Ted;



Since the heads I am building will have new seats/valves raised flush with the chamber roof, and the cam I will use has higher lift, I assume the rocker stand pedestals will likely need to be milled to obtain correct geomertry. In order to save a lot of trial & error assembly labor, I wonder if there is a rule-of-thumb formula you may have developed over the years (based on % of centering error?) to estimate the required cut?

6 VOLTS/POS. GRD. NW INDIANA

Dan,

       Think about it, if anything you'll need SHORTER pushrods, or raised pads. Actually I doubt if it will be necessary on a street engine.

Frank/Rebop

Bristol, In ( by Elkhart) 

Frank,



Have you read Ted's YBM geometry articles? Push rod length not relevant to rocker geometry (as such) with shaft system (only SBF and scrub relates).

Push rod length concerns head model/adjuster range/virtual rocker ratio with Y-block design.

Raising valve seats and increasing cam lift would indicate LOWERING shafts to compensate (unless Ted got it wrong).


6 VOLTS/POS. GRD. NW INDIANA

Isn't "necessary" a rather vague term? Is ballancing/blueprinting "necessary"? Are aluminum repro castings "necessary"? Is a "street engine" one that is only driven rarely, on sunny weekends, or one designed to run at max. efficiency for 100K+ miles without incident?

Sorry, but since this rebuild is a MAJOR SOURCE OF STRESS for me, I would rather go the extra mile and make it my last one!



Upon reflection, I must apologize to all (especially Frank, who supplies invaluable products), as nit-picking and attention to minor technical detail is just my way of warding off Alzheimer's!




6 VOLTS/POS. GRD. NW INDIANA

Daniel.  Unfortunately I don't have a generic rule for the height adjustment of the rocker stands and do still have to physically dry assemble the rocker assemblies on the heads and do an actual measurement to verify rocker arm geometry.  This is in a large part due to typically using none stock valves which have the valve tips sitting at a different height than stock.  But changing to a camshaft with a different lift would also require checking the geometry also assuming it hadn't been checked before the cam change.  After assembling the valves within the heads be sure to put a straight edge across the tips of the valve stems and insure that they are all at the same height.  Not doing this nullifies some of the advantages of correcting any rocker arm geometry issues that are being compensated for.

But here's some more food for thought.  Instead of going for perfect geometry where both the travel pattern across the top of the valve tips is minimized and valve guide wear scrubbing is kept to a bare minimum, the rocker arm geometry can be biased so that the rocker shaft is higher than the 'ideal' location.  What this does is promote a faster rate of initial opening at the valve and a subsequent increase in low lift flow numbers.  In some instances and mainly depending upon the head and camshaft combination, this increases overall performance and more than offsets any negatives with not having 'ideal' geometry at the top of the valve stem if not concerned about increased guide wear.

Lorena, Texas (South of Waco)

Dan,

        Remember, Ted deals with LOTSA lift and serious RPM. I didn't say don't check, I said you probably won't have to adjust much. Milling the rocker stands is going in the wrong direction. Most pushrod length checkers start with the assumption that you want the tip of the rocker arm on a the centerline of the valve at half the lift. This promotes even wear and somewhat less side thrust on the valve. Easier on guides and seals. He does make an excellent point on checking valve heights to be sure they are all the same.This also affects to the valve spring heights, installed. You could easliy wind up with 16 different heights, depending on how good the machinist was. 

Frank/Rebop

Bristol, In ( by Elkhart) 

Ted,



Assuming you wanted to trade guide wear for increased low-lift flow (by raising the shafts), wouldn't you also have to increase pushrod length to avoid giving back rocker ratio? Or would that just aggravate the guide wear in exchange for insignificant total lift?



After you have dry-assembled and checked geometry, if you DO decide the pedestals need to be milled, what do you do then? Just take them down .001" and reassemble/check? Could take forever without some idea re: how much correction needed vs. how much initial geometry error. Though I suppose going too far and having to shim them back could be worse.

6 VOLTS/POS. GRD. NW INDIANA

Frank,



According to Ted, centering the rocker not as important as reducing the length of arc/travel, which is what really causes guide wear.



Also, my machinist VERY aware his work will be scrutinized in detail. While I can check finished/assembled heads with straight-edge, I will trust him re: spring seat height, spring shimming/testing, etc., or I would have to buy a spring compressor and a bunch of other tools. I could do a rough check to make sure he has CC'ed and matched the chambers accurately. To avoid pulling the heads after they are installed, I DO want to have him remove a spring from each one so I can bring them home, install the shafts/rockers, and get a rough idea of how close the geometry is. I have a dial indicator and the cam lift specs., and will improvise a very weak spring to simulate rocker/valve motion. If the valve heights are even, checking the geometry of only one rocker should put me in the ball park as the valve gear is all new?

6 VOLTS/POS. GRD. NW INDIANA

Dan, Yup. Remember, the base circle of the cam may be smaller than the original while you're at it.

Frank/Rebop

Bristol, In ( by Elkhart) 

You’re correct in that altering the rocker arm pedestal height also mandates a change in push rod length simply to keep the rocker arm ratio as it was.  If the pedestal is raised 0.100”, then the pushrod length is simply increased the same amount to guarantee that the rocker arm adjuster stays in the same position thereby providing the same rocker arm ratio.  In this particular instance, if the pushrod is not lengthened, then the rocker arm ratio will be reduced if the adjuster can be screwed down enough to still maintain desired lash.  Guide wear gets more 'aggravated' as the lift increases but altering the valve train geometry to get some better low lift numbers compounds this wear factor regardless of the lift.

 When altering the stand heights, I’ll work in ~0.025”-0.030” increments.  I’ll do measurements with a single pair of valves (an intake and exhaust) so I’m only having to work with two pedestals while using the rockers that will ultimately be used for that particular set of heads.  If raising the pedestals, it’s simply a matter of shimming up the stands until I get the desired geometry.  Equal thickness washers works well for this.  If lowering, then I’ve got a pair of cut down stands that I simply keep raising or shimming up until I get the geometry where I like it.  Then it’s a matter of measuring the ‘cut down’ stand length and adding to that value whatever the amount of shim was added and then cutting a set of stands to that particular length.  I’ll cut all eight stands at one time with them mounted on a shaft to insure that they all end up at the same height.  If needing to increase the height, then I’ll use the longer Lincoln stands and cut them down to the desired length in lieu of using shims to raise the stock stands.

 

Trust that was clear.

Lorena, Texas (South of Waco)