Valve lash question....

[mike gerlach]

Valve lash question.

Maybe you could shed some light on a few questions about valve lash:

Why do we still see large hot lash number on roller cams?

Is there a problem with tight lash?

Seems like it would be much more beneficial to not have the valve train crashing at 20-30 thousands lash

Can a cam with a recommended valve lash of .020 (hot) be run at a tighter ??? say 10-12 thousands?

Just curious and hoping you could shed some light on the subject..

Thanks
mike

[Jay Allen]

Valve lash question.

Maybe you could shed some light on a few questions about valve lash:

Why do we still see large hot lash number on roller cams?

Is there a problem with tight lash?

Seems like it would be much more beneficial to not have the valve train crashing at 20-30 thousands lash

Can a cam with a recommended valve lash of .020 (hot) be run at a tighter ??? say 10-12 thousands?

Just curious and hoping you could shed some light on the subject..

Thanks
mike
Excellent question here.

When a cam lobe is designed, it is designed with a certain amount of lash at the lobe. The lash at the lobe is then multiplied by the rocker ratio to get lash at valve. If you just get a base cam card from cam company *X* and they don't know where the lash was designed, then the number on the card is like "Pin the tail on the donkey". Its a guess at best.

In this example, take a lobe with a HOT lash of .020". Let's say its an easy does it lobe. You'll be able to make incremental lash changes and see minimal if any effect on performance and cranking compression. Conversely, if you have a rocket ship fast lobe, .002" HOT can really make it or break it. Now factor in camshaft base circle vs designed lash, lifter diameter and wheel diameter, and you can create some real turds!

On running the lash tighter or looser, that kind of depends on how good of core you are using. Take a std off of the shelf roller core and grind it. You better stay withing .004" +/- of what it was designed for. If you get aggressive with lash outside of the parameters on a cheaper core, failure awaits you!

I use super high quality cores that are heat treated to hell & back. They take a REAL beating. Most of my cams are designed for HOT lash .008" to .022". I have an occassion where I need to get the lash loose, but I try real hard NOT to do that. My cams are CNC ground with wheel diameter, lifter size, and cam tunnel size taken into account. That way if I want a 270 at .050 .475" lobe lift, that is what I get. If you have a master that was based off of a 1.00" SBC base circle and you put it on a 55mm core with a .904" lifter and a .810" wheel, that cam is now 275-276 at .050 and God only knows at .200 how much it has grown. But again, I factor that in on the front side to avoid problems.

Hope this helps!

[mike gerlach]

Thanks Jay,

It's hard to believe just how complicated this can be.........see these are the things we never know or hear about.....

mike

[Jay Allen]

It's hard to believe just how complicated this can be
Mike you are so very right. This what most guys don't understand. It is very complicated and that is why it takes me HOURS to sort stuff out accurately. Your original question about lash is a question is something that is overlooked. And it does matter.

[Chuck L.]

Jay What the Ideal Temp for Valve train Lashing? (Mech. Roller)

[Jay Allen]

Jay What the Ideal Temp for Valve train Lashing? (Mech. Roller)
I identify *HOT* lash at the tempurature in which you are crossing the finish line. Whatever that is.

[899 coupe]

So after you have set the valves correctly HOT, Can you then go back the next day and see where you are set cold?
Can you use these cold numbers to set valve lash anytime the engine is cold? Will this be accurate?
This way my neighbors are happier LOL

[Jay Allen]

So after you have set the valves correctly HOT, Can you then go back the next day and see where you are set cold?
Can you use these cold numbers to set valve lash anytime the engine is cold? Will this be accurate?
This way my neighbors are happier LOL
Well, yes & no.

Yes, if you check them HOT more than a few times and the REFERENCE what that COLD setting is. This is the way I do it.

If you only check them once, then NO you cannot do this. Or, I guess a better way of saying it is, I wouldn't do that.

Tip: If you check them HOT & COLD let's say (5) times and you begin to eastablish a *trend*, then you baseline the cars performance. You then get to the track one day and the car's performance is mysteriously fast or slow. Check the lash, HOT. See what has transpired. Then check the cranking compression. Lastly, if you run a *good* Evac system, what's the vacuum?

Between HOT lash, cranking compression, or pan vacuum, you can almost always find a problem before it gets too bad.