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Pedestal rockers with new profile cam

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ivonius
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Pedestal rockers with new profile cam

Post #1 by ivonius » Sun May 17, 2020 8:53 pm

If I install a Comp 268 cam in my 300 (efi head) with pedestal style rockers, will I need to shim the pedestals or change the length of the pushrods, or do nothing, to handle the increased.456 lift vs .397 stock? How do hydraulic lifters handle more or less lift and base-line cam clearance with a non-adjustable pedestal rocker?
..bunch of Audis, + building a 67 Triumph Tr4a with 4.9 Ford. Gone but not forgotten: 66 Jaguar XKE (4.2 L6), several Triumphs (TR6 2.5 L6) , a couple of MG's, Datsun 280Z (2.8 L6), 64 Valiant Wagon (225 slant-six), three 57 Chevy's, 62 Impala, two Corvairs, etc; Worked high school & college at a Ford dealer as a mechanic (1968-1974). -Do all my own work

pmuller9
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Re: Pedestal rockers with new profile cam

Post #2 by pmuller9 » Sun May 17, 2020 9:36 pm

There was one case here where a Comp 268 cam was installed in an EFI engine without changing rocker height or pushrod length.
There didn't seem to be any mechanical problems.

The extra lift is not a problem for the lifters and pushrod length. It's the cam's base circle that needs to be considered.
If the new cam has the same base circle as the stock cam then the lifter preload will be the same.
If the new cam has a smaller base circle then the lifter plungers may run out of travel and longer pushrods would be needed.

With the side cover off look to see if the lifters get the .050" to .075" preload as you tighten the rockers in place.

ivonius
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Location: Cincinnati

Re: Pedestal rockers with new profile cam

Post #3 by ivonius » Tue May 19, 2020 10:00 pm

Is there not one serious mechanic among 82 viewers who will explain to me how a hydraulic lifter working through a no-adjustment pedestal rocker copes with a higher lift cam? I have worked with solid lifters and stud-adjustable hydraulic lifters my whole life, but I need some help here with bolt-down pedestal rockers.
..bunch of Audis, + building a 67 Triumph Tr4a with 4.9 Ford. Gone but not forgotten: 66 Jaguar XKE (4.2 L6), several Triumphs (TR6 2.5 L6) , a couple of MG's, Datsun 280Z (2.8 L6), 64 Valiant Wagon (225 slant-six), three 57 Chevy's, 62 Impala, two Corvairs, etc; Worked high school & college at a Ford dealer as a mechanic (1968-1974). -Do all my own work

pmuller9
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Re: Pedestal rockers with new profile cam

Post #4 by pmuller9 » Tue May 19, 2020 10:18 pm

I thought I did in my previous post but I'll try again.

Maybe you are thinking about correct rocker arm geometry with a higher lift cam.

If you paint the valve stem tip with a permanent marker or machinist dye and you find that the rocker tip is traveling behind the valve tip center then you can put shims under the pedestals to get the correct rocker arm height to center the travel on the valve stem tip.

At that point if the lifter plunger is up against the retaining clip because it has run out of travel then you have to get longer pushrods to regain lifter preload.

Anyway no matter what the case if adjustment is needed for any reason it is done by shimming pedestals and/or changing pushrod length.

Is this a good answer or am I still not getting it?
Last edited by pmuller9 on Tue May 19, 2020 10:53 pm, edited 1 time in total.

pmuller9
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Re: Pedestal rockers with new profile cam

Post #5 by pmuller9 » Tue May 19, 2020 10:32 pm

More info.
The heads with pedestal mount rockers use shorter valves than the early stud mount rocker heads.
Valves lifts near .500" and above will run the valve spring retainer into the valve stem guide.
The valves are 4.75" long while the early valves are 4.810" long.

Some of us drill and tap the head for stud mount rockers, add pushrod guides and use the SBC 4.910" long valves with 1.94" intake and 1.60" exhaust valve diameters and port the head accordingly.

Image

I'm not sure what the serious mechanics do.

ivonius
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Re: Pedestal rockers with new profile cam

Post #6 by ivonius » Wed May 20, 2020 8:59 am

Very helpful, Thanks muchly! One more question: The need for pushrod guides with the round-hole head.... because of higher lift, or is there something about a roller rocker that allows the pushrod to pop out of position?
..bunch of Audis, + building a 67 Triumph Tr4a with 4.9 Ford. Gone but not forgotten: 66 Jaguar XKE (4.2 L6), several Triumphs (TR6 2.5 L6) , a couple of MG's, Datsun 280Z (2.8 L6), 64 Valiant Wagon (225 slant-six), three 57 Chevy's, 62 Impala, two Corvairs, etc; Worked high school & college at a Ford dealer as a mechanic (1968-1974). -Do all my own work

pmuller9
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Re: Pedestal rockers with new profile cam

Post #7 by pmuller9 » Wed May 20, 2020 9:41 am

The pushrod guides align the top of the pushrod to keep the rocker tip centered on the top of the valve stem otherwise a stud mount rocker can turn sideways and off the valve stem.

There is no "off the shelf" guide plates that have the correct spacing for the Ford 4.9 six head so we use adjustable plates and weld the pairs to the correct center to center spacing.
Image

The Comp 268 cam has low valve lift and will work with the pedestal mount rockers but if you want to get serious about making power you need a high lift longer duration cam and would need to convert to stud mount rockers as shown.

Do you have the 1985 to 1986 large chamber pedestal mount head or the 1987 and later EFI head?

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MechRick
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Re: Pedestal rockers with new profile cam

Post #8 by MechRick » Wed May 20, 2020 10:33 am

I'm not sure I'm what you consider a serious mechanic, but I've done what you are asking about.

I installed a Comp 268 in an otherwise stock EFI cylinder head. I did check for valve spring coil bind and valve tip contact, and lifter plunger location. See my sig for details.

I don't recommend stock springs and retainers, I could float the stock valves (or the heavy retainers) at 5200 RPM.

The bolt-down rockers are adequate for these low RPM engines as long as there is no interference.
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
http://fordsix.com/forum/viewtopic.php?f=2&t=73244
Bronco II with a 2.3L swap http://fordsix.com/forum/viewtopic.php?f=13&t=72863
1988 F250 2x4, 460 ZF 5 speed.

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jgregg13
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Re: Pedestal rockers with new profile cam

Post #9 by jgregg13 » Wed May 20, 2020 5:10 pm

My engine is a '95 EFI engine with stock cam, valves & pedestal rockers. It didn't need to be decked or the head cut so the rockers didn't have to be shimmed up. But I also have an old carb engine for parts. So I used the valve springs and the Intake retainers off the carb engine to replace the exhaust retainers on the EFI engine. So I don't have rotators on the exhaust valves and I used some shims under the springs. The end result was some gain in seat pressure from the stock EFI springs. Just had to watch that I didn't get to coil bind with the longer carb springs.

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