I enjoyed reading the thread started by Jerkyboy on adjusting the lash on a solid roller cam. An all aluminum pushrod engine adds clearance to the lash as it warms up. The heads have hardened steel seats that do not wear much, not allowing the valve to sink.

My only concern for too little lash would be not permitting the valves to fully close which would burn them. The lash could close up on an iron engine as it got hot. Old iron heads had fairly soft valve seats which would allow the valve to sink into the head as the miles accumulated. You would want some safety margin with this combination.

I have one of the HPBooks, Big Block Chevy Performance that states "...lash can be reduced all the way down to .001 in. or .002 in. The only requirement is that some clearance must exist so that the valve firmly contacts the seat." (page 78)

My thoughts are that setting the lash at ZERO when cold would provide 10-12 thousandths when fully warmed up. It would be a much simpler process to adjust this way. I sent a quick email to GKull to get his opinion as to what the downside is to zero lash. His opinion has always been highly valued by me, even if he does drive a Vette with an automatic Anyway, his comment was much of what I have thought and would have expected for an answer. Basically, that he has never heard of such zero lashed solid cams either.

Is there enough variance in the valve train at high rpm’s that would cause continued contact between the cam and the valve even when the valve should be fully at rest? I'm thinking of possible continued spring vibrations, even when using the correct springs, that could be a problem if there wasn't sufficient lash clearance. Even if this is the case, wouldn't the normal thermal expansion to .010" of lash be enough?

I'm not looking for responses that the XYZ company gives these specs. I'm looking for a thoughtful explanation as to why.

Thanks,

Chuck

 

"Lash tuning" is somewhat of a myth. Typical mechanical lifter cams - flat or roller tappet - have constant velocity clearance ramps in the range of .0002 to .0005 inches per cam degree, so tightening lash will not really increase "effective duration" to any significant amount.

Some racing mechanical llifter cams have constant acceleration ramps and these ARE sensitive to lash tuning, but they can have short life - they are not designed for street engines. You would either have to (at best) have the engineering drawing or have a Cam Doctor analysis of the lobe to determine actual clearance ramp type/characteristics to make a rational determination of whether or not lash tuning is viable.

Duke

 

Duke,

Thanks for your quick and informative response. My reason for being tempted with a zero lash is not because of trying to make a small cam bigger, but really one of "would it hurt?" I am thinking of putting one in my ZO6 which unlike the normal small block Chevy is not exactly easy to get to to adjust the valves. Excess lash would tend to cause the lifter to slam into the side of the lobe without the benefit of much of a ramp. Yet I would think that a very small lash would not cause any harm as since virtually all of the ramp would be used.

The difference in valve timing would be, at least in my mind, be much of a difference since the lift would be so slight at the ramp ends that the effective timing probably wouldn't change much. Certainly not by enough to notice by seat of the pants.

Wouldn't a smaller lash create less of a hammering on parts, something many say is why solid rollers don't last on the street becuase of flattened roller bearings. The light weight Jesel lifters with their well designed adjustable rockers would seem to be a very attractive combination.

Thanks

 

Duke,

Thanks for your quick and informative response. My reason for being tempted with a zero lash is not because of trying to make a small cam bigger, but really one of "would it hurt?" I am thinking of putting one in my ZO6 which unlike the normal small block Chevy is not exactly easy to get to to adjust the valves. Excess lash would tend to cause the lifter to slam into the side of the lobe without the benefit of a ramp. Yet I would think that a very small lash would not cause any harm since virtually all of the ramp would be used.

The difference in valve timing would be, at least in my mind, not much of a difference since the lift would be so slight at the ramp ends that the effective timing probably wouldn't change much. Certainly not by enough to notice by seat of the pants.

Wouldn't a smaller lash create less of a hammering on parts, something many say is why solid rollers don't last on the street becuase of flattened roller bearings. The light weight Jesel lifters with their well designed adjustable rockers would seem to be a very attractive combination.

Thanks

 

Mechanical lifter cams have clearance ramps to gently take up lash and open/seat the valve at ramp velocity. The most lash you want is clearance ramp height times rocker ratio at the lash point which has to be measured because rocker ratios vary throughout the lift cycle. For example, everyone thinks vintage SB rockers are "1.5:1", but they are not. They start at 1.37:1 and peak at 1.44:1 at max lift with about a 0.3" lobe. The LS-SB rocker ratio is speced at 1.8:1 however, I can assure you they are not constant, though I have never measured their behavior.

The only way to determine optimum lash is to understand the clearance ramp architecture, detail design, and detailed rocker ratio behavior.

The vintage OE mechanical lifter cams have clearance ramp velocities of about .0004" per cam degree, so for a clearance ramp height of .016" the ramp is 40 cam degrees "long", which is 80 degrees of crankshaft revolution. So if you tight lash, the valve will spend considerably more time off the seat. This is critical because about 80 percent of valve cooling is through the seat, and unless the valve is firmly in contact with the seat, there is no cooling, which considerably increases the chance of burning a valve.

I've never analysed any aftermarket solid roller cams, but they are focused at racing applications. They may well have constant acceleration ramps. They require frequent valve lash checks and lack longevity. They are not designed for street use. Unless you are building an racing engine, which must rev over 8000 and produce peak power at 8000 or above, I would not recommend using a mechanical roller cam.

Duke

 

"The vintage OE mechanical lifter cams have clearance ramp velocities of about .0004" per cam degree, so for a clearance ramp height of .016" the ramp is 40 cam degrees "long",

Duke,

So if the warmed up lash is approximately .010 - .012 of an inch, then I would only be taking less than 20 cam degrees off the seat time. So a cam in the 220-230 degree range would still have more seat time than a cam with 250-260 (or more) degrees of duration for race application. Right?

 

Chuck - did you know that I'm doing something out side the box with my 427 small block?

It had a hot H-roller cam in it. So instead of having to be RPM limited to 6200 rpm or so rpm I decided the way to go would be to throw some solid rollers on it.

So I contacted the manufacturer and we talked about the differences in the lobe profiles and what would it take to try my idea. The answer was: To keep the valves set at .008 - .012 hot. It runs so quiet compared to my other billet cam which was .022 hot.

IMO - The milder the lobe the less adjusting!

Out of the 6 different roller cams that I have had in my vette over the years. The one that I never worried about checking or resetting the lash on was an off the shelf Crane 232/238. It was a street solid roller design. The ramps are not as steep as my radical cams. I took a 3000 mile trip and never even took any tools. With my good allen locking rocker nuts and stud girdle. Once it was settled in it just kind of stayed there. It's not rpm and the amount of lash. It's how hard your beating on the valves.

Look at the lobe of the street SR compared to the custom billets


This was the most radical cam I had in my 383 massive valve train noise

 

You forgot to add the loss of seat time on the closing side.

Take the following example, ALL IN CAMSHAFT DEGREES.

Seat to seat (NOT .050" lifter rise) duration 150, 40 degrees of clearance ramp on either side.

So the valve is seated, 100(360-150)/360, 58 percent of the time

Now lets' say you tighten lash so that the valve begins to lift, albeit VERY slowly, after 20 degrees of opening clearance ramp, and closes after 20 degrees of closing clearance ramp has passed. You have increased "duration" 40 degrees to 190 and now the valve is seated only 100(360-190)/360 = 47 percent of the time. The actual percent reduction in seat time is 1-100(170/210) or 19 percent.

To me that could a recipe for a burned valve. For sure it's going to run hotter under all operating conditions.

Duke

 

I recall my L88 cam having seat to seat timing of about 347 degrees. At lash I think it was over 300 degrees. Am I correct in thinking that a racing engine putting out over 500 horsepower during racing conditions is generating a bit more BTU's than my street motor? If such valves are designed to operate in such an environment, should I see burned valves being much of a problem on the street running seat to seat timing even if the total duration is 270 degrees.

GKulls experience is what I am talking about. To get his "Hot" measurements, I would have to zero lash cold. I still don't see any negative to trying this, other than the usual debate of solid vs. hydraulic which will not be decided here.

Thanks for all the input. This is the kind of discussion I love on the forum!

Chuck

 

I recall my L88 cam having seat to seat timing of about 347 degrees. At lash I think it was over 300 degrees. Am I correct in thinking that a racing engine putting out over 500 horsepower during racing conditions is generating a bit more BTU's than my street motor? If such valves are designed to operate in such an environment, should I see burned valves being much of a problem on the street running seat to seat timing even if the total duration is 270 degrees. But I will look into this more. There could be differences.

GKulls experience is what I am talking about. To get his "Hot" measurements, I would have to zero lash cold. I still don't see any negative to trying this, other than the usual debate of solid vs. hydraulic which will not be decided here.

Thanks for all the input. This is the kind of discussion I love on the forum!

Chuck

 

Chuck, i just use cams that get the valve to the max flow and hold it there for the most degees of cam duration. Last a long time and run fine are not the high point.

yes it has bothered me when I lost a whole block to valve train failure. Then I went back with better stuff and it lasted nearly 20,000 miles with a SR cam. Those Crane SR lifters in the picture are the best I've ever bought. I have them in both my motors. The comp cams sticker is just for the carbon poly dist gear.

You really have to talk to a cam guy about the clearance ramp. I run the best .080 wall high tech pushrods of custom lengths and have titanium intake valves with the not the best springs. You have to say to your self is $800 a set beehive NASCAR worth it or do some research and use just under shimmed to take seat pressure off .700 lift spring be good enough. Then they can take on .750 lift with less seat pressure.

Yes, I worry about something popping when it's pounding the rev limiter

 

You ask about burn valves. That is really a thing of the past with modern steel. I've ran the cheaper Manley race flows with maybe a 1/16 seat touch in heads that were so ported that nothing of the head casting was underneath them. It's a common circle track idea with cast iron head rules.

The Stainless exhaust valves x-mit enough heat with just the smallest contact.

With high flowing heads you don't need the massive cam durations anymore