Camshaft Base Circles

It seems to me (in a perfect world) that the clearance measurements for a given valve should be equal no matter where they are taken as long as they are taken between the base circle and the bucket or shim (depending on shim under or over bucket). This is because the base circle should be the same radius all the way from the end of the closing ramp to the beginning of the opening ramp (see pic below).

However, several folks have stated their clearance measurements vary depending upon where along the base circle the measurements are taken. What could cause this? I’m guessing the following:

The cam base circle is not ground correctly in that its radius varies or it is not concentric to the camshaft center.
The camshaft journals are not round or not concentric to the camshaft center.
The camshaft bearing surfaces are worn.
The camshaft is bent.
The valves are not closing uniformly each time.

Since the purpose of the cam is to open the valve a precise distance and permit it to tightly close, compensating for irregular base circles by adjusting the clearances could have an impact on the engine performance if the base circle if off by a wide margin.

So, the how much variance in the base circle is too much; how does one identify the cause as noted above; and if the variance is too much how should the problem be corrected?

The cause could be one or more of the combinations of the reasons listed above.
There is no correction or solution for your average mechanic and "these folks" don't know or really understand what they are doing.
Just set the valve clearance with the lobe 180 deg away from the shim and leave it,nothing untoward will occur.
Many cams have the quietening ramps that start and finish well before and after the example shown and taking measurements at points other than directly opposite the lobe can result in wildly incorrect readings.

zed1015 wrote: The cause could be one or more of the combinations of the reasons listed above.
There is no correction or solution for your average mechanic and "these folks" don't know or really understand what they are doing.
Just set the valve clearance with the lobe 180 deg away from the shim and leave it,nothing untoward will occur.
Many cams have the quietening ramps that start and finish well before and after the example shown and taking measurements at points other than directly opposite the lobe can result in wildly incorrect readings.

Amazing how people don't grasp simple things...

zed1015 wrote: ......Many cams have the quietening ramps that start and finish well before and after the example shown and taking measurements at points other than directly opposite the lobe can result in wildly incorrect readings.

Very interesting. That may indeed be the reason folks have complained about different measurements at different point along the "base circle" when in fact if they are measuring on a section of quietening ramp they are not on the base circle. I've not experienced that issue on my KZ650. Mine has stock cams and using the instructions in the Kawasaki manual has always worked well even though the lobes don't point 180* away when measuring, but clearly there are other cams where 180* is the only accurate position for measuring the clearance. Ed

With OEM cams, using the factory lash position lines on the sprockets will never put the lobe on the silencing or opening ramp. The only exception would be if the bolt holes have been slotted for finite timing adjustments. Another good reason to use the lobe away method to ensure lash is there when it needs to be.

What you see are the effects of very old engines with normal wear and tear on the valve train.

There is more than one cam on the shaft. The lobe circle may be perfect but the cam may be moving up and down with respect to the head based on spring pressure on the other cam and the amount of cam chain tension.... I have one valve that reads a solid .003" turning the cranksaft ONLY in the forward direction (which keeps the chain tight on the leading edge). If I reverse the crankshaft direction and let slack into the cam chain, the same clearance WITH THE CAM LOBE IN THE EXACT SAME POSITION increases to about .005" - .006".

That drove me nuts at first, but it is what it is. I used plastigauge to check all the cam journal clearance, all in spec.

On my cams, there is no cam position where ALL of the cams have no "tension" on them so one end or the other is going to be getting pushed on in some form or fashion at all times.

bountyhunter wrote: There is more than one cam on the shaft. The lobe circle may be perfect but the cam may be moving up and down with respect to the head based on spring pressure on the other cam and the amount of cam chain tension.... I have one valve that reads a solid .003" turning the cranksaft ONLY in the forward direction (which keeps the chain tight on the leading edge). If I reverse the crankshaft direction and let slack into the cam chain, the same clearance WITH THE CAM LOBE IN THE EXACT SAME POSITION increases to about .005" - .006".

That drove me nuts at first, but it is what it is. I used plastigauge to check all the cam journal clearance, all in spec.

On my cams, there is no cam position where ALL of the cams have no "tension" on them so one end or the other is going to be getting pushed on in some form or fashion at all times.

Curious as to why you would be turning the engine over backwards when checking valve lash?

I prefer the lobe being 180 degrees away from the bucket to check clearance. I expect the base circle is very accurately ground when the cams are new. But if the cams are bent at all, then all bets are off. I've always tightened and loosened the cam cap bolts in very small increments. Otherwise, maybe a cam could be bent? I don't know, but don't want to find out the hard way. Also to avoid stripping threads.
I believe, as bountyhunter said, that the camshafts are actually moving around slightly within the journals as they are rotated for checking clearances. The reasons being: upward pressures from other cam lobes pushing valves open; and the cam chain pulling the cams downward or forward within the journals. I expect that bountyhunter rotated the cams in reverse slightly, simply to test his theory about the movement, and yeah verily, it did make a difference in clearance. As LarryC said, most of these engines are not new any more and the clearances may be somewhat loose. In reality, I'll bet the clearance varies by a couple of thousandths as the engine runs, but unless it goes less than zero, then this is inconsequential. Just my theory.

LarryC wrote:

bountyhunter wrote: There is more than one cam on the shaft. The lobe circle may be perfect but the cam may be moving up and down with respect to the head based on spring pressure on the other cam and the amount of cam chain tension.... I have one valve that reads a solid .003" turning the cranksaft ONLY in the forward direction (which keeps the chain tight on the leading edge). If I reverse the crankshaft direction and let slack into the cam chain, the same clearance WITH THE CAM LOBE IN THE EXACT SAME POSITION increases to about .005" - .006".

That drove me nuts at first, but it is what it is. I used plastigauge to check all the cam journal clearance, all in spec.

On my cams, there is no cam position where ALL of the cams have no "tension" on them so one end or the other is going to be getting pushed on in some form or fashion at all times.

Curious as to why you would be turning the engine over backwards when checking valve lash?

It's hard to get the cam lobes pointed directly away from the shim bucket because of spring tension. A couple of times I "went by" and then bumped it back reverse a little bit to get the cam lobe pointed directly away as shown in the manual for clearance checking. That's when I found out clearances changed if the cam chain tension was released.

Thought I was going nuts until I figured out why it was changing.

CAM CAP CLEARANCE:

I just went to the manual and checked the spec for clearance between the camshaft cap and the can journal surface. It's:

.0065"

So, if you can have more than .006" slop between the camshaft and the cap holding it down..... that means the camshaft can certainly move up, down and around some as the camshaft turns. :ohmy:

bountyhunter wrote:

LarryC wrote:

bountyhunter wrote: There is more than one cam on the shaft. The lobe circle may be perfect but the cam may be moving up and down with respect to the head based on spring pressure on the other cam and the amount of cam chain tension.... I have one valve that reads a solid .003" turning the cranksaft ONLY in the forward direction (which keeps the chain tight on the leading edge). If I reverse the crankshaft direction and let slack into the cam chain, the same clearance WITH THE CAM LOBE IN THE EXACT SAME POSITION increases to about .005" - .006".

That drove me nuts at first, but it is what it is. I used plastigauge to check all the cam journal clearance, all in spec.

On my cams, there is no cam position where ALL of the cams have no "tension" on them so one end or the other is going to be getting pushed on in some form or fashion at all times.

Curious as to why you would be turning the engine over backwards when checking valve lash?

It's hard to get the cam lobes pointed directly away from the shim bucket because of spring tension. A couple of times I "went by" and then bumped it back reverse a little bit to get the cam lobe pointed directly away as shown in the manual for clearance checking. That's when I found out clearances changed if the cam chain tension was released.

Thought I was going nuts until I figured out why it was changing.

Ohhh... I see what the deal was now.