will see the cam gear only rotates 180 degrees, leaving the crank gear at 12:00..... and the cam gear at 6:00..... as the manual says.

What you do have is a typical problem with old nylon timing gears. Nylon is used to reduce cam chain noise, but it is weak and becomes brittle with age, hence the pieces coming off. Replace the chain and gears. You will find that virtually all replacement sets are all-steel, with no nylon to break.
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Old 11-04-2009, 12:58 PM   #3
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Re: Timing Chain?

Agreed with MR. At 12 & 12, cylinder #1 is "up" on compression. At 12 & 6, cylinder #6 is "up" (1 is on the "overlap" stroke). It's a bit easier to align the marks with a straight edge when they're at 12 and 12. "degreeing" the cam is the "right" way, but not always easy or convenient "in the car". Is that Pontiac factory manual that says 12 & 6? Or a Haynes/Chilton, etc.?

The nylon gear was discontinued by Pontiac in '70. Iron for them from then on. The oil pump relief ball can have trouble if the nylon pieces get in it.

To make sure you don't have to "visit" it again until rebuild time, get a Comp 2112 "roller" chain (not the high-end version, 3112 is, but more than adequate). Sealed Power/Speed Pro sells it as 224-2112.

Jim
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Old 11-04-2009, 06:53 PM   #4
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Re: Timing Chain?

Thanks! guys the trouble i'm having is before I pulled the engine I put a rag in the #1 spark plug hole and bumped the starter untill the rag popped out. ( at top dead center ) This should have put me at the #1 compression stroke. And i'm at 12 &12 on my metal gears that someone esle had changed over 15 years ago. Looking at my chiltons manual it says I should be at 6 & 12.
I don't want to start the engine and bend valves if I set the gears at 6 & 12.
If my cam gear is off 180 will the engine run or will I bend valves?
Thanks John H
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Old 11-04-2009, 07:36 PM   #5
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Re: Timing Chain?

If both gears are at 12 and 12, you are fine. The cam timing is correct.
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Old 11-05-2009, 08:27 AM   #6
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Re: Timing Chain?

To add to what MR says, you're missing something fundemental.

ALL (American) V8s have two cylinders at TDC at the same time. GM and Chrysler, when the timing mark is at "0", are "up" on numbers 1 and 6. When one of them is on "compression" (both valves fully closed), the other is on "overlap" (both valve SLIGHTLY open). When your gears are at 12 and 12, #1 is "up" on compression. When they're at 12 and 6, #6 is "up" on compression. There is no possibility of bending valves under either of these conditions. The position of the rotor in the distributor is the only point of possible "error", making it 180 degrees "out of time", if you put it in "wrong".

Chilton, Haynes, etc. are written for the novice to be able to make his/her way through simple and moderate-level repairs. By telling you to put the dots at 12 and 6, they believe it's easier to get it "in time". The factory service manuals ALL say 12 and 12. If there's a descrepancy between manuals, the factory manual ALWAYS "wins". The "right" way to do it is to put both at 12, and use a straight-edge across the bolt holes for both cam and crank, and being able to "see" 1/2 of each dot. By placing them as far apart as possible (12 and 12), "deflection" is less of a possibility. The "ultra-right" way to do it is to degree the cam, making certain the intake "centerline" is in the correct position in relation to the crankshaft and piston. Degreeing the cam can be considered extraneous when using stock cams and timing sets.

Just for clarity.

Jim
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Old 11-05-2009, 08:10 PM   #7
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Re: Timing Chain?

I would like to Thank you both. Mr. Pbody Thanks for going into detail that makes sence to me know!
If I'm off 180 degrees (on the cam gear) it wont start unless I move the rotor out 180 degrees.
Thank you very much John H
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Old 11-06-2009, 12:57 PM   #8
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Re: Timing Chain?

You're most welcome. (:-
Jim
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Old 06-21-2010, 03:33 PM   #9
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Re: Timing Chain?

Have a question somewhat along this line....I have a 400 WS cast in Dec 67.......will the timing cover's I see for sale that fit the 69 - XX fit the 68 blocks...I've tried to compare and they all look the same. I know the water pumps have more bolts on the newer ones but not a big deal since i have to replace water pump also. Thanks for any info.
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