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Author Topic:   rocker arm geometry
Mark From Detroit

Posts: 29
Registered: Apr 2001

posted 05-25-2001 09:20 PM     Click Here to See the Profile for Mark From Detroit   Click Here to Email Mark From Detroit     Edit/Delete Message   Reply w/Quote
I need advise on how to correctly check rocker arm geometry. It looks like I have a problem with mine and want to take care of this before destroy my rebuilt heads.


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S/Q 2204

Posts: 89
From: Ozark, AL(again after a year of being deployed)
Registered: Feb 2001

posted 05-26-2001 12:18 AM     Click Here to See the Profile for S/Q 2204   Click Here to Email S/Q 2204     Edit/Delete Message   Reply w/Quote
I use a home made adjustable push rod. Use checking springs on want to check (very light weight). Apply machinist dye to valve tip; reinstall rocker & rotate engine through. Should rub off dye in the middle of valve tip. Lengthen or shorten push rod to get it there then measure push rod to order correct length if your current length is wrong.
Hope this helps. I have had my share of valve train problems & it can wear you out.
Good luck,
Mark J.

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Posts: 733
From: Demopolis AL
Registered: Jun 99

posted 05-26-2001 03:44 AM     Click Here to See the Profile for 74merc   Click Here to Email 74merc     Edit/Delete Message   Reply w/Quote
I was going to post this same question sometime soon, that's a helluvalot simpler than I thought it would be...

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Posts: 31026
From: Saco, Maine
Registered: May 99

posted 05-26-2001 03:20 PM     Click Here to See the Profile for SteveLaRiviere   Click Here to Email SteveLaRiviere     Edit/Delete Message   Reply w/Quote
If you don't have machinist's dye, magic markers work well, too.

'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Loc
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC {Wife's car...}
All Fords since 1977

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Mark From Detroit

Posts: 29
Registered: Apr 2001

posted 05-26-2001 05:19 PM     Click Here to See the Profile for Mark From Detroit   Click Here to Email Mark From Detroit     Edit/Delete Message   Reply w/Quote
I just recieved my adjustable pushrod from summit and had to go much shorter than stock at .230 or almost a 1/4 inch to get the rocker to be in the center in the valve tip. This is a 351C with Harland Sharp roller rockers with a mild street cam and am adjusting the rockers 1/2 turn after zero lash. The heads were rebuilt and block was redecked by a quality engine rebuilder. I can understand some minor changes but this is a big difference. What gives?


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Posts: 666
From: Gadsden, Al
Registered: Aug 99

posted 05-26-2001 11:35 PM     Click Here to See the Profile for Dave_C   Click Here to Email Dave_C     Edit/Delete Message   Reply w/Quote
Mark from Detroit:

Two possible things: Are you centering the rocker with the valves open, closed? Where? It needs to be in the exact center at 1/2 lift. I use a dial indicator and find half lift and center it up there.

Also: decking the block will have a major effect, depending of how much. Decking the block is the same effect as putting a longer pushrod in. Same goes if the heads have been cut.

.230 would be not be unheard of. My 383W uses pushrods that are .200 short. Geometry is perfect. I mark the valve tip with a magic marker and shoot for a pattern that covers the center 1/3 of the tip.


David Cole

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Posts: 1367
From: Spencer, WV
Registered: Jan 2001

posted 05-27-2001 03:40 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
The last two engines I built were Ch*vys. Stock pushrods are 7.800"... and I had to use 8.050" on one, and 8.100" pushrods on the other.

ANYTIME you start using aftermarket cams, rockers, valves, heads, milling, etc... the valvetrain geometry NEEDS to be checked. 9 out of 10 engines (even mild ones with very little done) will need different length pushrods to correct valvetrain geometry.

Poor geometry wipes out guides and valvetrains really quick. It's something that NEEDS checked every time.

Good Luck!

Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3

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