I currently have a Comp 224/.581 cam. I picked up a set of LS6 heads, and had them cut to 60cc. From what I understand, thats "about" .030 off the heads. To compensate, I picked up a set of 7.350 pushrods.

Doin the math, this says I have lost about .020 off the lift of the cam. (a .050 shorter pushrod vs a head cut .030) Leaves me a net loss of lift of .020..

To compensate, I bought a set of 1.75 rockers to take up a little of the loss. From what I can see, these will increase lift about .015.

So,, net is about a .005 loss in lift..

First.... Is my "figuring" correct??

Second.. A few the pushrods touch the heads. Note, I only let it idle for a few minutes..

Third... Do the 1.75 rockers cause the clearance issue?

Fourth... Would you use the stock 7.40 pushrods??

Nothing seems to go easy!!!!!!!!!!!!!!!!!

Thanks for your time...

 

Well..

Its .030" x 1.7=.051 difference. this will be whats needed for pushrod length decrease. This does not change the overall valve lift only its position

The 1.75s wont really change the geometry much, only the lift slightly

.581@ 1.7 is now .598 overall lift

Phil

 

You did not lose any lift off of your CAM. The lift is built into the CAM lobe and is fixed amount with regard to the cam.
When you went to the 1.75 rockers that increases the TOTAL lift at the VALVE to MORE than the .581.
So you may have some clearance issues, you will have to check them.

 

with RoadRebel and CJS. Here are some of the reasons.

1: Aftermarket cams of the size you are running typically use a .030" smaller base circle. That is done to achieve the extra lift and keep the lobe ends from extending past the journal diameter so the cam can be installed and removed. That would give you +.015" of optimum pushrod length (distance from the top of the lifter to the pushrod cup on the rocker). .015 = radius of stock LS1 cam base circle - radius of your cam.

2: The LS6 heads have longer valves than LS1 heads. The LS1 cam base circle is 19.7mm and the '02 LS cam base circle is 19mm. This was done to achieve the same result as above. To compensate for that the 02 LS6 valves are .6mm longer than LS1 valves. That would result in -.023" optimum pushrod length assuming a stock LS1 cam and 02 LS6 heads and valves.

3: You have had your heads milled .030" which has resulted in -.030" optimum pushrod length.

4: So adding up the variables gives this result: + (.015") - (.023") - (.030") = -.038" of optimum pushrod length. This is assuming you are using the same head gasket thickness you were using before. For example, if you had a .052" gasket (graphite gasket) and went with a .060" MLS gasket you would need to add the .008".

5: The stock pushrod length is 7.400" so your ideal new pushrod length would be 7.362"

6: Hydraulic lifters do not work like solid lifters. They use what is called lash. A Solid lifter needs to have a small gap between the rocker tip and valve cap. A hydraulic lifter needs to be tight againt it and actually pushed some distance into it. The lash on the stock lifter is .020" and the lifter can tolerate up to .040" and as little as .010"

7: Since you now have a pushrod that is 7.350" and optimum is 7.363" you are out of spec on the lash about .013". You also have to set the preload on the rockers and that will affect your geometry. There is also one other variable to consider and that is the pushrod cup height of the new rocker. It may not be in the same place as the stocker so there may be a difference there.

8: The best way to find the optimum pushrod length would be to use a pushrod length checker. Comp Cams sells one for about $20. It is a pushrod with one end that screws in or out and is calibrated. You would install it and adjust the length by screwing it in or out until you find the optimum length. Then read that and call Comp Cams and order a set of custom length pushrods. They can overnight them to you.

Valvetrain geometry is not a simple subject and it gets more involved as you change more variables. The variable length pushrod checker is a simple solution to this problem.

 

That is some good info. Regardless of the pushrod length though, part of the issue is that 3 or 4 pushrods (same diameter as the stock pushrods) are just scraping the hole where they come through below the rockers. The rockers (comp rollers) are installed and torqued to spec like stock rockers. Yet there is a clearance issue but on only a couple of them.

Thanks

 

What happens if you use the stock 7.400 pushrods? The gent the heads came from said new pushrods weren't needed and the lifters would take up the slack.

Just another option.. Or is it??

(btw, had no clearance issues as far as the piston to the valves, only the pushrod to head issue)

 

Have you tried keeping the 7.350's in and putting the stock rockers in? I was thinking that if your new rockers had a shorter distance between the pushrod cup and the mounting bolt (to acheive a 1.75:1 ratio) that could cause the pushrods to scrape. I don't think you are out that much on the pr length with the 7.35's due to the .030" milling, but I think that trying to make up .037" on the lifter after preload might be a bit much.

 

Dumb question. By "after preload" I'm not sure what you are referring to. Unlike regular SBC H-lifters where you adjust the rocker preload (zero lash when you can't spin the pr any more and then another 1/4 or 1/2 turn depending) the rocker is torqued static (22ftlbs). So what am I missing?

 

[QUOTE=Fastbasser]I currently have a Comp 224/.581 cam. I picked up a set of LS6 heads, and had them cut to 60cc. From what I understand, thats "about" .030 off the heads. To compensate, I picked up a set of 7.350 pushrods.

cut .005=1cc
10.2:1 = 66.67 (Stock LS1 Head)
10.3:1 = 65.96
10.4:1 = 65.18
10.5:1 = 64.45 (Stock LS6 Head)
10.6:1 = 63.61
10.7:1 = 62.85
10.8:1 = 62.10
10.9:1 = 61.37 (Approximate Combustion chamber of 5.3L heads +/-)
11.0:1 = 60.66
11.1:1 = 59.96
A .030 cut gets you to about 11:1 with LS1 heads. With LS6 heads it's about .019. 60 cc would be around .022. Do the math again, it looks like stock rockers and 7.4 pushrods are what you need.

 

When the rocker is torqued to 22lb/ft static there should be about .030" +- .010" of preload on the lifter. If not, a longer or shorter pushrod can be used or shims can be installed under the rockers.

The issue that has me concerned is what you pointed out, some of the pushrods are scraping on the heads. That is clearly not normal. If they cleared before the swap it is being caused by the rockers, pushrods, or both. If that is so, I would go back to stock and try swapping in one component at a time until the issue shows up, then go back to the previous setup and stay there.

After looking at the numbers gojo posted I would agree with him, which is what the seller told Fastbasser also. If Fastbasser wants to keep the 1.75 rockers for the increased lift he may need to use a pushrod checker to find the proper pr length and go with a set of customs, unless there is a clearance issue introduced with the 1.75's.

 

I was lookin towards the stock setup. I guess its not a big deal to swap them out.

All it takes is time I guess!