I'm thinking about valve lash settings a lot in the last time. Given that on a 261 with solid lifters the lash gets wider when hot, I was questioning my self - why would you want anything more than zero lash (when cold)?
From what I know, hydraulic lifters always try to keep it zero lash, when the engine is running. So zero lash (or as close as possible) is a good thing - right?
With solid lifters we can't have that - the next best thing would be to set the lash to zero (cold) and live with whatever gap they end up when hot.
Did I miss something? Would there be any adverse effects?
Why is there a spec for anything more than zero (when cold) anyway, when the gap widens at operation temperature? Why would I want a gap in the first place - what is it for?
Solid lift cams require lash because their opening ramps on the lobes are more aggressive than on hydraulic cams, and require lash to soften the opening sequence down.
How is a lash softening this? If anything it creates an impact on the valve at opening, at closing it does nothing (at best) or even landing the valve harder, cause there is no rocker contact anymore, while the lifter is still riding on the soft ramp.
I don't get it. When I have a ramp already built into my cam, then what is the gap for (other than crating a sharp impact on my valve train at every valve event)?
Wouldn't that soft ramp work best with zero lash, so the lifter and everything else up to the valve would work like one solid piece and could closely follow that ramp, instead of the valve just sitting there and waiting to get a smack over the head with the rocker?
Dopey me! I always thought the lash got smaller as parts expanded when they heated up.
Yeah me too - but this real world test suggests that the opposite is true:
I mean they actually do expand, but - because of the sheer mass and dimensions of the head and the block, they expand much more than the valve train. If you have a aluminum block/head it expands even more.
Yeah, and when you use aluminum rods in a race engine, you have to allow more deck clearance for that as well. But the valve lash has more to do with it than just thermal expansion. I posted on the HAMB where you asked this also and told you where to have it explained better for you, so I wont repeat it here.....
A hydraulic lifter can collapse some because it has a plunger inside of it, a solid lifter has no give and requires the lash to do that instead. Other than referencing numerous sites that explain the dynamics of how these components were designed and engineered, there really isn't going to be much that I can expand on that is going to help you "get it"....
Long story shortened - the "clearance ramp" sections of a solid lifter grind were never intended to be used for lift - that's why the cam grinders card includes design specific lash specs. These specs are a compromise between duration, durability and chatter. As for the effects of thermal expansion - there are a whole lot of variables between cold winter ambient, summer day ambient and operating temperature. At initial startup the exhaust valves are the first component to heat up. If lashed too tight they cannot shed heat via the seats. Absent seat contact the heat will transfer up the stem - lengthening the valve in even further off the seat. The result will be burnt valves. The responses to your thread on HAMB include many other reasons to not experiment with zero lash on an iron block iron head engine.
That is quite a discussion over there. This all makes me glad my 153 has hydraulic lifters. My 270 has had a long good running life with the recommended lash, but the next time I'm in there I'll check it hot & cold.
Several years ago I had a noise that I was sure was in the valve train. I adjusted & re adjusted. I changed out the complete rocker setup. I checked valve height & seat pressure. I could never fix it. It ran fine but there was a noise that was definitely valve related. A few years later I changed from tube headers to cast Fentons. The noise went away. It ran better with the tube headers.