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Why does valve lash change?

General engine tech -- Drag Racing to Circle Track

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David_D.
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Why does valve lash change?

Post by David_D. »

I have a friend that seems to be getting different valve lashes at different points when setting the lash. He is experienced in setting lash, so it's not for a lack of know how. He has tried every method in the book, and still seems to find the lash changing. Every thing I can find suggests the EO/IC method.

What he is finding is when he sets the valve lash, then rotates the cam, the lash gets .004 looser. I have also seen this on my engine, but haven't really given it any thought.

Can anyone explain why this is? If the cam manufacturer has a method that suggests the lifter will be on the base circle, and you follow that method and come up with variations, then the lifter must not really be on the base circle.

Thanks for your thoughts gurus!
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Post by bill jones »

-what engine and valve train are you talking about?

-are you asking about a first time lash of a fresh component engine?----or after the engine has been previously adjusted and then run?

-if it's a first time adjust--are the valves being adjusted one at a time?----or all the valves/cam lobes being partially preloaded?

-are you hand rotating the crankshaft or using the starter motor?

-has anybody actually checked the center camshaft journals using a V-block on the two end journals?---looked at journal straightness?----straightened the camshaft?---then looked at the basecircles for runout once the cam was straight?

-If the engine has been run and is then being re-checked for lash----has anybody retorqued the head fasteners?

-what is being used for valves?---steel or titanium?----how much valve spring pressure?----if it has pushrods how wimpy are they?----how many times does the engine approach valve float between valve adjustments?

-does the cam have some sort of positive stop for forward thrust?

-does the engine use a timing chain/s or a belt/s?

-are the heads and block cast iron or aluminum?
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Post by jdavis »

Could be a whole long list of things. Cam base circle runout, differing amount of wear on the various needle bearings, cam not perfectly straight, things not being concentric.

When I set lash I bring each cylinder to tdc on compression and follow the firing order around the motor (man that ati damper is handy). That is the only way I have found that gets me results i'm satisfied with.

I am not old enough to have messed with them, but if i recall the old 30-30 cam if you use eo/ic method you will still be on the lash ramps not the base circle. Maybe I just dreamed that, surely somebody will know.
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Post by CamKing »

Either the cam has runout on the base circle, or the cam is flexing.
Have your friend pull off all the rockers, except one. Check that one , and see if you still have the rubnout.
You'd be suprised at how much some cams bend under the pressure of big springs.
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Post by David_D. »

This is on a small block chevy running a comp solid roller cam with matching springs. I don't think the springs are overly aggressive (400LBS open). Heads have just been freshened, new lifters. This is the initial lash adjustment. No run time on the engine. He's adjusted them, then went back to double check and they have been off. He has tried the TDC firing order method, then tried the eo/ic method and found variations in the lash.

It's not an issue for me, I'd run it, but he's just wondering why/what the possible causes are.

Thanks for the time/opinions.
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Post by Engguy »

So what happens when he goes through it, like 4 to 5 times?
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Post by bigjoe1 »

The most full proof method I have ever seen is this--- Turn the engine over by hand( bolt on front of crank ) notice when the valve you want to adjust is at full lift ( all the way open ) make a mark on the balancer with pencil ( whatever ) now turn the engine over exactlly one time ( 360 degrees ) to the exact same mark.. Now adjust that valve --- this is the only way to be exactlly 180 degrees from when the cam was at full lift--- This is absoluly the most accurate way to set valve lash on your engine

JOE SHERMAN RACING ENGINES
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Post by mike fabish »

Listen to Big Joe 1. Absolutely the best way, learned this from Racer Brown. We make that "mark" for each valve on the dampner while the engine is on the stand. We then make a chart listing all of these marks in rotational order and then use the chart every time we check lash. By following the chart the checking process is PRECISE and Fast. IMHO this is the best way to monitor your roller lifters. After using this method you will
notice how temperature affects valve lash. On cold or cool mornings you will notice the lash will be slightly tighter, warm temperatures loosen the lash slightly When your hitting the exact spot every time you'll notice things that you didn't catch with the "other" methods. Mike......
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Post by Racerrick »

The cam is bending like camking says. I use the when the intake has just closed set the exhuast, when the exhuast has just open set the intake
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Post by Tuner »

I’ve seen both cam follower and rocker arm rollers out of round .002” - .003”.
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Post by F-BIRD'88 »

What happens if you were to set the lash .004" tighter to begin with?
Does the valve lash then re-check where you want it?
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Post by David_D. »

Joe, thanks for that method. I'm going to use it and do as Mike has suggested and make a chart, etc. that way it's quick and easy.

He set the lash at .020 using the EO/IC method. He then checked a valve when the one next to it was at max lift, and got .024. With this information and the responses to my post, I'm fairly certain it's in that particular method of setting the lash.

Good info as always. This site is great!
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dan miller

Post by dan miller »

This thread begs the question of how much deviation from the intended lash is acceptable.

I'd suspect that the lash moves around more than one would think as the engine thoroughly warms and valves are getting brutalized during the pass.

We set both valves at TDC. The lash might open up or close down at some other position, but it's all relative anyway. We've determined the lash that runs the best, as measured at TDC.

I like Joe's idea of checking 180 degrees from peak lift, and will measure ours that way to see what differences there are.

Danny
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Post by John Wallace »

To me it sounds like the lash is different when using a different method of checking it?

If he sets the lash at .020" by the EO/IC method, then goes back and re-checks it with the EO/IC method , has it changed?
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Post by 2xmod »

Do the roller tips on the rocker have slight flat spots. Ive seen that once.
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