Lurbie

iTrader: (3)

So I used the Comp Cams 7702 pushrod length checker today with checking springs on my 383 with XFI280 cam, LS7 lifters, 1.6 Comp Cams roller rockers and here's what I came up with. I measured 7.225" on the Exhaust and 7.275" on the Intake. There's no pushrod at those exact length, so what do I choose? Also, is it normal to need 2 different length pushrods? What I found in Summit are these lengths:
7.210 in. 7.250 in. 7.300 in.

Should I choose a slightly shorter or slightly longer pushrod?
Should I choose two different lengths, or just pick one since nothing directly matches either of my measurements?

Also, I did the procedure like Comp Cams explains in this video:
But in reading some threads on TGO tonight, I think I need to go back and pay more attention to the width of the pattern, more than the location of the pattern, like the video told me to do. So I'd like some thoughts on that too. I get that you want a narrow pattern because there's less movement of the parts, but it seems to me that spreading the load across the widest part of the valve stem, being the center, would be more important than having that same load concentrated in a smaller area near the edge of the valve stem. Particularly with using a roller rocker.
(Hope this all makes sense, my internet has been dropping out all night long, so it has been difficult to make this post.)

sofakingdom

iTrader: (1)

Doesn't matter. (within reason)

What DOES matter, is side loading on the guides; the rocker pushing the valve stem sideways. That DESTROYS heads.

Obviously if the rocker tip is sliding across the stem, it's pushing it sideways. Bad.

Getting the pattern the narrowest possible, minimizes this source of FAIL.

https://www.google.com/?gws_rd=ssl#q=mid+lift+geometry Read some of this stuff.

Since the force on everything is greatest when the valve is at full open, I prefer to set the push rod length slightly on the short side; that way the half of the sweep that has more force, is also straighter.

As long as the pattern is within the center third of the stem, it's fine.

AlkyIROC

It's sort of a toss up. You want the wear pattern as close to the center of the tip as possible however you also want the narrowest wear pattern as possible.

If the wear pattern is way off the center then you have more serious issues. If you have a wide pattern close to the center then adjusting the pushrod length to get a narrower pattern shouldn't be far enough from the center of the tip to cause any issues.

My BBC heads have around 800 pounds of spring pressure when the valves are open. My pushrod length gets me the narrowest pattern possible. At mid lift, the valve stem needs to be at a 90* angle to the rocker.

Having a wide pattern means the rocker is moving more across the tip so it's pushing side to side more. Having a narrow pattern even if it isn't directly in the center of the tip means the rocker isn't moving side to side as much and is pushing downward more than to the sides.

With all the tolerance variables, it's unlikely you will get a perfect pattern across each valve. Select a pushrod length that will give you the closest pattern. You may have 2 different pushrods lengths for intake and exhaust though.

An adjustable pushrod and length checker tool is the easiest way to determine the proper length without having to decide if the pattern is good enough. You already have an adjustable pushrod. You just need one of these now.



Since your measured length isn't a standard length offered, subtract the difference from the next closest length and choses the one that is the closest.

Lurbie

iTrader: (3)

Thanks for the replies, I'm going to work on this now that I understand it more and I'll make sure I get the right length. I hadn't thought about side load and that explains very well why you want very little movement. I know I can get a tighter pattern just off center, but had been concentrating on the wrong thing before now. And if I don't have a standard length, I'll pick the one(s) closest to my measurements. Appreciate your help! Thanks!!!

Gumby

push rods are push rods, if you wanted a custom length go into the Buick world of parts to find them.

Most still have and use the factory rocker shafts where there is no adjustment, just torque shafts proper n done.
[ why I love mine, I hate adjusting vales, checking and rechecking again,]

But if someone does machine work, you have to re-do the push rods to proper length to make up for it.

Lurbie

iTrader: (3)

Yep, I had my block decked down, switched from flat tappet to roller lifters, cast to aluminum heads, stamped to full roller rockers, so a lot has changed from what this engine was when it came out of a Suburban, so that's why I'm verifying the proper length.

Lurbie

iTrader: (3)

I've been out of town on a business trip, but is this video showing the correct way to use the tool you mention?

AlkyIROC

Well that was a lot easier to watch a demonstration of how it's done than trying to explain it.

You just need to buy the correct tool for the engine you're working on. The SBC tool comes in 2 different rocker stud sizes. 3/8" which is standard size and 7/16" which is the larger aftermarket stud size. That also needs rockers designed for the larger stud. Most people who are building a SBC will be using 3/8" rocker studs and rockers.