Is My Cam Going Flat?

[rogeroadster]

I've got a rebuilt B block with a Cragar head. Virtually all of it is rebuilt or new including re-ground cam and new lifters. I've had a lot of break in problems with the side draft weber carb. I've got one loud lifter at all rpm. So with the exhaust valves set at .013 on the number one cylinder and I back the engine off about 180 degrees so the lifter is on the heel of the cam, the valve lash grows to .025. I can't say that I've ever experienced this. Any ideas out there?

 

[Patrick L.]

I guess I don't understand. How are you setting the lash ? It should be set on the heel. Is this something just noticed or has this recently changed ? Are you having a tight guide issue ?

 

[Kurt in NJ]

If the clearance changes on the base circle that lobe is not ground correct

 

[rogeroadster]

I guess I don't understand. How are you setting the lash ? It should be set on the heel. Is this something just noticed or has this recently changed ? Are you having a tight guide issue ?

You are correct, is didn't explain this very well. I set it a .013 on the heel and rotated the crank in reverse about 180 degrees and check the clearance and it has grown to .025.

 

[J Franklin]

? Between what and what, and what are you measuring?

 

[johnneilson]

You are correct, is didn't explain this very well. I set it a .013 on the heel and rotated the crank in reverse about 180 degrees and check the clearance and it has grown to .025.

one of two things;

1. you are setting the clearance on one of the ramps and then rotating off

2.the cam lobe base circle is not concentric

put no1 piston at tdc, you can look thru plug hole.
check to see which is on comp stroke, 1 or 4 by looking at dist rotor
set that pair of rockers at .015
rotate crank 180 degs and set next pair 1-2 or 4-3
rotate again 180 and set opposite of first pair
rotate again and set final pair.

John

 

[John]

If you are rotating the crankshaft 180 degrees, then the camshaft is only rotating 90 degrees due to the cam/crank gear ratio. You are checking the valve lash on some point other than the heel of the cam lobe.

 

[johnneilson]

If you are rotating the crankshaft 180 degrees, then the camshaft is only rotating 90 degrees due to the cam/crank gear ratio. You are checking the valve lash on some point other than the heel of the cam lobe.

John,
you are right, but the point is to set the valve lash when the engine is in cylinder firing position.
By setting one cyl at a time, in firing position, rotating the crank 180 degrees each time you are rotating the motor through the full cam rotation.
The old "rule of 9" for setting the valve lash works pretty well for a stock cam shaft grind. It can work for performance reground cams, I have just found it easier to go thru the four positions and not take too many chances.

It is possible that the reground cam and a larger than stock lifter has the foot rubbing on the cam and not allowing the lifter to ride on the heel of lobe. This usually happens when Chrysler lifters are used and high performance cam grind.

Be safe, John

 

[rogeroadster]

If you are rotating the crankshaft 180 degrees, then the camshaft is only rotating 90 degrees due to the cam/crank gear ratio. You are checking the valve lash on some point other than the heel of the cam lobe.

Yes I agree with you and from my experience I believe the valve lash should decrease not increase. I would expect the valve lash to diminish once the lifter moves off the base circle.
Anyway, so far I haven't done anything other than scratch my head and wonder. So I guess I'm going to have to delve a little deeper.
I thank you guys for your replies.

 

[100IH]

The place to adjust is: the center of the heel. This is also TDC for that hole and the companion hole. If you start the adjusting procedure for the two valves on # 1, both cam lobes will be swung down (with lash on each). This won't be the center of the heel but close enough. If you look at #4 cyl. both lobes will be up, sorta, this is the overlap when one valve is nearly closed and the other is just opening. Kinda like a teeter totter where one is opening and the other is closing, both equal lift( just barely). Here the front cyl. is Top compression and the rear is top of the exhaust stroke. With this method you are using the rear cyl. in overlap to locate the front cyl. at top, "centered on the heel - Time to adjust both valves at #1. Every cyl. can rely on the overlap of it's companion to locate the exact correct position on the heel to adjust. 360 on the crank is 180 on the cam. !&4 are companions 2&3 are companions.

 

[Jim Brierley]

I use the rule of 9's. You could also set the valves while idling the engine. It is not uncommon for the base circle to not be totally concentric on performance grind.

 

[briphaeton]

I've used the rule of 9 on my Miller also but recently picked up a P & G valve gapping tool.
I'm looking forward to trying it out.
https://www.ctci.org/pg-valve-gapper-model-111-instructions/

 

[mike657894]

In an l head. You can only have so many things affecting lash . If a lifter does not rotate it will wear fast and so will the lobe. If there was an obstruction between the valve and seat. if the valve was bent it might hold the valve up slightly. sounds like a problem that is not going to go away with the engine still together.

 

[wmws]

To be sure you are on the heal I rotate the engine so the valve I am checking is fully open. Then rotate the crankshaft 360 degrees. That will rotate the cam 180 degrees and you will be sure you are squarely on the heal.

 

[Dan McEachern]

Jim- I don't understand your statement about the base circle not being concentric? I would think if that's the case, the cam grinder got in a rush on his finish grind . . . . .

 

[CWPASADENA]

With performance cams, I have found the easiest way to adjust valves is to do one cylinder at a time. Start with #1 cyl. Rotate the crank until the exhaust valve just starts to open, then adjust the lash on the intake. Then rotate the crank until the intake just closes, then adjust the exhaust valve. If you plot it out, when the exhaust just starts to open, the intake lifter is essentially at the center of the intake base circle. Likewise, when the intake just closes, the exhaust lifter essentially in the center of the exhaust base circle.


This is the way we set up our competition engines for many years. Once you get used to it, you will find very consistent valve settings.


I have also found some re-ground cams where the base circle is not concentric with the center line of the cam shaft. In such a case, we had to adjust the valves with the engine idling as slow as possible. This works for most overheads and a few flat heads and can be somewhat of a mess but at times you have to do what you have to do. In the "Old Days" we would adjust the Chevy sixes and some other OHV engines this way.


My experience,


Chris W.

 

[johnneilson]

Jim- I don't understand your statement about the base circle not being concentric? I would think if that's the case, the cam grinder got in a rush on his finish grind . . . . .

Dan,

I agree, sloppy machine work, you get what you pay for.

The size of the lifter can also be issue if the cam is not clearanced for the larger foot properly.

The list goes on.

There is another possibility, the cam is flexing.
In the case of having large valve springs and high ratio rockers, the intake lobes can flex the cam shaft. Especially when the cam is a regrind where the heel/base circle is smaller than the cams shaft (between lobes) diameter.

I doubt that this is the case on this one, but have solved with supports.

John