1990 Valve Spring Shims
Thread Starter
Intermediate
Joined: Dec 2019
Posts: 35
Likes: 11
1990 Valve Spring Shims
Question...
I'm installing the FIRST TPI Intake and rockers, and as a "while you're in there", I'm swapping the springs and valve seals as well. The original spring assemblies have the weird cap/cup on top. I've seen several threads mention removing them, but I'm seeing some mixed info on whether or not to add the LT4 style shims in return. The original cups won't fit with the new retainers. The hole is a little too small.
Any wisdom?
Springs: COMP Cams 981-16
Retainers: COMP Cams 750-16
Keepers: COMP Cams 611-16
Proposed Shims: GM# 10212809
I'm installing the FIRST TPI Intake and rockers, and as a "while you're in there", I'm swapping the springs and valve seals as well. The original spring assemblies have the weird cap/cup on top. I've seen several threads mention removing them, but I'm seeing some mixed info on whether or not to add the LT4 style shims in return. The original cups won't fit with the new retainers. The hole is a little too small.
Any wisdom?
Springs: COMP Cams 981-16
Retainers: COMP Cams 750-16
Keepers: COMP Cams 611-16
Proposed Shims: GM# 10212809
Thread Starter
Intermediate
Joined: Dec 2019
Posts: 35
Likes: 11
I checked the install height and it's almost exactly longer than the adjacent spring (original assembly) by 0.03" which is the shim thickness for the LT4 shims referenced above.
I guess my question is, what am I adjusting by adding the shim? Is it the amount of force the the spring is applying to the valve to seal? If so, is it necessary if this spring is stiffer compared to the stock spring (which I think it might be)?
Sorry pros. I'm still wrapping my head around this.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
The cap may be a oil deflector that Chevrolet used because they didn't use a positive valve seal on the guide. If you are not changing cams I would leave them on. Valve rotators are usually only found on exhaust valves, they aren't necessary in most cases. I would leave them on tow vehicles, if they are operational.
A common mistake of installing the little O ring on valve stem is : installing O ring in groove before retainer and keepers. When retainer is compressed many times, the seal is cut or pushed out of groove. Install retainer, then seal and keepers. Without positive seal on guide; once O ring seal is out of place, oil will run down valve stem and guide where it burned
A common mistake of installing the little O ring on valve stem is : installing O ring in groove before retainer and keepers. When retainer is compressed many times, the seal is cut or pushed out of groove. Install retainer, then seal and keepers. Without positive seal on guide; once O ring seal is out of place, oil will run down valve stem and guide where it burned
Thread Starter
Intermediate
Joined: Dec 2019
Posts: 35
Likes: 11
The cap may be a oil deflector that Chevrolet used because they didn't use a positive valve seal on the guide. If you are not changing cams I would leave them on. Valve rotators are usually only found on exhaust valves, they aren't necessary in most cases. I would leave them on tow vehicles, if they are operational.
A common mistake of installing the little O ring on valve stem is : installing O ring in groove before retainer and keepers. When retainer is compressed many times, the seal is cut or pushed out of groove. Install retainer, then seal and keepers. Without positive seal on guide; once O ring seal is out of place, oil will run down valve stem and guide where it burned
Thread Starter
Intermediate
Joined: Dec 2019
Posts: 35
Likes: 11
The required spring pressure is determined by camshaft profile and projected engine RPM operating range.
Shims under the spring increase the spring's force against the valve, but also the valve train all the way to the camshaft. The primary objective of valve spring tension is to keep the lifter's base or roller in contact with the cam lobe. As the cam lobe opens the valve by pushing the lifter - pushrod - rocker - valve, the spring keeps all the bits in contact with each other. The higher the RPM, the higher the spring's force needs to be to accomplish this. You may be familiar with the term "valve float". This happens when the springs are not 'strong' enough, or the engine RPM exceeds the intended upper limit.. With hydraulic lifters, if valve float occurs, the lifters take up the difference; after RPM comes down the engine runs like crap (because the valves aren't completely closed) until the lifters bleed down.
Springs have a specification for pressure at a specific installed height. Builders that pay attention to details check springs in (at least) two ways. That the spring supplies the spec pressure at the spec height. Those that don't are discarded. Then each valve's retainer is installed on its valve without the spring. The dimension between the retainer and the bottom spring seat surface is measured and recorded, this dimension is usually larger than the specified installed height. Shims are selected to compress the spring to its specified installed height.
Find the installed height specification for your springs. Measure a few valve positions with only the retainer / rotator / cap that you are going to run. Come up with an average shim thickness that gets closest to the specification. Put that amount of shim under all the valves with that retainer / cap combination. Intake and Exhaust may be different.
There's a TL;DR, but the answer to your question is in there.
Shims under the spring increase the spring's force against the valve, but also the valve train all the way to the camshaft. The primary objective of valve spring tension is to keep the lifter's base or roller in contact with the cam lobe. As the cam lobe opens the valve by pushing the lifter - pushrod - rocker - valve, the spring keeps all the bits in contact with each other. The higher the RPM, the higher the spring's force needs to be to accomplish this. You may be familiar with the term "valve float". This happens when the springs are not 'strong' enough, or the engine RPM exceeds the intended upper limit.. With hydraulic lifters, if valve float occurs, the lifters take up the difference; after RPM comes down the engine runs like crap (because the valves aren't completely closed) until the lifters bleed down.
Springs have a specification for pressure at a specific installed height. Builders that pay attention to details check springs in (at least) two ways. That the spring supplies the spec pressure at the spec height. Those that don't are discarded. Then each valve's retainer is installed on its valve without the spring. The dimension between the retainer and the bottom spring seat surface is measured and recorded, this dimension is usually larger than the specified installed height. Shims are selected to compress the spring to its specified installed height.
Find the installed height specification for your springs. Measure a few valve positions with only the retainer / rotator / cap that you are going to run. Come up with an average shim thickness that gets closest to the specification. Put that amount of shim under all the valves with that retainer / cap combination. Intake and Exhaust may be different.
There's a TL;DR, but the answer to your question is in there.
Ok it's clicking with me. When I started to calculate the change in spring force (~10 lbs) over 0.035" on the 981-16 springs, it dawned on me that it was a substantial difference (~10%) in force at the installed height. For these springs, 1.70" is the listed install height from Comp Cams. I understand these to be mostly drop-in for the stock springs, so I will work to match what is the current installed heights and use shims as needed.
Thanks for everyone's help. I'm thinking the plan is to ditch the deflectors since they don't fit and add in the LT4 style shims. They are about the same thickness as the old deflector, so that should yield the same install height as stock.
