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Drop in Z28 springs needs many shims?

9K views 13 replies 6 participants last post by  BOBCRMAN@aol.com 
#1 ·
I went to do my first valve spring upgrade today on my 882 sbc heads. My new springs are rated at 110lbs at 1.710" and 305lbs at .500 lift.I bought the proper valve spring height micrometer but when I went to do my first valve I found that I needed four .060" shims and one .030" shim to get to the installed 1.710 valve height. Is this right to need so many shims?
 
#2 ·
Sounds like you are not using the mic. correctly.

Most spring mics, when used on 1.25 retainers, require the use of a shim under the stock retainer. Otherwise the small retainer falls to the inner step on the mic.

The heads will probably need extra shims in the exhaust spring pockets if you remove the rotators (recommended) and use conventional retainers.

Try mic, then a shim, then retainer, then locks. Determine height then subtract thickness of the top shim. This will give you true spring installed height.
 
#4 ·
Exhaust Valve Rotators

Hey Man, If your are eliminating a valve rotator, shims are not the best way to do it. On big blocks the rotator is .300", you need to replace them with a cupped spring locater seat. That much shim is going to make your springs start doing a dance, which is not good. You may want to look into this more on your small block heads. I personally have never come across a small block head with them, but I have used Comp rotator eliminators on more than one big block w/781 heads. ;) olnolan
 
#5 ·
On stock SBC heads w/rotators, the installed height is lessened by the added thickness of the rotator, so there shouldn't be a need for a bunch of shims to make up for the rotator's thickness.

I use a snap gage (just haven't been able to justify the cost a spring height mic :( ), so I can't speak to the need for a shim under the retainer, but it sounds plausible- and this would throw the measurement off by the amount of the step that's on the retainer. I don't have a stock retainer in my hand, but the step isn't very large, maybe 0.080" at a WAG?

But there's no earthly way that you should need over 1/4"(!!!) of shims to obtain a 1.710" installed spring height unless there are other extenuating circumstances.

Are the valves, retainers and locks stock-spec parts?
 
#6 ·
Yes , just stock style stuff. Last night I reviewed how to use the mic in the Competition Products catolog. Seems that I'm doing it right. I also noticed that the more expensive drop in springs have an installed height of 1.850".If I had got these springs I would only need a shim or two. I'm going to measure the mic today, maybe it should have been a reject :rolleyes: . If it's ok I'll stop at the machine shop on the way home for a little hands on class.
 
#7 ·
"Most spring mics, when used on 1.25 retainers, require the use of a shim under the stock retainer. Otherwise the small retainer falls to the inner step on the mic."
Yes this is what is happening! But the instructions say to simply put the mic in place of the spring. Nothing about putting a shim under the retainer and subtracting it.
 
#8 · (Edited)
Just to recap, what you're measuring is the distance between the retainer- where the outer spring actually contacts it- to the spring seat on the head itself. This can be measured fairly closely w/a machinist's rule or a caliper's depth function- at least close enough to see that the height mic is accurate or not.

Often, it's just a matter of learning to read an individual tool, so be sure of what you think you're seeing IS what you're seeing! lol

Longer than stock springs will have an effect on the installed height, and are commonly used on performance heads and cases where additional spring installed height is needed. You should check the overall length of the valve and compare it to a stock length spring of 4.910".

The retainer design and even the locks can alter the installed height.
 
#10 ·
upstand2 said:
Well, I just measured the mic, Bobcrman hit the nail on the head. Crazy, the sbc retainer fits perfect in the second step of the valve spring mic but that throws the measurment off 1/8" .
OK, so you have accounted for 0.125" of the 0.270" that you measured earlier. Any idea where the other 0.145" is or went?
I needed four .060" shims and one .030" shim to get to the installed 1.710 valve height
 
#11 ·
The reason to eliminate the rotators is. They are quite heavy. They are also known to come apart at high rpm.

Any lightening of the valve train is a plus. Also the shields are usually eliminated with lifts over .470. They too have been known to break and cause problems.

Good luck.
 
#12 ·
cobalt327 said:
OK, so you have accounted for 0.125" of the 0.270" that you measured earlier. Any idea where the other 0.145" is or went?
I think F-bird covered the missing .115"( my first valve was the exhaust).
I'll still need an additional .010" shim to get it at the recomended height. I do have the Comp #4753 shim set.Thanks alot everyone!
 
#14 ·
NO, as a matter of fact. Once I change a head to high performance status, (no rotators and plain retainers, no shields). I use rubber umbrellas as a minimum (customer with tight pockets). The rubber O-ring is very ineffective after the metal shield is removed. Most budget springs clear the small universal rubber umbrella seals.

If the head is a later model that can handle "positive" seals. I use the Blue type seal on the intakes and an umbrella on the exhaust.

GM quit using the O-rings in the 90's. Most of the aftermkt valves are now single groove.
 
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