Valve Lash

[Joe C.]

All:

So I notice that my L76 seems to idle somewhat higher than it did, and a tad smoother, too (that nice rumpety-rump is a wee bit muted). The engine is the slightest bit more prone to detonation, too. This is after 900 miles on the original engine build-up using Duke/John H's adjusting procedure. The engine was built using "Posilocks" to retain the adjusting nuts. The valves are sprung with slightly stiffer than stock "Z28" springs (I can find the number if that is necessary).

I get out the old vac gauge, and, sure enough, it reads 13 in-hg @ 850 RPM, a full 4 inches higher than it did when I set the lash initially.

I'm gonna reset the lash today, but, meanwhile, any ideas on why the lash got wider in such a short amount of time? I'll record the readings after I "go in" today.

I hope that the Posilocks may have loosened.
I hope that the rocker studs are not pulling out due to the stiffer springs.

Joe

[Clem Z.]

the correct way to adjust "posilocks"

is to set the lash .003 looser than the required setting,tighten the setscrew while holding the "nut" with a boxed end wrench. after tightening the set screw turn the "nut" tighter with the boxend wrench til you get to the proper lash setting. "posilocks" like the have the top of the stud machined flat so the setscrew get a good "bite"

[Duke W.]

Re: Valve Lash

I don't know what you mean by "Z-28 springs". Production Z-28s use the same springs as all other SBs of the era. If you used the springs designed for the 140 cam, that may be the source of your problem, especially if you did not convert to screw in studs. They are ABSOLUTELY UNECESSARY assuming you have a 30-30 or other production mechanical lifter cam.

With the gentle dynamics of these cams they will spin reliably to 7000 with the production springs and proper lash.

I've changed my recommended cold lash spec on the 30-30 cam to .023" and .016"/.023" for the LT-1 cam.

As long as the nuts still have good drag torque, the lash should hold well and not pull the studs.

As in the past I continue to recommend NOTHING but OE or OE equivalent parts in the valve train including the late production springs, which have slightly higher rate, in earlier engines.

The old adage, "if it works don't fix it" is highly applicable to the SB valvetrain.

Duke

[Joe C.]

Re: Valve Lash

Here are the specs on the valve springs, that I have just now obtained from the manufacturer. These were not installed at my request, rather, as recommended by the machinist who installed the hardened exhaust seats in my stock 461 heads. The machinist insists that the studs do not need to be pinned in order to use these springs, as he did three years ago when he did the job. This was the first question I asked him when he recommended them, and if he wanted to make more money on the job, then I assume he would have pinned the rocker studs, especially since I asked about it.

Manufacturer: ERSCO Inc.(Engine Rebuilders Service Co. Inc.)
Location: N. Muskegon, MI
P/N: SS943-7
O.D.: 1.253 - 1.267
I.D.: .869 - .883
Wire Dia.: .192
P and L (closed): 104 - 116 lbs. @1.70"
P and L (open): 273 - 297 lbs. @1.21"
Solid Height: 1.16"

The ERSCO representative stated that "these are what all of the engine builders call the Z28 springs, or the fuelly springs" I don't know the specifications for the stock springs, to compare, or whether these are, in fact, the same as the stock springs. I have not yet removed the top covers and set a straight edge across all of the Posilocks to check for pulled studs.
If these are not stock, then any info you have on these specifications will be most appreciated.

Joe

[Clem Z.]

Re: Valve Lash

those sound like a copy of the GM "142" spring as the stock SBC springs are 80# @1.700

[John H.]

Re: Valve Lash

The "stock" springs are still available from GM - P/N 3911068; 80# closed, 200# open @ 1.25", 1.239" O.D., 2.027" free length, 0.177" wire diameter.

[Joe L.]

Re: Valve Lash

Joe-----

These springs are quite a bit higher rate than the original springs installed on your engine. The original springs were of GM #3735381. These springs have a rating of 175 lbs @ 1.26" (open) and 75-78 lbs @ 1.70" (closed). They're still available from GM for $8.52/each. These springs were used for all 1956-66 Corvette small blocks and have an outside diameter of 1.22".

The 67-91 Corvette small block valve spring which was also used on the vast majority of other small blocks over the period, including all PRODUCTION Z-28 engines, is GM #3911068. These springs are rated at 195-200 lbs @ 1.25" (open) and 80 lbs @ 1.70" (closed). They're still available from GM, too, for a current GM list of $7.77. These springs have an outside diameter of 1.24".

As others have mentioned, the springs that you have installed are about equivalent to the GM #3927142. These are a SERVICE-only spring designed for use with off road Z-28 camshaft GM #3927140. I think that they might be a bit too stout for your application.

I do think that you could use the GM #3911068 spring with no problems, at all, and possibly to some advantage. They will fit and install perfectly in your heads.

Also, I do not recommend the use of posi-locks with stock steel rocker arms. It "just ain't the way to go" in my mind. For stock steel rocker arms with stock rocker studs, I would use stock-type, self-locking rocker nuts. In fact, I'd use these with roller rocker arms were it possible to do so (but, it's not). Use GM #12557390. $1.23/ea GM list.

[G B.]

I would use stock .030/.030" settings

I don't see anything wrong with 13" vacuum at idle. In fact, that should give you much stronger low end power on the street.

I'm sure you're going to suit yourself though. If you want a really tough sounding idle at the drive-in, consider disconnecting two plug wires. That's what I used to do to my mama's Electra in 1963. I think many people then thought it was a top fueler.

[Mike McKown]

Do you suppose the Chevy engineers in 1964

knew something that is not being talked about today? I'm quite sure they knew what the "real" rocker ratio was and when it got the lifter up on the clearance ramp. They weren't unknowing or "ignorant".

I'd just about guarantee one thing, they wouldn't recommend a lash adjustment that would be detrimental to engine longivity. You can take that to the bank and that goes for the LT-1 and 097 cam both. There is a little lash leeway to play with on all those cams, depending on your desire for top or low end power.

[Clem Z.]

orignal GM cam lash setting

for the 346(30-30) cam was .025

[Mike McKown]

The mystery is...................

why did they change it?

They knew .025 would work and so would .030. Same as .023 will work. You just get different results from different settings.

[Clem Z.]

Re: The mystery is...................

i think jerry b hit they needed more vacuum at idle for the FI

[G B.]

Since you didn't ask...

This is what I tell '64-5 fuel injection owners. They can run
any cam they like as long as it makes at least 12 - 13 inches of steady
vacuum at an 850 rpm idle speed. If it makes less vacuum than that, then
they will have an unstable idle and some off-idle stumbling problems no
matter how I calibrate their FI unit.

To get this much idle vacuum out of a 30-30 cam grind you have to run at
least .030" on both the intake and exhaust. Sometimes it's more. One man
in MA has to run .035" just to get 12" with his engine. Maybe he really has an Isky Super LeGuerra cam, or maybe his pistons are swapping holes with each other, I just dunno...

Those with whiz-bang tight valve lash ideas are welcome to do their thing as
long as they don't expect me to make their FI units run right. I just can't do it.

[Joe C.]

Re: the correct way to adjust "posilocks"

Clem:

Thanks. That's the answer.

Joe

[Joe C.]

Re: the correct way to adjust "posilocks"

Clem:

The heads are installed on the engine, and the studs are untouched, as stock, so they are certainly not flat on top. Is it a decent bet that if I adjust them as you said, that they'll hold, or should I just go ahead and use stock GM nuts.
I can file all the tops flat.........but those pesky iron filings...........

Joe