LT4PWRD

I had a reputable engine builder assemble my short block (LT4). I provided them with the LT4 hot cam as well as the Heavy Duty Timing Chain and Sprockets. When I picked it up, I noticed that the dot on both the crank sprocket and cam sprocket were both at 12 o'clock while #1 was @ TDC. From memory, I thought that the crank dot should be at 12 o'clock and the cam sprocket at 6 o'clock, but I wasn't positive. I asked the builder this question and he assured me that he had degreed the cam in appropriately and this is where the dots ended up. I'm not sure if he looked at the manual or not. This configuration also put the cam dowel pin at 9 o'clock. When I got home, I looked at 3 different manuals (Helms, Chiltons and Haynes). Each indicated that the cam sprocket dot should be at 6 0'clock and the cam dowel pin at 3 o'clock.

Now for the question, is this alternatre configuration ok. I am thinking that I need to turn the cam sprocket 180 degrees, but what do I know. If I do this, do I need loctite for the cam sprocket bolts? By the way, in the manuals, the 1991 and earlier models do have the 12 o'clock cam sprocket configuration. Any advise appreciated.

Thanks,
Chris - 1996 LT4

Bill Reid

Chris,
I'd think the cam gear alignment dot is supposed to be in the 6 o'clock position with the crank alignment dot in the 12 o'clock position. May want to call someone at Exotic Muscle here in town...verify it.

tdock

The crank turns twice for each rotation of the cam shaft, so that when the dot on the crank sprocket is at 12:00, the dot on the cam shaft sprocket can be at either 12:00 or 6:00 (that's 6:00pm), so it sounds like you're OK.

Goody

Your LT4 is not a L98 so don't use that for comparison. You looked it up in the Helms book and that is the bible...go with that. Talk to Chris at Speed demon. :seeya

JAKE

You're good to go.

I had this same concern when I first started out screwing on these engines 40 years ago.

Most of the manuals show the cam gear at the 6 o'clock position and the crank gear at the 12 o'clock position. What they fail to tell you is that when the gears are in those positions, the #1 is NOT on the compression stroke, but the #6 is. The #1 is on the exhaust stroke.

When the #1 is on the compression stroke (remember the #1 - and all the others for that matter - are at TDC twice, once on compression and once again on exhaust) the camshaft gear will be at the 12 o'clock position and the crank gear will be at the 12 o'clock position also.

This has tripped up guys lots of times, so don't fret. The manuals should, but seldom do, explain this.

Manuals use that illustration because it's easier to see if the two "dots" are in alignment when the cam gear is at 6 o'clock and the crank gear is at 12 o'clock.

Hope this helps.

Jake

0ski_dwn_it

They were both at 12 O'clock when I did my engine with the 219 cam. Your kinda locked in with both of them as to their position because of the following:
The crank has a keyway and unless you have the type that has the 3 keyways (one for advancing and retarding the timing) you have to put it on only one way. Keep in mind this is assuming you are at TDC with #1. Then for the cam. There is a pin that you want at 9 O'clock. This then positions the cam sprocket at 12 O'clock. The trick is getting the timing chain on without moving the cam sprocket off a tooth. There is a bit of a trick to that I found, but too much info for here. Unless someone wants it. Once together you can run a straight edge through the centerline of the crank and cam and the two dots should be right in line also. I would say you are fine.

LT4PWRD

I'm still a bit concerned. The point that got me is that on one page (91 and earlier LT1's) it emphasized that the dot's should both be at 12 o'clock, while on the very next page (92 and later models LT1/LT4) it shows and describes the dots as being at 6 o'clock for the cam gear and 12 o'clock for crank. The builder did explain the compression cycle as well as the exhaust cycle for TDC and ensured me that he was correct on this one. But why would three different manuals, including the Helms all describe it as 12 o'clock and 6 o'clcok? I'm still not sure which road to follow. Anyone else with any experience on this subject?

Thanks,
Chris

0ski_dwn_it

You can also tell when you rotate the engine around it should go from compression (w/ both valves closed) to Past TDC. then somewere past that point the Exhaust should begin to open. Then remain open till the piston comes back up. There should be a bit of overlap were the exhaust is closing and the intake is opening and the piston will go back down again to fill the chamber with fresh air and then begin its journey back to TDC with both valves closed for compression/combustion. If he had these off the sequence of events would be all screwed up from the way they are descrided here. if the valve covers are off, play stupid and ask him to rotate the engine by hand and you will see what I have described. Not sure exactly what sequence would occure if he had it backwards, but I would imagine the intake would open past TDC and the exhaust would open on the intake stoke of the piston. Hope I wrote this all correct, I was kinda rushing. Hope it helps. Also, comp cams has a nice explination of the above with diagrams. Check it out if you like.



[Modified by ski_dwn_it, 9:11 PM 3/8/2002]

EricVonHa

Here's another spin on how to figure this out (besides posting in the "Engine Mods Section")

My understanding is that at TDC, both exhaust and intake valves are closed. Then, rotate the assembly and the intake valve should open... this is the intake stroke/compression stroke. Next 360 crank rotation and the exhaust should move... Don't need the heads installed.. just lube 2 lifters and drop them in.. Watch the results!

0ski_dwn_it

haaaa haaaa beat you to it! :lol:

EricVonHa

No way, my time is "Stamped" the same as you're time!! That means... we both gotta split the fees on this one.. Last time a tie like this occurred, I only got $5000.00 of the $10k purse. Man, imagine if they started posted down to the exact second... we wouldn't be splitting any of the fees!! :D

0ski_dwn_it

Yeah but I typed more, doesn't that count for something? :p:

EricVonHa

No way dude, typing more doesn't mean more dough. Whoever makes the most meaning with the fewest words (ancient chinese proverb rule applies) takes the cake... we gotta refer this one to the judges for discussion... I only made $15k this week from all this consultation stuff!! ;)

JAKE

Chris, relax. When the dots are at 6 (cam) and 12 (crank) that's just for installation of the camshaft gear; just to make it easier to align the dots. Being so close together it's easier to see if they're off one or more teeth. With the dots at 6 and 12 that configuration is NOT #1 compression stroke. That configuration is #1 exhaust stroke, or said another way #6 compression stroke.

If you set the dots at 6 and 12, then drop in the distributor, align the distributor rotor with #1 and fire the engine you'll get one hellofa backfire. Why, cause the phasing of the cam and crank is wrong.

All you'd do then is remove the distributor, turn the engine untlil the cam dot is at 12, then drop in the distributor and it'll fire right up.

With the #1 on the compression stroke, here's what you'll see: Crank Gear at 12 o'clock, cam gear at 12 o'clock, cam dowel pin at 9 o'clock.

With the #6 on the compression stroke, here's what you'll see: Crank gear at 12 o'clock, cam gear at 6 o'clock, cam dowel pin at 3 o'clock.

I wish I had a dollar for every time I've put a distributor in 180 degrees off before I learned the right way. As i said the manuals, Chilton, Motors, etc., don't explain this.

I don't know how else to explain this. Maybe someone else can do a better job than me.

Hope this helps.

Jake