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Discussion Starter #1
Hello,

I have a MkIII with a 390FE that I wanted more RPM out of. I pulled the Comp Cams 256Hyd (5200rpm about it) cam out and put in a 270Solid. At the same time I installed Comp Cams FE roller rockers (since I needed adjustable rockerarms). I have new push rods (Stock Mech Cam Length).

With the adjustment nuts fully backed out, at TDC I have no clearance to adj the valves. I think I may need shorter pushrods then stock, or can I shim the rockershafts without losing lift or valvetrain strenght?
 

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Rocker stand shims

Yes, you can shim the rocker stands, but must all be the exact same shim thickness and no more then .040. Uneven shims would "tweak" the rocker shafts into a bind. This train of thought is not the recommended way to go with push rods available in different lengths or custom made ones.

What push rods are you running? Stock push rods are probably not the right ones to use if you are using a known vendors cam and rockers. I suggest that you call Comp and ask what push rods are required for the set up you have put together. They are usually pretty well versed in what works together and what will cause you headaches.

How mush more RPM are you expecting to get? 390's are not small block "Buzz Bombs" and start to get some block casting shift when you get real close to 6000 - 6200 unless you have done your homework on preparing the block for higher. 427's would go just past 7000 with the LeMans rods back in the day but also had cross-bolted mains, 390's don't.

HTH

Doc
 

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Discussion Starter #4
what push rods do you have???
Comp say's to use 7533-16 which are 9.157".
In summit there substituted by 7530-16
http://store.summitracing.com/partdetail.asp?sub=1&part=CCA-7530-16&N=700+115&rsview=sku.
Direct from Comp there cheaper.
http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=CC&Product_Code=7530-16&Category_Code=

I've got Cranes in mine with a hydraulic that are 9.234" (#34645)

Where's the rocker tip on the valve??
Thanks for the quick responce!
The Pushrods are the Comp Cams from Summit 9.157" #7530. I just looked up on Summits web site and they list a 9.0 " (roller cam?), a 9.157" (mech cam) and the 10.65"(hyd cam). The roller tips appear to be centered on the valve. I put a washer under the stands to raise up the Shafts, I think they are about .080 or .100" thick. Seem to be the minimum to allow for me to do .022 mech cam adjustment. Do you think I should get the 9.0" rods?
 

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Discussion Starter #5
Yes, you can shim the rocker stands, but must all be the exact same shim thickness and no more then .040. Uneven shims would "tweak" the rocker shafts into a bind. This train of thought is not the recommended way to go with push rods available in different lengths or custom made ones.

What push rods are you running? Stock push rods are probably not the right ones to use if you are using a known vendors cam and rockers. I suggest that you call Comp and ask what push rods are required for the set up you have put together. They are usually pretty well versed in what works together and what will cause you headaches.

How mush more RPM are you expecting to get? 390's are not small block "Buzz Bombs" and start to get some block casting shift when you get real close to 6000 - 6200 unless you have done your homework on preparing the block for higher. 427's would go just past 7000 with the LeMans rods back in the day but also had cross-bolted mains, 390's don't.

HTH

Doc
Hello Doc,

I tried shimming with some washers and guessing they are .080/ .100 thick which does the trick, barely. I am not too comformtable with shimming, as the oil pressure goes up the the shaft stands, and I am concerned that I may affect oiling to the rockers too. I am thinking picking up an adjustable pushrod to test without shims and see if I can figure up the correct lenght. As I posted above, with the info from convincor, summit has 3 lenghts listed and it's looking to me the 9.00" may be what I need, but measuring to be safe might be prudent at this point.

As far as RPM I figured 5200 seemed a bit lower then I was aiming for. With the hyd cam, I had no noticalbe powerband, and great low end torque. I have Edelbrock heads, RPM intake a 750DP and FFR headers, I was hoping for a 6000 RPM motor to take advantage of the combo a little better. The engine is professionally balanced when built also.
 

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Push Rod Length

gwfortune,

Definately get an adjustable push rod and test for the best length to put the rocker thru full contact with the valve stem tip thru one full rotation. In other words, you want to have the rocker half way thru it's travel arc at half way thru the cam cycle. Adjust the adjustable push rod to get as close to this senerio as possible. When you get the most contact with the center of the valve stem, that is the length that you should order . . . or as close to that measurement as possible.

If your push rod length is too long, the rocker will wipe the outer edge of the valve stem and cause excessive side loading of the valve . . . too short and it will wipe the inner edge and cause the same problem in the opposite direction. The plan here is to get the rocker to press the valve stem straight up and down as close to perpendicular as mechanically feasible. Too short also runs the risk of rocker to spring retainer interference also.

In my last responce, I mentioned that "shimming" is NOT the way to go and the oiling issue is why. With standard washers found at the local hardware store, you risk oil leaks and the rocker stands being "tweaked" in one direction or another . . . causing the shafts to bend and making the rockers work off to one side as opposed to perpendicular to the valve stems. I know we're only talking about a few thousands here, but any "errors" in the valve train will show up at higher RPM's . . . not where you want a failure.

Get the adjustable push rod checker and go from there . . . the difference in the sizes from Summit and other places is close to ¼", that's a lot.

What does your cam grinder say? What does your Head vendor say?

Good Luck . . .

Doc
 

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With shaft mounted rockers pushrod length will NOT change valve tip location as the shaft is a fixed position.
With STUD mounted rockers you can effectivly raise or lower the pivot point of the rocker to get the correct geometry, but not with the shaft mounted without raising or lowering the whole shaft.
 

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If your pushrods are too long you can shorten them.
The cups are pressed in and can be removed and have the rods turned in a lathe to desired lenght and press cup back in.
 
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