Valve to piston clearance info sought

[mike sampsel]

When I place my cam at or close to the maximum valve depression (intake) into the cylinder, I can only get about 1.2 turns (1.2 mm). It does not look like there is and valve to piston contact as I can get a pipe cleaner to slide under the bottom lip of the valve.

So it seems to me my valve spring might be squished as far as it will go?
At TDC I can easily spin off three total turns so I know my test screw can go deeper.

The cam I’m checking has a intake lift of .485 inches and add about another 0.05 inches and the valve is pushed down 0.535 inches.

How far can the stock (or OEM springs) be pushed before they cry uncle?
Suspect the piston is way down close to the bottom of the stroke, but have not studied it.

Just trying to be thorough in my rookie rebuild journey

[mike sampsel]

Well checking for valve to piston collision at maximum valve lift seemed like a smart thing. Of course the crank is heading towards BDC, so there is no chance for valve-piston collision there. As I thought the reason the screw stops after about 1 turn plus tax, is the springs are squished.

[Jonny042]

As you have surmised, the issue with valve to piston clearance is more at or around TDC between the exhaust and intake stroke.

As the piston is approaching TDC the exhaust valve could be still open or (more likely) the intake could be on its way to open, enough to interfere with the piston.

For instance, my DC43-102 cams are supposed to be timed to 4.4mm, meaning the intake valve will already have opened that much when the piston is at TDC and starting the intake stroke. That's probably one of the more extreme examples.

PS - the term you are looking for is "coil bind". Depending on your springs and retainers and cam lift, yes you might be reaching it with the adjuster screw turned in 1 turn. Your retainer could also be hitting the top of the valve seal?

[Jonny042]

You are a few steps ahead of me in your build, but I'm catching up! Hope to be going through the same process as you next week. In the meantime, here's a pic I took which might be meaningful.

[mike sampsel]

Thank Jonny42,

Yes, I learn more from mistakes than not. Coil bind at max lift is to be 0.045”. I could turn about 1.2 turns (little more) and took out the lash (.1 mm) so I’ve got 1.3 mm clearance. Need 1.14 mm, so I’m good there.

I’ve learned all my measurements were not required with the 20/21 and JE pistons, but it kept me from spending money