72 LT-1 Valve lash adjustment

[Ed H.]

My 72 LT-1 is running well with 43k miles with no indication or sounds that the valves are need in immediate adjustment. However adusting the vales is an easy thing to do. My question is should I follow the factory specs for the valve lash adjustment or adjust to a tighter spec - I read the thread in the archives from Duke and others suggesting going to a tighter spec and the benefit of going to a longer mileage interval between adjustments. Are there other benefits and does this accelerate valve and cam wear? What are the recommended valve lash adjustments? I do plan to have the car judged.

[Steve G.]

Re: 72 LT-1 Valve lash adjustmentTh

My personal opinion is that the teams of engineers that came up with the specs did so after research and development done under far more controlled conditions and with much more experience than the "field engineered" specs.

The oe engineers weren't stupid and they were drawing on a lot of years of archival information when they came up with those numbers. When you are not privy to the r & d data you are at a high risk of missing an important detail.

A very good example of that, which may also have relevance to your question, can be found in a TDB discussion of the value of roller tip rockers. In this case the author had failed to recognize the fact that the Chevrolet engine is one of the few, if not the only, to adjust valve lash by moving the fulcrum point rather than altering the contact point at the pushrod. And what was discovered in the field, which I'm sure the engineers already knew, was that altering the design fulcrum point and with it the angle of the rocker relative to the valve from design has a negative impact on valve guide life.

Steve

[Leif A.]

Re: 72 LT-1 Valve lash adjustmentTh

Ed,
You state that your LT1 is "running well with no indication of sounds from the valves". Why not leave the hood closed and leave well enough alone? But, if you're dead set on tinkering with the valves, I second Steve's advice.

[Ed H.]

Re: 72 LT-1 Valve lash adjustmentTh

Thank you Leif and Steve for the good input. I certainly will not plan to adjust the valves until it is needed. I found the information for adjusting the valves per Duke's recommendation and read that the method and spec was inclusive of the LT-1 in the updates. My LT-1 is running very well as it is and hard to make a case to change it.

[Joe C.]

Re: 72 LT-1 Valve lash adjustmentTh

My personal opinion is that the teams of engineers that came up with the specs did so after research and development done under far more controlled conditions and with much more experience than the "field engineered" specs.

The oe engineers weren't stupid and they were drawing on a lot of years of archival information when they came up with those numbers. When you are not privy to the r & d data you are at a high risk of missing an important detail.

A very good example of that, which may also have relevance to your question, can be found in a TDB discussion of the value of roller tip rockers. In this case the author had failed to recognize the fact that the Chevrolet engine is one of the few, if not the only, to adjust valve lash by moving the fulcrum point rather than altering the contact point at the pushrod. And what was discovered in the field, which I'm sure the engineers already knew, was that altering the design fulcrum point and with it the angle of the rocker relative to the valve from design has a negative impact on valve guide life.

Steve

That would be a valid point if the engineers weren't constrained by economic considerations as well as the fact that OE specs were often meant to apply to multiple engine applications. Optimizing parameters to specific applications always plays second fiddle to interference by corporate bean counters.

[Steve G.]

Re: 72 LT-1 Valve lash adjustmentTh

That would be a valid point if the engineers weren't constrained by economic considerations as well as the fact that OE specs were often meant to apply to multiple engine applications. Optimizing parameters to specific applications always plays second fiddle to interference by corporate bean counters.

Absolutely right Joe, but unless you have the lab facilities and the research capabilities to determine whether the specs were the product of the accounting team versus the engineering team, it's a guess.

I've seen enough bad field and internet engineering in my almost 40 years as a journeyman to cause me to put my faith in the oe first, particularly in the absence of specific information about the contributors lab facility, data sampling, other peer reviewed research publications etc.

Steve

[Paul D.]

Re: 72 LT-1 Valve lash adjustmentTh

I owned a '72 LT-1 and the factory specified lash settings proved to be fine.....No need to reinvent the wheel.......P..

[Mark E.]

Re: 72 LT-1 Valve lash adjustmentTh

Can you really accurately tell if all valves are adjusted to spec by just listening?

Periodic checking per the maintenance schedule is a best practice for a well tuned engine. It's not a difficult task; when I had a solid lifter car, I followed a routine during each oil change that included checking valve lash, plugs, points, dwell, timing, carb adjustments, and general poking around. This habit rewarded me with a great running engine.

[Steve G.]

Re: 72 LT-1 Valve lash adjustmentTh

Can you really accurately tell if all valves are adjusted to spec by just listening?

It seems periodic checking per the maintenance schedule is a best practice for a well tuned engine. It's not a difficult task; when I had a solid lifter car, I followed a routine during each oil change that included checking valve lash, plugs, points, dwell, timing, carb adjustments, and general poking around. That was when gas had lead, so plugs tended to foul sooner than today...

I can't tell by ear if they are to spec, unless of course something has gone wrong with one making it louder than the rest. I advocate for routine adjustment according to the service manual interval.

Steve

[Joe C.]

Re: 72 LT-1 Valve lash adjustmentTh

Absolutely right Joe, but unless you have the lab facilities and the research capabilities to determine whether the specs were the product of the accounting team versus the engineering team, it's a guess.

I've seen enough bad field and internet engineering in my almost 40 years as a journeyman to cause me to put my faith in the oe first, particularly in the absence of specific information about the contributors lab facility, data sampling, other peer reviewed research publications etc.

Steve

The original engineering design drawing for the 3849346 cam showed lash @ .025/.025. The lash was increased to make the cam compatible with the Rochester system on the L84 engine. Lash .025/.025 worked perfectly with the L76 for 1964-65.

[Steve G.]

Re: 72 LT-1 Valve lash adjustmentTh

The original engineering design drawing for the 3849346 cam showed lash @ .025/.025. The lash was increased to make the cam compatible with the Rochester system on the L84 engine. Lash .025/.025 worked perfectly with the L76 for 1964-65.

That doesn't sound like accounting dept changes. What were their specific reasons for increasing lash and why is that reason no longer valid?

Not sure I'm following.

Steve

[Joe C.]

Re: 72 LT-1 Valve lash adjustmentTh

That doesn't sound like accounting dept changes. What were their specific reasons for increasing lash and why is that reason no longer valid?

Not sure I'm following.

Steve

Driveability concerns with the L84 is the unofficial reason. Lashed @ .025/.025, a 327 cu in engine equipped with the 346 cam does not develop enough idle vacuum to keep the injected engine from stalling unless idle is increased significantly. There was no official reason published.

Reason is still valid for a fuel injected engine, unless the owner doesn't mind increasing idle speed to about 950. Reason was never valid for L76 and DZ engines.

If lashed @ .030/.030, the lifter is past the cam's clearance ramp, where it is not subject to acceleration. It is past the clearance ramp where it is subject to acceleration and thus the valves and seats experience harsher closing of the valves with more possibility of valve bounce.

[Steve G.]

Re: 72 LT-1 Valve lash adjustmentTh

Reason was never valid for L76 and DZ engines.

Sounds like there may be more to the storey.

Steve

[Ed H.]

Re: 72 LT-1 Valve lash adjustmentTh

Mark, I agree it makes sense to include the valve lash as a periodic maintenance check along with the normal tune up cycle. Thank you for the suggestion.

[Duke W.]

Re: 72 LT-1 Valve lash adjustment

In case anyone is interested, attached is the Hinckley-Williams paper on small block valve lash adjustment - the latest revision, September 2008. This is also on the Web as are earlier, obsolete editions, so be careful.

The paper includes my analysis of rocker ratio behavior and hot/"cold" (idling) valve lash change (effectively nil) based on measurements I made back in the seventies and my dynamic analysis of the lobe data from the Chevrolet engineering drawings (every cam degrees in inches to five decimal places) to determine the velocity, acceleration, and jerk of the OE lobe profiles. This also allowed me to determine the tops of the constant velocity clearance ramps with precision well within one mil.

The paper was jointly authored and "peer reviewed" by John Hinckley and me. We could not find any other "peers" that had our combined level of in depth engineering understanding of camshaft design and valve train dynamics to provide any additional reviews or critiques.

Duke