verano29

I'm torquing down the rockers according to the GM service manual. According to the GM service manual picture TDC #1 is when both dots (the crank and the cam gear) are pointing up (12 o'clock).

It says when #1 is TDC torque exhaust of 1,2,7,8 and intake of 1,3,4,5
Then rotate engine 360 and torque exhaust of 3,4,5,6 and intake of 2,6,7,8

It seems there is some disagreement as to where TDC #1 is, some say its when both dots are facing each other others say its when both dots are at 12 oclock. Which is it?

I followed the manual and did the procedure when both dots were pointing up (12 oclock) at it certainly seemed that the torquing sequence was correct (that all rockers were off of lobe lift).

Check this thread for the GM Service manual description:

http://forums.corvetteforum.com/show....php?t=1921903

BatmobileSS

iTrader: (8)

TDC for number one (first one on drivers side) is the dots facing eachother. Dot on the cam gear straight down and dot and the crank straight up.

ChrisH821

Well they are both TDC... the crank spins twice for every one revolution of the camshaft, thus the #1 piston is at TDC two times (dot to dot and both dots up). I think the spinning 360* part is so you arent torquing down against the valvesprings causing false numbers.

Last C5

Both positions are TDC, but only the position with both dots at 12 o'clock is TDC of the compression stroke which is what you want for valve work.

DaddySS

They are telling you to torque both intake and exhaust for #1 so that should be compression TDC. Take the plug out, put your finger over the hole and turn it over until you feel the compression, that should be dot to dot TDC.

verano29

Yes I want TDC of the compression stroke. I can check that easy enough I guess. If TDC of the compression stroke isn't when both dots are at 12oclock (as opposed to both dots facing each other) I'm going to be sending GM a bill for wasting my time for thier poor quality control on thier products (the GM service manual being one of thier products).

Randy WS6

iTrader: (69)

Compression stroke on #1 is at 12 o'clock and compression stroke on #6 is dot to dot ...

verano29

Awesome! Compression stroke TDC is both dots at 12 o'clock...looks like I won't be sending a bill to GM after all!

69GMC/99LS1

I think you better look at that picture again...that shows TDC for #6. Further down in the thread someone else noticed this too.

TDC #1 piston is cam gear mark at 6 oclock, crank gear mark at 12 oclock.

The FBody service manual has the correct photo.

eallanboggs

iTrader: (1)

It's encouraging to see so many people who are willing to tear their engines apart who don't even know basic 4 stroke theory. Maybe encouraging isn't the correct word. Scary is probably a better work.

KCS

iTrader: (20)

The whole point here is to torque down the rockers when both lifters (intake and exhaust) are on the camshaft base circle. Pull the plugs, rotate the engine until neither pushrod moves (just after the intake closes) and there is no more air being pushed out of the spark plug hole. Torque #1. Turn the engine 90* and then torque #8, and continue on through out the firing order turning the crank 90* before moving on to the next set of rockers. This is about the safest way to do it.

code4

iTrader: (30)

Just watch the rockers when they both go down tighten them. You are just trying to get the spring pressure off of them so they torque properly.

verano29

Does anybody have their motor apart (at least the cam/crank gear showing). Can you quick put an end to this and determine if TDC for #1 of compression stroke is when both dots are at 12 o'clock or when they are next to each other (12 o'clock for crank and 6 o'clock for cam). I could have determined it quite easily but since I knew the position of my crank when I began the process I just assumed tdc on compression was when both dots were at 12 o'Clock.

I'm betting that its when both dots are 12'oclock. I say this because following the procedure it sure seemed like the correct lifter was off lobe lift.

verano29

Where's vettenuts, he'll know the answer for sure.

-Joseph-

iTrader: (12)

With both dots at 12 and 12 its the firing position for cylinder #1 or TDC of compression stroke.

When the dots are together that is the start of the intake stroke.

verano29

If you look at the service manual clip in the link you can see why its so confusing. It shows the two dots at 12 o'clock but the cam dowel would indicate its at 6:00 o'clock. It then goes on to describe the crank key as it would if you were looking at the motor but then describes the cam dowel as "it will be at 10:30 if your looking from the rear of the motor." I bet this nuget of technical writing was part of an off-shore outsourcing project.

verano29

That's what I want to hear....but does that make it correct?