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Old 08-16-2007, 09:47 AM
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Valvetrain geometry problems on SBC, need some help!

Guys,

I have an issue with my stud length - read below for how I got here. Sorry for the length - just trying to get the info out there.

On my 68 Camaro, I decided to replace the valve springs for the correct ones for my cam. THat's a whole 'nother story, but now I have double springs with the correct pressures. Installed at 1.850 (using a +.050 valve locks), I have about 135 lb pressure closed, 385 or so open at 1.250

So then I decided to replace my cheap skate no name rockers (that I received with a pile of parts when I bought the car in pieces) with some good ones. I heard good things about the PVS rockers from Miller Engineering, did some research and bought some from a mail order place for a great price.

Miller's website has a great way to measure where the rocker trunion should be at relative to the valve tip, so I proceeded to measure my geometry with the 7.400" pushrods that I had used before. The geometry was not correct (according to Jim Miller anyway), so I bought longer 7.600" pushrods (determined with length checker rod).

I have Dart Pro1 230cc heads, with standard SBC screw in studs and guideplates. Now with the longer pushrods, I can only get about 2-3 turns of thread engagement on the rocker adjusting nuts, + 1/4 or 1/2 turn for lifter preload.

Is just 2-3 turns enough or will I pull the threads off the rocker stud / adjusting nut???


If 2-3 turns is not enough what should I do to make this situation correct?? Everyone I've asked around home is stumped as to why I'm having this issue.

What would happen if I installed another set of guideplates (wallowed out) under the studs so that I had more thread engagement with the polylocks?


ANy help would be much appreciated, as I've not run into this before.

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Old 08-16-2007, 09:57 AM
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What length stud? The ARP Pro series studs are longer and may help.
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Old 08-16-2007, 09:59 AM
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Quote:
Originally Posted by lmsport
What length stud? The ARP Pro series studs are longer and may help.
I measured from the Guildplate face to the tip of the stud and it's about 1 7/8" long.

The speed shop's ARP studs were the same length. DOn't know if they were "Pro" series or not.
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Old 08-16-2007, 10:40 AM
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Pick-a-Stud

http://www.arp-bolts.com/Catalog/Cat...atalog046.html
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Old 08-16-2007, 11:02 AM
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Quote:
Originally Posted by lmsport

Only 1 problem: most of those studs are 7/16" dia., which would be fine had I known and bought the 7/16 rockers. ALas, mine are 3/8", and the longest thing that ARP lists there is the 3/8" stud for small blocks with roller rockers - and they are 1.895" long, which is what I have currently.

Back to the drawing board . . ..
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Old 08-16-2007, 11:18 AM
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Quote:
Originally Posted by hotrodf1
Guys,

I have an issue with my stud length - read below for how I got here. Sorry for the length - just trying to get the info out there.

On my 68 Camaro, I decided to replace the valve springs for the correct ones for my cam. THat's a whole 'nother story, but now I have double springs with the correct pressures. Installed at 1.850 (using a +.050 valve locks), I have about 135 lb pressure closed, 385 or so open at 1.250

So then I decided to replace my cheap skate no name rockers (that I received with a pile of parts when I bought the car in pieces) with some good ones. I heard good things about the PVS rockers from Miller Engineering, did some research and bought some from a mail order place for a great price.

Miller's website has a great way to measure where the rocker trunion should be at relative to the valve tip, so I proceeded to measure my geometry with the 7.400" pushrods that I had used before. The geometry was not correct (according to Jim Miller anyway), so I bought longer 7.600" pushrods (determined with length checker rod).

I have Dart Pro1 230cc heads, with standard SBC screw in studs and guideplates. Now with the longer pushrods, I can only get about 2-3 turns of thread engagement on the rocker adjusting nuts, + 1/4 or 1/2 turn for lifter preload.

Is just 2-3 turns enough or will I pull the threads off the rocker stud / adjusting nut???


If 2-3 turns is not enough what should I do to make this situation correct?? Everyone I've asked around home is stumped as to why I'm having this issue.

What would happen if I installed another set of guideplates (wallowed out) under the studs so that I had more thread engagement with the polylocks?


ANy help would be much appreciated, as I've not run into this before.
Depending on what kind of nut you're using:
- For factory crimp locking style nuts, you should have stud threads all the way through the nut, a couple above the nut is a good plan.

- For Polynut styles where a counter screw is theraded into the end, the stud should be positioned in the nut such that the allen screw can be turned all the way into the nut when locked.

The proper way to set the push rod length is to have:

- With the lifter on the base circle of the cam, the contact point of the roller on the valve stem should be about the 1/3 point of the stem diameter toward the pushrod/intake side of the engine.

- With the lifter at 1/2 the max lift of the lobe, the contact point of the roller on the stem should be in the middle of the stem diameter.

- With the lifter on the top of the lobe, the contact point of the roller on the stem should be toward the 1/3 diameter point of the stem toward the exhaust side.

See Comps web site for a simpler expanation than Jim Miller's: http://www.compcams.com/Technical/FA...inGeometry.asp

When the roller moves as outlined above, the pushrod length that makes this happen is the best it can get.

ARP sells a 1.9 inch thread length stud P/N 134-7104 if some extra length on the nut screw side is needed.

Bogie
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Old 08-16-2007, 11:29 AM
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Quote:
Originally Posted by oldbogie

- For Polynut styles where a counter screw is theraded into the end, the stud should be positioned in the nut such that the allen screw can be turned all the way into the nut when locked.

The proper way to set the push rod length is to have:

- With the lifter on the base circle of the cam, the contact point of the roller on the valve stem should be about the 1/3 point of the stem diameter toward the pushrod/intake side of the engine.

- With the lifter at 1/2 the max lift of the lobe, the contact point of the roller on the stem should be in the middle of the stem diameter.

- With the lifter on the top of the lobe, the contact point of the roller on the stem should be toward the 1/3 diameter point of the stem toward the exhaust side.

See Comps web site for a simpler expanation than Jim Miller's: http://www.compcams.com/Technical/FA...inGeometry.asp

When the roller moves as outlined above, the pushrod length that makes this happen is the best it can get.

ARP sells a 1.9 inch thread length stud P/N 134-7104 if some extra length on the nut screw side is needed.

Bogie
I'll check out COmp Coms, but the Jim Miller way makes a lot of sense to me. He's the only one with a patent on valvetrain geometry. How crucial it is, is another matter I suppose, but I wanted to get close.

Unfortunately, that part number that you referenced above is the same length as the stud that I have on the heads now.

Can I put a second set of guideplates under the stud to effectively made the stud "longer"?
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Old 08-16-2007, 12:05 PM
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Quote:
Originally Posted by hotrodf1
I'll check out COmp Coms, but the Jim Miller way makes a lot of sense to me. He's the only one with a patent on valvetrain geometry. How crucial it is, is another matter I suppose, but I wanted to get close.

Unfortunately, that part number that you referenced above is the same length as the stud that I have on the heads now.

Can I put a second set of guideplates under the stud to effectively made the stud "longer"?

If your have clearance between the bottom of the rocker and the hex, all you need is some HD washers, like head bolt washers. Just dont decrease the thread engagement in the head too much.
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Old 08-16-2007, 01:15 PM
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Quote:
Originally Posted by hotrodf1
I'll check out COmp Coms, but the Jim Miller way makes a lot of sense to me. He's the only one with a patent on valvetrain geometry. How crucial it is, is another matter I suppose, but I wanted to get close.

Unfortunately, that part number that you referenced above is the same length as the stud that I have on the heads now.

Can I put a second set of guideplates under the stud to effectively made the stud "longer"?
I wouldn't get too carried away with Mr. Miller's geometry lessons. So many guys selling aftermarket parts berate the OEM engineers like they're a bunch of dummies and they act as if no common specifications exist. However, all these engineers in Detroit and else-where all belong to the SAE and they through the SAE write industry wide spec. Plus, each company has it's own internal specs which may or may not agree with the SAE spec.

When you open your own store front you can pretty much technically say and do as you please. When you work for a big company you first are confronted by having to design with-in corporate and SAE guidelines. Then for every engineer, there are 3 or 4 finance guys a couple packaging engineers, and a handful of manufacturing guys hanging around your desk. The former telling to get the cost out, the middle telling you it won't fit the frame and body profiles, and the latter telling you he can't manufacture your design. It's more fun than you can imagine.

Now for the 1/3 rule, it's a good rule. No matter the tip roller's position to the trunnion, it always swings an arc. The general rule is to place the middle of the arc, which should occur at the middle of the lift, on the middle of the valve stem. This is simply to minimize rocking loads on the stem. Generally when the rocker to stem is set up this way, the contact point is within the inboard 1/3 diameter position of the stem when the valve is closed while at the 1/3 out board diameter position on the stem at full open. A mild cam set up at mid lift will probably not excision as far as the 1/3-1/3 distance. A high lift cam, say above .6 inch will traverse somewhat beyond these outer points. So long as the contact point doesn't fall off the stem, you're OK. For those extreme lifts where the contact gets on the edge, there are hardened lift caps or longer stem valves, or both are used to put the contact geometry back into the preferred location.

Yes the effective rocker ratio changes through-out the lift range. This can't be helped, the valve stem intersects the rocker at a given tangent. As the rocker rotates, while the valve only goes up and down, the tangent point to the rocker moves in relation to the valve stem. You can play some dimensional games here that minimize or accentuate the tangent movement, but you can't eliminate it.

Yes you can put a washer under the hex of the stud to raise it a bit, but the rocker should be position correctly without this messing around. I'd recommend going back over the pushrod length measurements using what I discussed here and Comps pictures as an aid to visualizing what this should look like. You might have to consider some lash caps to work this, I don't know what cam your running nor how long the stems are in the heads you have. The Miller PVS mid lift is really intended for GM production heads, when you get off onto aftermarket heads issues can be discovered.

With problem solving in mind, you can make your own stud using high quality 7/16x14 bolts. Turn the shank above the base dia thread to 3/8s then cut a fine thread. On the coarse thread run a jam nut on it, slip on the push rod guide, screw the coarse end into the head with some anti-seize then torque the nut to spec. Then assemble the top end. But you shouldn't be having to do odd things like this for a street engine.

Bogie
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Old 08-16-2007, 01:32 PM
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Stud thread engagement should be mimimum the diameter of the stud. 3/8" stud, 3/8" thread engagement. Remove one of the studs from the head. Screw it back in to just where the threads begin to catch. Measure the gap between the head and the bottom of the hex on the stud. Subtract 3/8" (0.375"). The remainder of the measurement will be the thickness of the washers or shims you can use under the stud to space it up farther, assuming no interference between the stud hex and the bottom of the rocker.
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Old 08-16-2007, 01:47 PM
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If you put any kind of spacers under the studs you risk pulling the threads out of the head, however the thickness of guide plates usually wont make a difference. You say your spring pressure tops out around 400lbs thats some pretty dramatic forces. If you cant find any studs, you might look into having a machine shop make you some, the guys that run the milling machines around here can make just about anything, and can harden them if they have a magnetic heating coil.

EDIT: yeah tech beat me to it - thats some real good advice
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Old 08-16-2007, 03:45 PM
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Quote:
Originally Posted by techinspector1
Stud thread engagement should be mimimum the diameter of the stud. 3/8" stud, 3/8" thread engagement. Remove one of the studs from the head. Screw it back in to just where the threads begin to catch. Measure the gap between the head and the bottom of the hex on the stud. Subtract 3/8" (0.375"). The remainder of the measurement will be the thickness of the washers or shims you can use under the stud to space it up farther, assuming no interference between the stud hex and the bottom of the rocker.
Good idea. Makes sense to me.

I think mine are .700" into the head, but the thread is 7/16 on that end I think, so perhaps spacing them out a little bit will be the fix for me. Will be close, but since I've only got a hyd. roller, it should be okay. If I had a wicked solid roller with open pressures of 600 or more, I might worry a little more about it.
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Old 08-16-2007, 03:59 PM
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Quote:
Originally Posted by oldbogie
I wouldn't get too carried away with Mr. Miller's geometry lessons. So many guys selling aftermarket parts berate the OEM engineers like they're a bunch of dummies and they act as if no common specifications exist. However, all these engineers in Detroit and else-where all belong to the SAE and they through the SAE write industry wide spec. Plus, each company has it's own internal specs which may or may not agree with the SAE spec.

When you open your own store front you can pretty much technically say and do as you please. When you work for a big company you first are confronted by having to design with-in corporate and SAE guidelines. Then for every engineer, there are 3 or 4 finance guys a couple packaging engineers, and a handful of manufacturing guys hanging around your desk. The former telling to get the cost out, the middle telling you it won't fit the frame and body profiles, and the latter telling you he can't manufacture your design. It's more fun than you can imagine.

Now for the 1/3 rule, it's a good rule. No matter the tip roller's position to the trunnion, it always swings an arc. The general rule is to place the middle of the arc, which should occur at the middle of the lift, on the middle of the valve stem. This is simply to minimize rocking loads on the stem. Generally when the rocker to stem is set up this way, the contact point is within the inboard 1/3 diameter position of the stem when the valve is closed while at the 1/3 out board diameter position on the stem at full open. A mild cam set up at mid lift will probably not excision as far as the 1/3-1/3 distance. A high lift cam, say above .6 inch will traverse somewhat beyond these outer points. So long as the contact point doesn't fall off the stem, you're OK. For those extreme lifts where the contact gets on the edge, there are hardened lift caps or longer stem valves, or both are used to put the contact geometry back into the preferred location.

Yes the effective rocker ratio changes through-out the lift range. This can't be helped, the valve stem intersects the rocker at a given tangent. As the rocker rotates, while the valve only goes up and down, the tangent point to the rocker moves in relation to the valve stem. You can play some dimensional games here that minimize or accentuate the tangent movement, but you can't eliminate it.

Yes you can put a washer under the hex of the stud to raise it a bit, but the rocker should be position correctly without this messing around. I'd recommend going back over the pushrod length measurements using what I discussed here and Comps pictures as an aid to visualizing what this should look like. You might have to consider some lash caps to work this, I don't know what cam your running nor how long the stems are in the heads you have. The Miller PVS mid lift is really intended for GM production heads, when you get off onto aftermarket heads issues can be discovered.

With problem solving in mind, you can make your own stud using high quality 7/16x14 bolts. Turn the shank above the base dia thread to 3/8s then cut a fine thread. On the coarse thread run a jam nut on it, slip on the push rod guide, screw the coarse end into the head with some anti-seize then torque the nut to spec. Then assemble the top end. But you shouldn't be having to do odd things like this for a street engine.

Bogie

Thanks for the info. That method of the 1/3 1/3 rule is kind of the same principle as the Miller measurement method. Geometrically, my rocker should be almost exactly in the middle of the valve tip at mid-lift. Miller's principle, though is to get to that level by having the roller tip half of the net valve lift above the center of the trunion, which also means that at full lift, the rocker will be half the net valve lift below the center of the rocker trunion.

To me, even if the roller tip doesn't end up EXACTLY in the middle of the valve tip, the geometry is better than if I would shorten the push rod to get it there, which creates the "over arcing" effect that Jim Miller talks about.

He did a geometric example in CAD that showed a possible scenario in real world application. The example he showed started from an "over arcing" .685 net valve lift with improper rocker geometry, then he showed it with the proper 50/50 above and below the trunion type setup, and the net valve lift increased to .709".


And like you said, setting it that way helps to minimize the tangential movement of the roller tip across the valve tip.

I am wondering if perhaps the PVS rockers are just enough "different" than the normal rocker design, and that's throwing me this curve ball.

Oh well. Always learning new things. Mostly about what "not" to do.

On a side note: why does ARP make so many lengths in the 7/16" stud but only 2 in 3/8"????



Thanks everyone for your input so far!
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Old 08-17-2007, 06:50 PM
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I had a similar issue on my last engine. I am using a low-lash solid roller with 1.65:1 Harland Sharp roller rockers and lash caps. With this setup, a stock length pushrod was way to short, as you can see in the picture the sweep across the top of the valve was about 3/16" or so. Way more than I wanted! So, I just worked on extending the pushrod length until my pattern was minimized. This is basically the same idea as the Miller reference. I ended up with 8.15" pushrods, a 0.35" increase over stock. This also required going to a longer stud to provide proper thread engagment. This reduced my contact pattern to about 1/16" or less. It is slightly past the centerline, but that will not be a problem since there is effectively no side loading.
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Old 08-17-2007, 07:46 PM
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Are the valve stems standard length?
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