SuzukiSavage.com - How Important is Valve Clearance?

DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	How Important is Valve Clearance? 07/31/22 at 19:34:40 How Important is Valve Clearance? None of us want to end up with this.
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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #1 - 07/31/22 at 19:35:15 Or this.
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DragBikeMike

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Re: How Important is Valve Clearance?
Reply #2 - 07/31/22 at 19:38:20

On this forum, I see valve failures far too frequently. Way too much. It doesn’t give me that warm, fuzzy feeling of confidence in my faithful steed. I always want to feel like I can get from “A” to “B” without sendin the old powerplant to the graveyard.

What causes these valve failures? They seem to be confined to the exhaust valve. Is the valve overheated? Is it a fatigue fracture? Did the valve stick and then get hammered by the piston? Did the cam chain slip and cause a valve-to-piston collision? Are the failures limited to the early model valves that have reduced stem diameter? Does it ever happen to the intake valve?

IDK. I’m not gonna claim that I have the answer. But I will admit that it worries me. That valve snappin off could conceivably put you on your a$$. At a minimum, it sure as heck will ruin your motor (and your day).

One possible cause could be valve clearance.

On one hand, if the clearance is insufficient, the rocker arm will hold the valve off its seat. Good contact with the valve seat is essential for valve cooling. If the valve doesn’t contact the seat, the valve will overheat. If it gets too hot and reaches the tempering point, the material might yield and fail. But I’ve always heard that the valve will burn. I’ve never seen a burnt valve on a Savage. Also, I have never found my valves tight. Every time I have checked my valve clearance it has been loose, not tight. It always seems to be about .001” to .003” loose, and the exhaust valves always seem to be looser than the intakes. Any of you have a problem with valve seat recession? Any of you finding tight valves?

On the other hand, loose valves might prevent the opening and closing ramps on the cam lobe from doing their job. The opening ramp gently moves the valve off its seat instead of just banging the whole shiteree open. The closing ramp gradually decelerates the valve so it can be gently placed on the seat rather than slamming home (like a sledgehammer). The cam curve below shows how the valve moves much slower at the very beginning and end of the cycle (circled in red) than it does during the general opening and closing phases (circled in green). At the beginning and end of the cycle, the valve moves a lot less per degree of crank rotation than it does during the general opening and closing phase.

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Re: How Important is Valve Clearance?
Reply #3 - 07/31/22 at 19:41:04

For instance, during the closing phase, the valve moves about .150” from 40° before top center (BTC) to top center (TC). So, in 40° of crankshaft rotation the valve moves .150”. At 6000 rpm, that’s about 11.4 feet per second (fps) or 7.8 miles per hour (mph).

But from 10° after top center (ATC) to 50° ATC, the valve only moves .012”. At 6000 rpm that’s about 0.9 fps or 0.6 mph. Way, way, way slower. That’s due to the deceleration ramp that’s ground into the cam lobe. That ramp is a very important part of every cam design.

What effect does valve clearance have on those opening and closing ramps? I figured too much clearance would screw things up. I decided to find out.

My Big-Bore 4-Speed project is underway and it seemed like a good time to look at the valve clearance issue. The plan was to do timing checks on a stock cam. I wanted to see how increasing clearance affected valve timing, and more importantly, what it did to the opening and closing rates (crank rotation vs valve travel).

I only tested the exhaust valve since that’s the one that seems prone to failure. The effect on intake timing should be similar.

I installed one exhaust valve with a light checking spring. That way, the tendency for autorotation would be minimized. It would be much easier to rotate the engine and stop precisely on the data points (10° increments).

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Re: How Important is Valve Clearance?
Reply #4 - 07/31/22 at 19:42:05

I then installed the cylinder head and proceeded to set up my degree wheel. I use a piston stop to establish the top dead center (TDC) position on the degree wheel. You thread the stop into the spark plug hole. I took this measurement for record purposes. The length of the stop will vary from one engine to the next, so it’s a good measurement to have on hand. It’s not critical, you just want the stop long enough to hit the piston about 10° to 20° before TC. Don’t ever use a piston stop with the rocker arms installed. You run the risk of running a valve into the stop. Use the stop to adjust your degree wheel, then remove the stop before you install rocker arms.

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Re: How Important is Valve Clearance?
Reply #5 - 07/31/22 at 19:43:10

You install the stop and rotate the engine until the piston hits the stop. Then rotate the other direction until the piston hits the stop. When I rotated mine clockwise (CW) until it stopped, the degree wheel read 17°, and when I rotated it counterclockwise (CCW) until the piston hit the stop, the degree wheel read 15°. You want it to read exactly the same for each direction, so adjusted the wheel on the crank until it reads exactly the same in each direction. In this case, 16° was the target.

On the stop CW at 16°.

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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #6 - 07/31/22 at 19:43:52 And on the stop CCW at 16°.
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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #7 - 07/31/22 at 19:44:22 Now when you position the crank at 0° on the degree wheel, the engine will be exactly at TDC.
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Re: How Important is Valve Clearance?
Reply #8 - 07/31/22 at 19:45:34

I don’t know about yooz guys, but I am tired of running back & forth from left to right when I set up a cam. Plus, the timing marks on the alternator rotor and alternator cover are too far apart for any reasonable degree of accuracy. I had this thing dialed in to TDC, so I decided to capture that location with an appropriate mark on the right-hand side of the engine. I did that with a combination square, a scribe, and a simple stop for the square. Voila, easy peezy timing in the future.

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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #9 - 07/31/22 at 19:46:05 I installed the cam and proceeded to dial it in. For my current build, the crank sprocket had to be retarded three-splines to achieve the best possible cam timing.
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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #10 - 07/31/22 at 19:46:56 Tough to get any closer than this.
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Re: How Important is Valve Clearance?
Reply #11 - 07/31/22 at 19:48:15

Then it was time to throw on the head cover and adjust the lone valve. I decided to use a dial indicator to do each valve adjustment. I figured it would get pretty dicey once I started getting up around .020” lash because the checking spring is so soft, and the thicker feelers are so stiff. The indicator worked out good. BTW, every 1/8 turn on the adjuster is .0037”. That’s a useful tidbit of info. Quick & Dirty adjustment: screw in until just touches valve tip, back out 1/8 turn, lock.

Here’s what the setup looked like. The indicator was reading on the adjuster screw. All the lash is taken up such that the follower pad is bearing against the cam lobe. So, the indicator was zeroed when the adjuster tip was touching the valve stem, and it read .020” when the follower pad was brought into contact with the cam lobe. It was set to .020”. I like this process.

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Re: How Important is Valve Clearance?
Reply #12 - 07/31/22 at 19:48:57

I checked valve timing with the lash set to .004”, .008”, .012”, .016” & .020”. That would give me a good picture of how the lash affects acceleration and deceleration of the valve, and also how lash affects duration & lift.

Valve travel was measured with a one-inch travel indicator reading on the spring retainer.

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Re: How Important is Valve Clearance?
Reply #13 - 07/31/22 at 19:50:06

After a few hours of work, I had a clear picture.

As expected, valve lift and duration were decreased each time I increased lash. I don’t see that as too big of a deal. At .012” lash, duration was decreased about 60° and lift was reduced about .009”. It just keeps getting worse as lash is increased. Unfortunate but not life threatening.

But the deceleration ramp on the cam was grossly diminished at .012” lash, and completely inoperative at about .016” lash. I think that’s a Big Deal. Crashing the valve into the seat 3000 times every minute is brutal. The spring exerts about 55 lbs of force, and it’s trying to stretch the valve stem. Expecting a red-hot valve to endure that sort of abuse is asking a lot.

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DragBikeMike Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 4406 Honolulu	Re: How Important is Valve Clearance? Reply #14 - 07/31/22 at 19:50:53 This is a graphic depiction. At .016” lash, that sucker crashes into the valve seat like a lead brick dropped out of a second story window. Ouch!
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