LT1 Cam - Lash Adjustment w/1.6 Roller Rockers

[6T5RUSH]

OK, I'm thinking that using JOHNZ's cold lash adjustment of .021 Intake and .026 Exhaust makes that task somewhat straightforward.

Wouldn't I be able to arrive at the correct lash cold using 1.6's by taking the .021 & .026 settings for 1.5's and doing the math this way:

.021 / 1.5 x 1.6 = .02240

and

.026 / 1.5 x 1.6 = .02773

Am I being too simplistic here? Help me out 'cause my heads are being checked for flow #s and if it is benefecial to use 1.6's, I'd like to use 'em. I just know the correct lash settings bantied about are with the correct factory 1.5 rockers.

Thanks all!

Jim

 

[JohnZ]

Jim, the "cold lash" procedure and revised clearances for the "30-30", LT-1, and "097" Duntov cams that Duke and I developed only apply to cars with stock stamped rockers, which are nowhere near the stated 1.5:1 ratio; they're actually 1.37:1 at initial lift contact, and only get to 1.44:1 at full valve lift, and it's that measured ratio differential that's used to factor down the factory-specified lash numbers, which assumed (incorrectly) a nominal rocker ratio of 1.5:1.

We haven't measured the actual ratio of any aftermarket rocker arms, although it would be easy enough to do using a dual dial indicator setup as we used for our measurements (one on the pushrod end of the rocker, the other on the valve spring retainer). If a given aftermarket rocker arm ratio was actually measured at 1.6:1, you'd want to increase the clearance spec slightly (rather than decreasing it) by 6.7% to accommodate the difference between the 1.5:1 ratio the factory spec assumed and the actual measured 1.6:1.

 

[allcoupedup]

This bring up a difficulty that I did not think about. I'm going to use a set of Comp. Cams true 1.52 roller tip rockers in my build. How would one go about setting the proper lash?

Why do I have a sinking suspicion that this will have to be done hot with engine running!?!?!?!

Brian

 

[6T5RUSH]

JohnZ, I think I understand. Would this be the correct way to think this through:

.021 Intake Cold x .933 (1.00 - .067) = .01959
and

.026 Exhaust Cold x .933 (1.00 - .067) = .02426

The above, assuming the 1.6's truly are 1.6's

Thanks JohnZ.

Brian, just another small "hook" in this process.

Jim

 

[WayneC]

6T5RUSH... I'm not convinced you need to change those lash settings. Here's my reasoning:

The lash setting is to reduce the tendency of the rocker to destroy itself or the end of the valve by slapping down too hard on the valve stem, minimize valve train noise, yet maintain enough clearance to assure that the valve closes fully and remains seated until the cam opens it again, and that the contact surfaces get lubricated. The "cold" lash setting is a setting that should hopefully result in a correct "hot" lash setting.

A larger rocker ratio would traverse the (original) clearance and start opening the valve just a bit sooner as the pushrod moves up, and finish closing it just a tad later, an argument in favor of increasing the lash clearance; but, then the possibility of damaging the valve stem argues against increasing lash clearance. I would think we aren't concerned once contact is made, because we WANT the rocker to open the valve further (that's the purpose of going to a wider ratio, under the assumption that the larger ratio rocker will actually enhance performance... in fact, it may not).

However, I can't convince myself that the lash is directly related to the rocker ratio. If they ARE directly related, then your first calculations are probably correct. Did JohnZ and Duke actually "calculate" the cold lash setting somehow, or is it simply that a measurement was made of the cold lash after having set the lash with the engine hot?

I'd say to use the .021 and .026 cold settings initially, then do a lash adjustment to check for the proper "hot" settings (per Chevy spec) with the engine running at temperature. I haven't had a solid lifter car for many years... but having done (messy) lash adjustments on running engines, I don't think I'd fully trust a cold adjustment.

 

[6T5RUSH]

WayneC, Good points you're bringing up. Hope JohnZ responds to your question of how this measurement was made. I'm on a big learning curve (hopefully it isn't flat!.

Thanks WaynC.

Jim

 

[JohnZ]

The issue here, and the reason we did the research, is the relationship between the point where the clearance is taken up between the rocker and the valve stem and the position of the lifter foot on the lobe profile of the cam. The factory-published .030" (hot) lash clearance was based on the nominal (design) 1.5:1 factory rocker arm ratio, but, as manufactured, they aren't 1.5:1 - they vary from 1.37:1 to 1.44:1, and never reach 1.5:1.

We did complete crank angle/cam lift diagrams for both "30-30" and LT-1 camshafts, in engines with stock stamped rockers, using the dual dial indicator setup I described above; this involved taking readings 720 times for two crankshaft (one camshaft) revolutions, one measurement/reading at every degree of crank rotation. What this told us was that the clearance ramps (the low-acceleration initial and ending portions of the cam lobe just off the base circle that take up opening clearance gently and then set the valve down on its seat gently) are exactly .020" high before the serious lift acceleration portion of the lobe profile starts (and ends, as the "30-30" lobes are symmetrical). This would be right on the money with 1.5:1 rockers, the valve stem end of which would take up the factory-spec .030" clearance exactly at the point where the lifter transitioned from the clearance ramp to the acceleration ramp portion of the cam lobe, to avoid "pounding" the valve train.

However, with the "real" as-manufactured/installed rocker ratio (1.37:1 at initial lift), at .020" lifter lift (the height of the clearance ramps), that movement only moves the valve stem end of the rocker .0274", so there's still two-and-half thousandths clearance at the valve stem when the lifter hits the acceleration portion of the cam lobe profile, which "pounds" the valvetrain and makes a lot of noise. On the closing side of the lobe profile, this means the valve is dropped back on its seat at greater than clearance ramp velocity as well. This also "pounds" the valvetrain, can contribute to valve seat recession, can cause valve "bounce" at high rpm, and also creates excessive mechanical noise.

Closing the running clearance down from .030" to .026" allows the rocker tip-to-valve stem clearance to be taken up at the gentler clearance ramp velocity (with a one-and-a-half thousandths safety margin) before the lifter transitions to the acceleration portion of the lobe profile, and also allows the valve to be returned to its seat at clearance ramp velocity, as designed. The original design parameters are correct, but the "real" stamped rocker ratio doesn't meet the design parameters, which assumed a 1.5:1 rocker ratio.

This procedure, done with the indexing process (which sets the intakes at 90 degrees ATDC and the exhausts at 90 degrees BTDC due to the unique-to-the-"30-30" cam's extreme overlap which has both valves still on the ramps at TDC instead of on the cam's base circle), has been done on many L-76 and L-84 Corvettes and '67-'69 Z/28 Camaros (including mine) since we developed it, and it results in a nice mechanical "singing" sound, no "clacking" noises, they run better, sound better, the idle is more stable, and throttle response is improved. If the factory stamped rockers were 1.5:1 at initial lift, it would be the same way with .030" clearance, but they're not. The best intentions of the cam designers were altered by the tolerance spread of the rocker arms as manufactured, but the published clearance specs in the shop manuals were based on the design assumption of 1.5:1 rockers, which didn't happen.

I hope this clarifies the rationale behind the change from .030" clearance to .026" with the "30-30" camshaft; it lets the system operate exactly as the designers intended, in spite of the inaccurate rocker arm ratios.

 

[6T5RUSH]

JohnZ, Thank you for the explicit walk through of the rationale. I just need to reread this a few times.

So, assuming an actual lift of 1.6 ratio, I would want to "increase the clearance spec slightly". I'm sorry, I'm still struggling with this. Are you saying I would be setting my lash with a lower # (intake cold...lash setting .01959)?? I'm thinking the first calculation would give me the "increase" (intake cold...lash setting .02240). The bigger the # reflects an "increase" in clearance, right? Sorry I'm thick in the head on this JohnZ.

Appreciate any response to my thinking on this.

Thanks!

Jim

 

[WayneC]

6T5RUSH, I guess a larger lash setting is the proper conclusion using JohnZ's reasoning. He assumed GM calculated for a 1.5 ratio, and he reduced the setting 0.0025 because he measured a smaller rocker ratio than GM intended. I calculate 0.032 lash for exhausts if I assume original ratios are 1.5 as GM intended and new rockers have a true 1.6 ratio.

I wasn't quite clear, though, on how he measured the rocker ratio, since I suspect the rocker contacts the valve stem head at different points during it's movement (sort of a wiping action, offset slightly to rotate the valve), while the pushrod socket is pretty much stationary. Seems to me that to be sure you'd have to measure to see if the valve itself moved at all before the end of the approach ramp (and perhaps he did that); but you may be kidding yourself that the measurement will be the same on a hot engine... the pivot ball expands, moving the rocker closer to the valve stem, the rocker stud lengthens, probably more than offsetting that, and the pushrod and valve stem lengthen, moving their ends closer to the rocker.

Although I think I mostly understand the measurements and agree with the rationale about the clearance from the cam "ramps", that's an argument for altering the hot lash spec, not an argument for setting the lash with the engine cold. GM specifies a hot lash setting, not a cold lash setting (which they could easily have measured after letting an engine cool down), and I suspect the reason is that there are heat expansion and other (moving parts, oil films, etc) issues at work here, and I would personally feel safer doing a hot engine lash adjustment. Too tight a lash burns valves, too loose a lash loses power and gradually destroys parts.

Since setting the lash cold is much easier, I doubt GM would go out of it's way to make things more difficult for their mechanics by specifying only a hot lash spec, unless it was really necessary (having worked in Chevy's engineering department long ago, I gotta believe they made lots and lots of measurements of their own, not to mention endurance testing and subsequent teardowns of many engines, and I know they were very cost-conscious). Cold lash, to me, is only a reasonable starting point for initially setting up the engine... and perhaps I'm making a leap in assuming JohnZ is recommending setting the lash cold; maybe he is just recommending an initial setting for engine assembly.

 

[JohnZ]

The valve side of the "dual dial indicator" readings were taken directly from the valve spring retainer, in the same axis as the valve stem, not on the valve end of the rocker arms, so actual valve lift was compared directly to actual lifter lift on the cam lobe. The expansion differential of a hot vs. cold rocker arm has virtually no effect at all on the effective ratio (we measured that too).

The hot-vs.-cold adjustment scenario was developed simply as a matter of convenience; clearance checks on engines that were set both hot and cold showed .001" or less difference. The iron block and heads and steel pushrods all have essentially the same thermal coefficient of expansion - aluminum heads would alter this relationship, but we only worked on stock iron/iron combinations. The different thermal coefficient of expansion of aluminum would introduce another variable into the equation, and we didn't do any measurements on engines with aluminum heads.

The ONLY reason that the shop manual specifically requires engines with the "30-30" cam to be set "hot and running" is that the "30-30" clearances can't be set by the same traditional shop manual method as other Chevrolet solid-lifter cams (engine hot, but not running, adjusting 8 valves at #1 TDC and the other 8 at #6 TDC), as you could do with the "097" Duntov solid-lifter cam used from '57 through '63. This is because the "30-30" cam is the only one Chevrolet ever made with VERY long ramps and VERY long duration, to the point that at TDC on the compression stroke, both the intake and exhaust valves are NOT closed - they're both still on their ramps, not on the cam's base circle. Rather than confuse the mechanics with the progressive indexing procedure required to set them properly on a static engine (hot or cold) for just this one camshaft, it was easier for the guys who wrote the shop manual just to tell them "valve adjustment must be done with the engine hot and running" instead of going through the rationale behind "why" it's different than any other cam they ever made. I spent 21 years in Chevrolet Product and Production Engineering (1964-1985), most of it in engine and chassis development, and I was quite close to this issue at the time, including the re-release of the '64-'65 Corvette "30-30" cam for the '67-'69 Z/28 application.

After decades of lashing these things hot and running, with cut-in-half valve covers and rocker arm deflector clips for tools and hot oil everywhere, we just decided there had to be a better way, and that's what started our research project three years ago to understand the "30-30" cam and valvetrain and why its unique valve timing events required the unique "hot and running" lash method. Now we know the "why", and have developed a "better way" to get the job done that achieves the original design parameters and valvetrain dynamics without dealing without hokey cut-up tools, clips, and hot oil everywhere. It works

 

[bigbear]

JohnZ, this is very very interesting stuff. This post is 5 1/2 years old, and makes me wonder where the hell I've been all this time I've had my '65 327/375 for 40 years now, and was fortunate to have a friend who worked for Doane Chevrolet's B production team back in the early days. Setting valve lash "hot and running" was a real pain, due to the fact that you have to disassemble part of the intake ducting and the sheet metal attached to the air meter, just to remove the rocker cover. I've been warming the engine, then setting the lash to .026 - intakes as the exhaust just opens, and exhaust as the intakes close - for 40 years now.
Right now, I have a really strong urge to buy a set of 180 or 195cc AFR heads, and am playing with the idea of using a 1.6 roller rocker, due to the fact that these heads will flow much better at higher lifts than the old double humpers do. JohnZ, do you remember, or have access to flow capabilites on the rochester air meters? I think I remember that was always a problem. There were some "not so successful" attempts at getting more flow, by adding an additional air meter, due to the changes the additional air meter created to the vacuum signal sent to the fuel meter.
Having several of these old Rochester units, I would like to modify one of them to electronic. I already have O2 sensor "bungs" on my header collectors for tuning with a wide band sensor.