I finally got started on the 406 again and switched over to a solid Isky cam,and while I was at it roller rockers ,7/16" studs ,and was checking the valve train geometry. I made an adjustable pushrod,and by trial and error,ended up .250" longer than the stock 7.8" rod. The pattern I see is when the valve is closed the roller is slightly off center of the valve tip,favoring the intake side .When the valve is fully open it's slightly favoring the exhaust side.From what I've read this is what should be the case,but holy cow.How can the stock pushrods be that far off the mark.The cam is a standard base circle cam,and I've always run 1.60 rockers,(though these are full roller,the previous set was only roller tip)
Is it possible the shorter pushrods would have caused the pushrod to break?(That's what started this whole saga.)

 

After market heads, milled heads, decked blocks, head gasket thickness, valve stem length, camshaft base circle, are just what pops off the top of my head, quickly, that alters the valve train geometry. Even though I don't think Chevy obsesses with the geometry , I believe the basic, as, designed, geometry must be pretty close. Your 1/4" deviation must have an explaination.

 

This is obviously not an original motor, deck height differences and past milling of heads can all contribute to the offset of vavle train geometry. Also, I know that the base circle of a cam is supposed to be "X" but it is really up to the manufacturer what the actual base circle of the cam is ... lax tolerences here and there can add up quickly. Thats why you always check all of the clearances when building a motor, remember what you didn't look at will probably come back to bite you.

 

I'm using 8 inch rods on my solid roller 406 with Brodix heads. Cam has 1.10 base circle, block is not decked, .027 head gasket, with 1.6 pro mags. Dont know the size of the valve stems off hand, but the spring install height is 1.95. Hope that helps.

 

Different lifters and different rockers, dosen't matter what you used before, they could have been the right length who knows, just make sure the new ones are correct length

 

This is a subject I can never get a good answer on. Seems to me that a measurement of the distance between the valve stem and the rocker stud as well as the length of the rocker trunion to rocker tip is critical to really nail this thing down. The million dollar question is should the rocker at mid lift be perpendicular to the valve stem? If this was the case, the rocker tip will start at the intake side, rolling over at mid-lift to the exhaust side, then returning to the intake side at max lift. This condition would also give you the least (lateral) motion of the rocker tip over the valve stem.

In your case, your geometry suggests that the rocker to valve stem angle never reaches a perpendicular condition even at max lift. This is evidenced by your max lift condition still pushing the rocker tip outwards. I’m not saying this is wrong, I just don’t know if it’s right. By changing the distance from the rocker stud to the valve stem, you could achieve many different conditions that would track over the valve stem like you described.

Hopefully there is an experienced engine builder out there who may have answers to these questions. I’ve got to believe that pro builders do more than just look how the rocker tip tracks over the valve stem. There are too many other variables that need to be taken into account to get the geometry really nailed down.

 

Are you using a checker spring on the heads?
Usually if the center mark is .060 or less in the center, You likely nailed it.

 

I never bothered checking any of the other engines I've built over the years and never had any valve train related ploblems. I'm just being **** with this one due to the level of power I'm after and of course the expense of it all.I guess the witness mark is around .060 or so (didn't measure it)but even if it's off a smidgen,it's clearly more on target than the stock length pushrods. Those had the roller tip almost off the edge of the valve tip when the valve was closed. Another thing I noticed was with the guideplate back on(took them off with the homemade checker) the body of the roller rocker hits the rocker stud with the stock length,but with the extra .250" there's ton's of clearance.Stock definately would not have worked.

 

Apparently you have thought a lot more about the geometry, than some have. For maximum valve lift, a line through the center of the fulcrum (pivot point at the stud) through the center of the roller on the rocker arm, would be EXACTLY at 90* to the valve stem at mid lift. That makes the valve stem a tangent to the circle the arc the rocker arm tip travels. If this occurs at mid lift, the valve stem is closest to being the tangent throughout the cycle, and would allow for the greatest lift, at the valve. Ideally, that would place the contact point to the exhaust side of the valve stem at mid lift. The contact point would be to the intake side with the valve closed, pass the center point on the way to mid lift, and be beyond the center point (toward the exhaust side) at mid lift. The contact point would then move back inward as the valve approaches max lift, and repeat the pattern during closing.

That would be "ideal" and may be impossible to achieve with available parts. One frustrating factor, is that the valve stem and valve stud aren't parallel to one another. A longer valve changes everything. I think, as a practical matter, one needs to seek out a combination of parts (read, push rod) that keeps the smallest amount of roller travel, the closest to the center of the valve stem, as possible.

The concept can be difficult to visulize. I hope I stated it in a way that can be understood.

RACE ON!!!

 

Here's what I ended up with using a 8.050 length pushrod. The valves are closed,and you can see the roller is slightly off center,favoring the intake. When fully open the roller favors the exhaust side,but still fairly well centered.

 

CFI-EFI:

Nice explanation, I understand exactly what you are describing.

I have never seen any of these issues discussed in an engine building book. The height of the valve stem seems like a critical parameter in this mix. Also the fact the rocker stud and the valve stem angle are different makes this a doozy to ever get truly right. (Right being the rocker is perpendicular to the valve at mid lift.)

You bring up another interesting point that the rocker ratio will change depending on the angle the rocker sits to the valve stem. This is also affected on the other side with the push rod to rocker angle.

I've gotta believe the GM guys must have thought about this long and hard. Judging by the lack of detail on this subject, it tends to indicate that all this stuff is not too critical. Set the rocker height to put the average position of the rocker tip on the center of the valve. Though, with all the **** race motor builders out there, I can't believe there isn't some hot rod tweak to get this exactly right.

 

I have seen pushrods from various manufactures vary as much as .050 each side of the specified length. The 7.300 comp cams pushrods measure 7.320 (smallest) to 7.348. Your pushrods could actually be .050 shorter than specd out. Food for thought.

 

Yes Pete, my Trick Flow p-rods where off by .005-.010" too - and the advertised size was even stenciled on the p-rod (but wrong).

Anyways i gave up trying the adjustable p-rod for a tool with the block in the car. Used the checker tool from ProForm and Moroso (yes both) - much easier and the rocker tip travel (using new p-rod length) looked good by eyeball calibration. Moneypit if ur still worryed u could double check with one of these tools. But that #1 (exh) rocker looks good from the pix.
Good luck and report how that new Isky solid cam works out. cardo0

 

The 406 dropped in Saturday,but I still have some last minute hook ups. I was happy with the geometry I ended up with valve train,and I expect to break in the cam next weekend.

 

Good news! Keep us posted. I am 1/3 of the way thru the build up of my latest 383 to replace my new (blowed up ) 406.I hope to be ready a week or so after you.

 

I agree great info CEFI a low hp factory engine had stamped rockers. Obviously the radius on the end was there for valve train life. I wonder what the tolerance was or how many engines ran with geometery that was way off and no one ever knew the wiser?