dan6712cc

Im putting in a new timing chain and gears and I want to find the most accurate way of finding TDC. I have a piston stop but another post said its not accurate. I was wondering if I could bypass the whole method of finding TDC by just looking at the camshaft dowel pin?

I have new gears put on the cam and crank but the crank isnt in sync with the cam (it was moved during sprocket removal). So I was wondering if I could put the new chain on and crank the engine until the cam dowel pin is at 9 oclock which should put #1 at TDC. Then remove the chain and rotate the crank until the dot on the gear is at 12 oclock then put the chain back on. Would this work or is the cam dowel pin not a good way of judging which piston is at TDC?

Corvette Kid

The same pistons (1 & 6) will be at TDC whether the cam mark is at 12 o'clock or 6 o'clock. The difference will be which one is in firing position. It doesn't matter as long as you know this and install the distributor accordingly.

CFI-EFI

Top dead center will find itself. Get the engine to near TDC. watch the dot on the crank sprocket. When it points straight up, it will be where you want it. Place the cam sprocket on the cam without the chain. Line up the dots. You may have to turn the crank, slightly, to get the dots to line up correctly. You can use a straight edge to confirm the dots are aligned properly. Then remove the cam sprocket and reinstall it with the chain. There is no need to independently establish TDC before hand. You will be where you want to be when the dots align. Although I didn't say it, you want the crank dot at 12:00 o'clock and the cam dot at 6:00 o'clock when you do this. Once the cam sprocket bolts are tightened, you're done. Be aware with the dots at 12:00 and 6:00, that the engine is at TDC for the #6 cylinder. If you need to go to TDC for #1 for resetting the ignition timing, rotate the crank one full turn until both the crank and cam dots are at 12:00 o'clock. Simple, eh? That'll save a bunch of the monkey motion you had planned.

RACE ON!!!

Dartvader

I confess, I have never rebuilt an older Chevy smallblock, much less one of the newer ones with modern engine management systems, computer timing and so-forth. :o I have done a Pontiac V-8, a Mopar small block, a Ford small block and a Mopar slant six. Please set me straight here if my understanding is inappropriate for these newer Chevy small block set ups. I will welcome being brought into the 21st century. I hope some of my experience with these other engines is transferable. Maybe this is old school, but on my engine rebuilds I degreed in the cam with a degree wheel. I have always done this, and found the timing of the cam sometimes off a degree or two. I like to establish the true top dead center with a degree wheel also. To do this mount a dial gauge on the deck surface so it can read the movement of the #1 piston up and down at the very top of it's travel, rotate the engine one way until #1 gets to the top of it's travel, and notate the mark on the degree wheel. Then rotate the engine the other way until the #1 is all the way up, and again notate the degree mark on the wheel. The two readings will not be the same. True TDC is in the middle of these two readings. Then you can center your degree wheel and proceed to measure the cam opening and closing times for an intake and exhaust.You can also place whatever marks you need to indicate TDC on your flywheel or harmonic balancer. When I did the Slant six, I found the timing was two degrees retarded, so got a bushing to advance the cam. You can do this with off set keys as well. Again, I confess my knowledge is limited to old engines, and I dont; know for sure how the newer engines are timed, or if the timing can be changed.

You can then calculate over lap, center line, etc, and see if your cam is what you thought it was. It is alot of work, but very interesting. It forces you to get the old grey matter going to keep track of what is before TDC, and after, etc. I am sure your engine will run well without this, but it gives one peace of mind to know how these things work, and that your engine is set up right. It will also allow you to modify your cam for a certain kind of application if you want. If you are running high numerical number gears, and turning high RPM's you might want to retard the cam a degree or two. If you are running low numerical number gears, and building a torque engine, then advancing the cam a degree or two, might help it. It is just a hobby, and it is can be fun to take the time to check out this kind of thing, if you are so inclined. I'm sure someone here will give us useful feedback about my missing something here with these new engines, and I will feel thankful for the new information.

dan6712cc

So it doesnt matter where the current piston positions are at now? If I line up the cam and crank sprockets at 12 and 6 oclock, put the timing chain on, and crank the engine a few times #1 and #6 will align to TDC at the 12 and 6 oclock sprocket marks irrelevant of their previous position? I dont understand how that happens, I didnt know they self align. I thought I couldnt go by the crankshaft position since it was rotated after the timing chain was removed. It seems like its alot easier than Im making it out to be.

rick lambert

CFI-EFI stated it perffect. Alot of guys used to degree it when they were making major mods-but I don't really know anyone who has done it on a basically stock motor.

Pete K

If the dots line up with a straight edge it is fine. Off 1 tooth will move the dot either way quite a bit.

dock351

So when settting intitial valve lash, is it prefereable to have the crank and cam marks at 12 and 6; or rotate to 12 and 12? Or does it matter?

The watson guide on the web says: Bring the engine to No. 1 firing, both No 1 valves closed and timing mark zero on the timing tab, adjust certain valves, and then rotate a complete revolution to number 6 firing and adjust certain other valves.

Sort of confused. and im doing this process right now...

CFI-EFI

First, he is doing a timing set change. Degreeing a cam is not usually a part of this operation. For the purposes of getting the cam and the crank into the proper relationship with one another, having the crank sprocket at 12:00 o'clock WILL be TDC for the number 1 and number 6 pistons. It is the position of the camshaft that determines if it is TDC before the power stroke or TDC before the intake stroke. With the cam sprocket dot at 6:00 o'clock, it will be the #6 piston beginning the power stroke. Since the cam turns at half the crank speed, one revolution of the crank will put the pistons back in the same position, but the cam dot will now be at 12:00 o'clock, which puts the #1 piston at TDC before the power stroke. There is nothing "magic" or "self aligning" to this operation. The prior, independent setting of TDC is unnecessary and can only confuse and frustrate the process. Especially if there is even a tiny error in the setting. You are trying to make it more complicated than it is.

The pistons don't care what stroke they are on. It's the camshaft that determines that. For setting the valves it does matter which stroke you are on because the camshaft position is different with #1 at TDC than with #6 at TDC.

RACE ON!!!

dock351

One more humble question:

For purposes of setting lash using the Watson method (described above), can one tell if No. 1 firing position, that is when both No 1 valves are closed is when the cam pin is at 3 o'clock or nine o'clock?

CFI-EFI

I've never noticed, and why would one notice? I rarely pull the front of my engine apart just to adjust the valves. The position of the dots, tells all. If the engine is at TDC for #1 and #6 and both valves for #1 are closed with some semblance of adjustment, it is at TDC for #1.

I don't have a lot of faith in that "Watson" method, especially for a non-stock cam, with more duration. Lifters that you THINK should be on the base circle of the cam, may not be. The Intake opening - Exhaust closing is much more reliable. Of course adjusting them while running is fool proof.

RACE ON!!!

dock351

Thank you CFI EFI

the reason I asked is that i currently am setting the initial lash while this new engine does not have its timing chain cover on. As the thread started with a question about the timing pin location, I wondered if its position in its rotation would be a way of determining where one was with regard to valves in no 1 being at the compression, firing or whatever position it takes to correctly set the lash using the approach I described.
I did watch the exhaust valve opening and closing and the relative heights of the lifters as it rotated. Seems to make some sense. Now that the lash is set Ill check it again with the noise method when the engine is running w/o valve covers.

CFI-EFI

If the timing cover is off, then you can see the dots on the sprockets. I've explained how to tell #1 TDC from #6 TDC by the timing marks. As with the cam sprocket dot, the dowel pin would be in a position 180* from the one TDC to the other. I have never heard of anyone using the dowel position to determine what cylinder was at TDC. Since the cover is off, and you can see the dots on the sprockets, and you know how to tell #1 TDC from #6 TDC, isn't the dowel position obvious? I don't understand why you asked the question. Or maybe I've misunderstood the question.

RACE ON!!!

dock351

Nevermind. i dont want to irritate you any further.

CFI-EFI

The answers are staring you in the face. Take notes and you can be the expert the next time someone askes about dowel positioning.

RACE ON!!!

dock351

Don't worry, i'll never bring that subject up again.

JAKE

I've been reluctant to dive in on this one since I believe you've already gotten all the answers you need, but there are a couple of points.

We need to try to be accurtate in our terminology so that everyone's on the same page. Unless you're running a mechanical cam, you won't be setting "lash", instead you'll be setting lifter preload. Mechanical cams call for 'lash' and hydraulic cams call for "lifter preload". It may seem like a minor point, but when time comes to make the actual adjustment, the difference become readily apparent.

Setting valve LASH with the engine running is a very inaccurate way of doing it and, unless you've got preferred stock in some feeler gauge company, stay away from trying to do it that way. It'll kill feeler gauges in short order.

As far as the dowel pin location issue:

With the dowel pin the the 3 o'clock position, which puts the dot on the cam gear at 6 o'clock, the #6 piston will be on the compression stroke and the #1 on the exhaust stroke.

Now, when the dowel pin is at the 9 o'clock position, this has the cam gear dot at 12 o'clock position, the situation is then reversed. The #6 will then be on the exhaust stroke and the #1 will be on the compression stroke.

The main reason for having the CAM gear dot at 6 o'clock and the CRANK gear dot at 12' o'clock position is to make it easier to align the dots. Much less chance of being one tooth off when the dots are so close together.

Hope this helps.

Jake

JAKE

BTW, did the Rockford track ever re-open?

I remember showing up there with my 8 second roadster right after the track had been re-paved.

Whoever re-paved that track didn't know "jack" about paving 1/4 mile tracks. I did almost a full 180 about 50 feet off the starting line.

Guess you know my racing was done at that track. Good thing I had a big supply of clean underwear. LOL

Jake

dock351

Hi Jake:

I' m not sure if theRockford track is open--I work in Rockford but live nearer to the Quad Cities--near Iowa. I think it is, but I will find out if you want to know...I plan on using the Cordova Dragstrip later this year. I hope i dont do a 180 on the strip!

thanks for the info. Im new to auto mechanics, but find myself, alone, in the middle of a 383 upgrade. I realize im referring to lifter pre-load as lash. I did set the timing chain at the 12 oclock of the crank shaft and 6 o'clock on the timing chain. it works right. I understand that at 12-6 No. 1 is at TDC. and it is related to #6 at TDC at the same time. What you said is that not all TDCs are the same in relationship to the valve action. After reading CFI-EFIs commentary, i was wanting to make sure that I had everything correct, and not bass ackwards...I examined the opening of the exhaust valve at, if i recall, cam pin at 3 o clock. That would mean, using the "watson method" that I could set various pre-loads, rotate the crank once around, and do the other set. I just wanted to make sure that i was at the correct one (of two #1 TDCs) to go off and set the pre-loads--by examing the location of the pin or, by viewing the exhaust valve in #1. i don't know if this makes sense, but that is what I was trying to do. I felt like I was at TDC for setting the perload when i saw that the exhaust valve on the #1 was at a certain point.
As CFI-EFI said, it was staring me in the face; i just wanted to make sure i had it right. From what you have added, i think so. When i start the engine up, i will re-do the preload. Thanks again. Mark

JAKE

Reason I asked is I was born and raised in that neck of the woods and raced that area a lot.

Okay, glad you've got a handle on it.

What confuses many is that the piston is actually at TDC twice during each cycle/stroke (That's why you'll sometimes see our engines referred to as "four stroke" engines); Each piston is at TDC once on compression and once again on exhaust.

As CFI said, it's where the cam lobes are and what they're doing to the valves that makes the difference. That's what distinguishes TDC on the compression stroke from TDC on the exhaust stroke.

Jake