Valve Lash Update from GM

[brad hibbard]

Chevrolet Performance has just issued a bulletin regarding the recommended valve "lash" for the circle track 602 and 604 engines.



this was prompted at the request of several engine builders that have discovered the previous recommended adjustment of "zero to 1/4 turn" was not enough preload and left a window open for failure during valve float and in many cases the overall engine performance can be enhanced with additional preload.



The recommended valve lash or pre-load has been changed to 1/2 to 3/4 of a turn. by Chevrolet Performance (GM)



Race-1 and other associated engine builders have been using and recommending this amount of pre load for over a year now



the valve cover sticker and technical manual will be changed in all future printings.


***special note***

as racers, our mentality is if a little is good more will be better

NOT the case here-----stick with recommendations from your engine builder or manufacturer

increase in preload affects rocker arm geometry -------the latest recommendations are within acceptable limits but more may create wear and possible failure.



Brad Hibbard

https://www.race-1.com

[brad hibbard]

here is the actual bulletin ----includes the new balancer description>>>>>>>>>>


Date:
12/11/2014
Ref. number:
Parts / Product / G_0000206538

Subject:
Chevrolet Performance - Circle Track (CT) Tech Manual UPDATE - Balancer Appearance & Valve Lash



--------------------------------------------------------------------------------

To: Chevrolet, Buick, GMC, and Cadillac Dealers

DESCRIPTION:

Chevrolet Performance announces the latest update to the Circle Track (CT) Tech Manual.

To obtain a CT Manual: Refer to page 369 of the 2015 Performance catalog for GM ordering number.

EFFECTIVE DATE:

Product is available immediately.

GENERAL INFORMATION:

UPDATES

IMPORTANT NOTE: Some of the early installed Circle Track Silver Balancers may come with the name Pro/Street on it (See attached photo). This is a Chevrolet Performance, Engineering validated, balancer installed at the time of engine build by Chevrolet Performance.

Balancer (19318604 - CT400 & 19258602 - CT350):

Beginning with MID-2013 production, both the CT400 [ 19318604 ] and the CT350 [ 19258602 ] include a new harmonic balancer. This change was due to part availability. The new balancer has the same dimensions and is also sold separately under P/N 19301706. This new balancer is silver in color, may come with no name or "Pro/Street" on the product. As previously mentioned, both are Chevrolet Engineering approved.

CT Valve Lash Procedure:

This covers the proper procedure to lash the valves. It is imperative to set the valve lifter lash properly on each of the engines. All three engines use the same procedure. It is also critical that the rocker arm nuts are properly secured so they do not loosen during operation. The 602 & 603 engines use “Kool-Nuts” and the 604 uses aluminum roller rockers. Each has a locking nut with set screw. The set screw is the only way to keep valve lash secured. Refer to Tune-up section for photo of cool nut and set screw.

Recommended Lash is ½ to ¾ turn Hot:

[ric78]

quick question for you Brad. I want to clarify exactly what is meant by 1/2 to 3/4 pre load. This is the way I was taught to adjust the valves on a hydraulic/zero lash camshaft. It is the same exact way I do the 602 we race. Exhaust starts to open adjust the intake just until you start feel some resitance but the pushrod will still rotate in your fingers. roll engine over until intake opens and starts to close then adjust the exhuaust in the same manner. Are you saying that when i get to the point of what I am calling zero lash I should rotate the nut another 1/2 to the 3/4 of a turn then lock it down?

[Matt49]

quick question for you Brad. I want to clarify exactly what is meant by 1/2 to 3/4 pre load. This is the way I was taught to adjust the valves on a hydraulic/zero lash camshaft. It is the same exact way I do the 602 we race. Exhaust starts to open adjust the intake just until you start feel some resitance but the pushrod will still rotate in your fingers. roll engine over until intake opens and starts to close then adjust the exhuaust in the same manner. Are you saying that when i get to the point of what I am calling zero lash I should rotate the nut another 1/2 to the 3/4 of a turn then lock it down?

Yup. 1/2 turn to 3/4 turn AFTER you feel zero. And the method you are describing is also exactly how Brad recommends. He has a ton of good information and recommendations on his website FAQ page: http://race-1.com/faq.html

[LITE-INN]

The only way you'll ever get laid is if you crawl up a chicken's ass and wait

[PushinTheLimit]

Can we ban this idiot?

[brad hibbard]

thanks for the plug Matt
ric78, ----the recommendation from us has never been below 1/4 turn pre-load and did increase to more in the recent year
zero lash has a high lifter failure rate and in most cases will produce less power than a preloaded lifter

[ric78]

Thanks Matt and Brad. I just wanted to hear it from some people in the know. I read it in the manual but could just not wrap my head around preloading the lifter. I was always afriad of when it pumped completely up it would be to tight. Shoot I may have been leaving a little on the table.