Engine/Transmission (1994 - 1998) Changing Valve Springs

[CThompson]

What is the best way to find TDC for each cylinder when changing the springs of the exhaust valves. I am planning to put heavier springs on so as to increase the efficiency of the PacBrake.

 

[PHaller]

You don't need to find the perfect TDC you just need to find close to TDC so valves don't fall into the cylinder because the piston is at the bottom of the stroke.

When I replaced the exhaust springs, I used the alternator nut and ran the engine backwards, the only way it will turn without the belt slipping, and watched for valve overlap on the cylinder I was working on. Watch one valve open and close and just as it is closing you will see the valve next to it begining to open too. That is valve overlap and very near TDC for that cylinder.

After you remove the keepers and pull the spring you can push the valve down until it hits the piston when set at overlap. You can push them about 1/2 inch or so, not enough to lose them. Actually, my valves never fell at all. They were pretty tight and required some force to overcome the friction of the stem seals. I found overlap for every cylinder anyway just for safety sake.

-Paul R. Haller-

 

[bighammer]

Just to avoid confusion, the valve overlap (intake opening just as exhaust closes) is at the END of the cycle. (or begining of a new one, depending on how you look at it)



To be (close to) TDC, mark where that happens, and turn the engine 360* to your mark again. You can also use the companion cylinder (look at #6 valve overlap for #1 TDC, 5 for 2 or 4 for 3) and not have to rotate the engine over.

 

[Jim Fulmer]

All the above info is correct, additionally on mine my valves were loose enough too drop down and when I had the cyl close to TDC I would compress the spring and leave the keepers installed and let it drop the 1/2" or so on to the piston then roll the motor over till it pushed the valve to the top, by doing it like that you know without a doubt the piston is at TDC cause I had to use the piston as a wedge to pop the keepers loose, then do the other valve then on to the companion Cyl as stated above, you time doing it will go like 30-40 mins for 1 and 6, 20 mins for 2 and 5 and 10 mins for 3 and 4, once your done you will wonder why you thought it was such a big deal to do.



Jim

 

[JGK]

bighammer,



While I agree with you that valve overlap occurs at the BOTTOM of the stroke, you need to roll the crank only 180* to get the piston to the top, right? 360* will put the piston at the bottom again, but it will be at the end of the next stroke. So if you first had it at the end of the power stroke, 360* puts it at the end of the intake stroke. Conversely, if it was originally at the end of the intake stroke, 360* would have it at the end of the power stoke.



And no one here wants be at the end of any PowerStroke, right?



-Jay

 

[JNailor]

No, no, no, valve overlap happens when the piston is at the top of the stroke. It's finishing the exhaust stroke and starting the next intake stroke. The companion piston on the same plane on the crank will be at TDC to fire the charge on the power stroke.

 

[Joe G.]

Dking is right. Do the valve sping replacement on the companion cylinder to the one with overlap. The firing order for all in line six cylinder engines that turn the normal way is 153624. So the companion cylinders are 1-6, 5-2, 3-4.

 

[JGK]

Overlap

Yep. Bad call on my part. Overlap at top. I was too focused on the degrees of rotation issue.



-Jay