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Old 02-20-2011, 01:48 PM
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Need help with SBC cam install (timing marks question)

In the disassembly phase and I'm noticing something strange with my timing marks. At TDC on cyl. #1 on the compression stroke, the cam gear "dot" is at 12:00 (not 6:00 as it should be). The crank gear "0" is at 12:00 (good). Cam dowel is at 9:00. Distributor rotor at #1. Engine ran fine like this. I have a new Crane cam ready to go in, but I'm concerned, because I had expected Cyl. 1 to "hit" when the cam gear timing mark was at 6:00, and one of my books mentions the dowel at 3:00 (not 9:00). Any idea what gives? This is my firs cam job, and every photo I have seen shows the cam mark at 6:00 and crank at 12:00. The engine was rebuilt. Builder cannot be reached. I can certainly duplicate this setup using the new cam, and it seems I would have to if I expect the engine to run. Advice appreciated. Thanks!

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Old 02-20-2011, 02:01 PM
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It is correct as you took it apart. With the cam gear at 12:00 and the crank gear at 12:00 you are at TDC firing #1 cylinder. With cam gear at 6:00 and crank gear at 12:00 (as recommended for ease of aligment at assembly) you are at TDC exhaust stroke on cylinder #1...and you are actually at TDC firing #6 (1/2 way around the firing order).

Dots aligned at 6:00 and 12:00 is not TDC firing #1, it is just for cam to crank assembly. If you drop the distributor in the engine at this time pointed at #1 cylinder on the cap you have just installed the distributor 180 out of alignment. This fact trips up a lot of inexperienced mechanics who assume thet cam and crank dots aligned = TDC firing #1...it doesn't... it is really TDC firing #6.

You have to seperate cam to crank timing from distributor timing in this case.
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Old 02-20-2011, 02:02 PM
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The cam runs at 1/2 speed. Every other revolution cam is at 6 and crank is at 12 or the cam is at 12 and the crank is at 12. It makes no difference when assembling. Cam can be at 6 or 12..
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Old 02-20-2011, 05:14 PM
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ZEEB, having the crankshaft at #1 TDC is insignificant. You are aligning the camshaft with the crankshaft. Just put the crankshaft gear dot at 12 oclock and then the camshaft gear dot at 6 oclock. Install the timing gear set and you are good.
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Old 02-20-2011, 07:41 PM
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Got it guys! Thanks. You really helped out.
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Old 02-20-2011, 07:57 PM
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Quote:
Originally Posted by ericnova72
It is correct as you took it apart. With the cam gear at 12:00 and the crank gear at 12:00 you are at TDC firing #1 cylinder. With cam gear at 6:00 and crank gear at 12:00 (as recommended for ease of aligment at assembly) you are at TDC exhaust stroke on cylinder #1...and you are actually at TDC firing #6 (1/2 way around the firing order).

Dots aligned at 6:00 and 12:00 is not TDC firing #1, it is just for cam to crank assembly. If you drop the distributor in the engine at this time pointed at #1 cylinder on the cap you have just installed the distributor 180 out of alignment. This fact trips up a lot of inexperienced mechanics who assume thet cam and crank dots aligned = TDC firing #1...it doesn't... it is really TDC firing #6.

You have to seperate cam to crank timing from distributor timing in this case.
Not trying to hijack this thread, but where does he install the distrubutor for it to be in correct firing order on the start up?
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Old 02-20-2011, 08:50 PM
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12 and 12 .alot of people make same mistake including me many moons ago.
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Old 02-20-2011, 09:01 PM
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Although I haven't progressed to that point in the job, I think I understand. Once I line up the timig marks with cam gear mark at 6:00 and crank gear at 12:00 and torque the timing gear bolts, I'll place my finger over cyl. #1 and find the compression stroke, also observing both valves closed. At TDC on #1 cyl. compression stroke, cam gear mark and crank gear mark will both be at 12:00. I'll drop in the distributor and adjust it to align whatever "tower" I want to call #1 to meet the rotor in its current position. Which is the "fire 1" position at this moment. Once #1 is determined, just fill in the rest of the wires in the proper order.
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Old 02-20-2011, 09:31 PM
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Quote:
Originally Posted by Zeeb
Although I haven't progressed to that point in the job, I think I understand. Once I line up the timig marks with cam gear mark at 6:00 and crank gear at 12:00 and torque the timing gear bolts, I'll place my finger over cyl. #1 and find the compression stroke, also observing both valves closed. At TDC on #1 cyl. compression stroke, cam gear mark and crank gear mark will both be at 12:00. I'll drop in the distributor and adjust it to align whatever "tower" I want to call #1 to meet the rotor in its current position. Which is the "fire 1" position at this moment. Once #1 is determined, just fill in the rest of the wires in the proper order.
You are absolutely correct

You won't really need to do the thumb over the hole find compression trick. With the crank gear at 12:00 and the cam gear at 6:00 after you have installed the timing set,... all you need to do is turn the crank one full revolution, which will put the cam gear at 12:00 and crank gear back at 12:00(and the engine at TDC firing #1 cylinder) and you can then install the distributor just as you have outlined.
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Old 02-20-2011, 09:53 PM
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Quote:
Originally Posted by ericnova72
You are absolutely correct

You won't really need to do the thumb over the hole find compression trick. With the crank gear at 12:00 and the cam gear at 6:00 after you have installed the timing set,... all you need to do is turn the crank one full revolution, which will put the cam gear at 12:00 and crank gear back at 12:00(and the engine at TDC firing #1 cylinder) and you can then install the distributor just as you have outlined.
OK, it makes sense now...
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