Why must a solid roller cam have lash?

I'm not disputing it needs to be that way, not at all. But why do we have to set valve lash on a solid roller cam? The same question could be phrased as why can't it be zero lash?

Because things expand when they get hotter. Like: pushrods get longer, valves get longer...well you get the idea. If you started at zero, when the engine warmed up the accumulated expansion would not allow the valves to seat. They don't run well that way.

At first I must admit I laughed a little when I read the original post. But then, I got to thinking..... I don't know why so much lash is required.

What Chuck says makes perfect sense, but two things throw a wrench into this whole deal.

One, the lash is set while hot, so the cam grind actually wants to have that lash while at operating temperature.

Second, in the case of aluminum heads, the lash grows as the engine gets up to temp because the aluminum in the head expands at a greater rate than the iron and steel in the rest of the valve train.



Greg

Huh. I set the lash cold at .016. So does it tighten up as the engine gets warm? I planned on putting a few miles on it and then resetting the lash again.
The car is running like a champ. I mean its great.
The valve train does seem a bit noisy, but I did have a hydraulic cam in it before.
So the lash is always set hot?

Bizarre...I was wondering the same exact question last night while lying in bed.

Those tight lash guys say they run at like .004 - .006 how does that work out?

.001" or .030" lash , or still on hyd. cam - lifter preload-- still got to play with the wrench

but, does the lifter and pushrod not grow in length as well when heated??

I've never ran a solid lifter but I thought the lash was done cold so that when it warms up the lash all but goes away. But as stated, everything heats up and expands.

Bill

billandlori wrote:I've never ran a solid lifter but I thought the lash was done cold so that when it warms up the lash all but goes away. But as stated, everything heats up and expands.

Bill

Thats what I was thinking too... I'm hopeful someone has more info on this.

Cam manufacturers can give lash for hot, cold, or both. Really good cam manufacfturers give both, but not every one does. If there's no specification, assume it is hot.

Some will also give different specifications according to your combos, like aluminum rockers versus steel, the strength of the pushrods (thicker walls), how aggressive the lobe ramp is, and so on. Essentially, the "flex" of the valvetrain is taken into consideration, which is why car manufacturers not only use hydraulic lifters but for a while, stamped rockers-- they are lighter, cheaper to make, but also, they can "flex" more than solid rockers, so no lash adjustment needed.

Lash is mainly to accomodate for the expansion of metals in the valvetrain, but also for the things just mentioned.

Thermal considerations aside the reason lash is used on a solid lifter cam of any kind is so that when the valvetrain actually gets picked up it happens with a known velocity and acceleration. If the cam is run with no lash small errors/runout in the base circle of the camshaft would cause large variations in the duration of the lobe/how long the valve is off the seat and could even open the valve slightly while the lifter is on the base circle of the cam if there was any base circle runout. Remember that with the cyclic loading that's on the cam it's always going to bend and flex around to some degree in a running engine so without lash the timing would vary much more thus with some lash in the system these errors are eliminated where they matter the most and that's when the valve is closed. If the lift varies by a few thousandths once we're up on the lobe the error is much less important.

Even zero lash/lifter preloaded hydraulic cams have clearance ramps. These ramps are very short but they do pick the valvetrain up and set it down gently for a few degrees to allow for the amount that the lifter collapses when the valvetrain is operating.

Great explanation, Dave. Makes perfect sense.

That makes sense Dave.

One other thing I was wondering, doesn't the lash act as a hammer on the whole valvetrain? I guess the clearance ramps take this up.

Bill

my comp cam solid roller comes with a spec of .028 I used this during engine break in then reset the valves at .019 Did I tighten it up too much? I am usine shaft mounted roller rockers. Its with aluminum heads

How would this effect the relationship between rpm and peak torque?

My alum headed deal will increase .004 when hot.

I see a problem with the base circle runout -- setting the lash on the base circle would pick up the difference's , that is if all the opening ramps were cut the same spot (core radius)

Setting the lash with the roller on the base circle will pick up the differences so to speak but it allows the pick up point of the valve train to happen within a certain area where there is a clearance ramp that's designed to take up the slack with minimal shock.

Have you ever messed with a valvetrain that has NO clearance ramps? Some of those are very sensitive to lash. A few years ago I was building a Ford 8N engine for a customer. I had the cam and valvetrain in the engine and I had adjusted the lifters so that the valvetrain had quite a bit of lash. On the end of the cam I had mounted a handle so that I could turn the cam easily. Just turning it by hand at say 60rpm the amount of noise produced was AMAZING! It was VERY loud. After I set the lash to spec which was about 15 thousandths I believe it was much quieter. I don't think that modern solid lifter valvetrains are quite so lash sensitive.

This brings up another point about lash. I'm thinking that if the valvetrain needs some room for thermal expansion and to compensate for base circle runout why does the lash get set to some rather large number, 15-35 thousandths. But then I thought about it and maybe why these sort of numbers are used it because it's much easier to set instead of using a .003 feeler gauge which would be difficult. And it makes it less sensitive to minor variations in lash setting technique.

old grind solid were diff. than today . Today if you take a close look at flat cams hyd. & solid -- same lobes on a lot of them . fits all type of thing

The cam company i got my cam from said to set the lash at .026 hot . I set them cold at .023 and ran the engine , pulled the covers and checked them and they were dead on .026 hot . So the head expands away from the block and takes some of the lash out. Now i set them at .021 , just messing around and i found it liked alittle tighter lash.

The other big reason is, the ramp right at the base of the lobe has a deceleration area built into it to slow the lifter and as such the valve to keep it from slamming shut on the valve seat and causing problems , one being bounce. You can buy a tight lash cam , I have seen them a tight as .006 to .008 lash.

Hope that helped

The valvetrain will be happiest at the tightest lash you can get without holding the valves open. As previously mentioned, cam companies can't take everything into account so they give a number that will work with the lash ramps their cams have and call it good. Most cam companies list their lash hot with cast iron head/block combos. Going to aluminum heads typically means about .002" tighter than listed, if not more. Aluminum blocks require even tighter lash depending on the distance from the lifters to the deck. I'm sure there's a formula to figure it out based on aluminum expansion but I don't know what it is. If you run too much lash the lash ramps don't get used and then the valvetrain does indeed get hammered.

The valvetrain will not necessarily run quieter or have less wear and tear with tighter lash as long as it's being run within the correct range of lash for the lobe to begin with. In fact when you increase the rocker ratio you also must INCREASE the amount of lash so that the pickup point of the valvetrain remains the same. Divide recommended lash with the stock rocker by the ratio then multiply that number by the new ratio to get the correct starting point for lash.

Good info Guy's. Thanks to all.

Dave , thanks for your reply . At the end of the day , all one has to do is start with the recomended lash that's on the cam card and go something like .004" +/- from there to see what the engine wants .