solid roller cam knocking

[edfiero]

Newb here, working on a Pontiac 4 cylinder engine with solid mechanical roller lifers.

New rings installed, cylinder honed, new main bearings. I am reassembling the engine now and notice that when I rotate the engine over by hand that I get a knocking/clunk sound at the end of the cam, every time a lifter goes over the tip of a cam lob.... so basically 8 knocks per revolution of the crank. I confirmed by loosening the rockers I can make the noise go away, but as soon as 1 rocker is tightened (any rocker) the noise returns.

The engine was removed from the car after timing gear failure. (The valve springs on this engine are quite heavy and the aluminum cam gear was not up to the task. A couple of push rods and valves were replaced due to slight bends. Everything else about the engine is as it was with the exception of switching from the aluminum helical cut gear to a steel spur cut gear.

I don't know if this knock was there before and I never noticed it, or is this due to using the spur cut gear, or is something messed up??

What causes this clunk sound when the lifters are going over the cam lobs?

The block uses roller cam bearings and I have about .005 of space between the cam retaining plate and the first cam journal. I am thinking that is on the high side, but still within spec.

Thanks

[mag2555]

I would set the Cam up in Vee blocks and check if it's now bent!

[edfiero]

I would set the Cam up in Vee blocks and check if it's now bent!

I hadn't considered a bent cam. Educate me. Aren't cams brittle? I would expect it to BREAK, but not bend.
The cam slides easily into block (no binding), and without the lifters in, it rotates freely.

[DaveMcLain]

It is very normal for a cam to have uneven forces caused by the valvetrain opening and closing the valves. These can push the cam forward or backward in its rotation when you bar it over on the engine stand. These forces can be VERY strong especially if they are not balanced out by other valves pushing in the other direction. Maybe you're just hearing the backlash in the gear train.

[fdicrasto]

My guess would be if you put a helical cut timing set back on the noise would disappear. The straight cut spur gear style has no forward or backward bias so the cam can move in either direction very easily with even minute lobe differences. The .005" cam end play is actually on the tight side assuming your measurement is accurate. Phil D.

[PackardV8]

Educate me. Aren't cams brittle? I would expect it to BREAK, but not bend.

Is your solid roller cam a cast core or steel billet? Even cast billets can be straightened. It was enlightening to go through Ed Iskenderian's shop and observe his Mexican operators checking each finished cast shaft for straightness and then if needed, whacking it with a square nose chisel and ball pein hammer. Usually, took only one carefully eyeballed whack.

[cgarb]

Make sure to check side clearance between the lobes, maybe the lifters are rubbing the sides now that the cam is free to move with the straight cut gears.

[CharlieB53]

..............Maybe you're just hearing the backlash in the gear train.

Can you uncover the cam driver, back off the lifters then check and see just how much 'play' is in the cam drive?

As a lifter passes TDC of the cam lobe the valve spring attempts to rotate the cam forward, taking out any slack in the cam drive. You may be hearing this as your 'knock'.

If this is what you are finding then check into finding components that can fit a bit tighter, with less of this 'slack'.

[Zmechanic]

Not familiar with that particular setup, but last time I encountered this the push rods were hitting the lifter bodies over the nose, then eventually "snapping" back down into the cups when they started to straighten out on the backside of the lobe.

[Circlotron]

Disclaimer - I am not an engine builder.

Position the crank so that all the pistons are at half stroke then disconnect the cam from the crank.
Wind the cam over by hand and see if the knock is still there.
If the knock is gone it may be timing gear backlash as others have said.
If the knock is still there, could it be the lifters snapping from one side of the lifter bore to the other as the roller goes over the peak and the lateral force on the lifter changes direction?

[MadBill]

..............Maybe you're just hearing the backlash in the gear train.

Can you uncover the cam driver, back off the lifters then check and see just how much 'play' is in the cam drive?

As a lifter passes TDC of the cam lobe the valve spring attempts to rotate the cam forward, taking out any slack in the cam drive. You may be hearing this as your 'knock'.

If this is what you are finding then check into finding components that can fit a bit tighter, with less of this 'slack'.

Per this and fdicrasto's comment, maybe you could rest a finger on the mesh point of the gears and see if a 'twitch' is felt at each knock sound, thereby confirming that it's caused by load reversal over the nose of each lobe, amplified by gear lash. (or maybe take up all the lash with a feeler gauge and see if the noise is eliminated.)

[hoodeng]

If you can loose the noise by backing off the rockers ,it would suggest there is mechanical clearance in the cam/ lifer to rocker parts until full lift is applied ,but binds and knocks when lash is taken out and full lift achieved would suggest to me valve spring coil bind or valve to piston clearance , as this came apart with bent push rods , valves and drive damage would suggest the problem existed before you got to it.
You have identified very heavy valve springs , what are the installed heights , seat pressures , full lift pressures ,bind height and lift clearance? Or confirm valve to piston.

Cheers,

[cjperformance]

..............Maybe you're just hearing the backlash in the gear train.

Can you uncover the cam driver, back off the lifters then check and see just how much 'play' is in the cam drive?

As a lifter passes TDC of the cam lobe the valve spring attempts to rotate the cam forward, taking out any slack in the cam drive. You may be hearing this as your 'knock'.

If this is what you are finding then check into finding components that can fit a bit tighter, with less of this 'slack'.

Per this and fdicrasto's comment, maybe you could rest a finger on the mesh point of the gears and see if a 'twitch' is felt at each knock sound, thereby confirming that it's caused by load reversal over the nose of each lobe, amplified by gear lash. (or maybe take up all the lash with a feeler gauge and see if the noise is eliminated.)

And again !! This

[superpursuit]

Check to make sure the pushrods are not fouling the rocker. If this is happening the pushrod can be forced out of the pushrod cup in the rocker at full lift and snap back into place at a lower lift. Also if your timing gears are dry (no oil on the teeth) any noise from the backlash will be amplified.