BBC Roker Arms

[Stockerchevy]

What ratios are you guys getting out of the Nitro stamped rocker arms when you are checking them? At this time, we just measured a batch of them at 1.62 to 1.64. Am I the only 1 seeing this? They are on aluminum heads and we are down on lift as well. No I haven't cam doctored the lobes yet so I no we could have some alternative issues.

Jim

[Alan Roehrich]

Originally Posted by Stockerchevy

What ratios are you guys getting out of the Nitro stamped rocker arms when you are checking them? At this time, we just measured a batch of them at 1.62 to 1.64. Am I the only 1 seeing this? They are on aluminum heads and we are down on lift as well. No I haven't cam doctored the lobes yet so I no we could have some alternative issues.

Jim

Jim,
The only thing we've used successfully in recent years are Clark Holroyd's rockers. They're consistently around 1.7:1, provided you have the correct pushrod length. You'll find that as little as 0.025" of pushrod length can move the maximum lift you get by a considerable amount.

[Stockerchevy]

Originally Posted by Alan Roehrich

Jim,
The only thing we've used successfully in recent years are Clark Holroyd's rockers. They're consistently around 1.7:1, provided you have the correct pushrod length. You'll find that as little as 0.025" of pushrod length can move the maximum lift you get by a considerable amount.

Thanks for the reply Alan. It is just a lot to give up when we would love to run more camshaft.

Jim

[Alan Roehrich]

Originally Posted by Stockerchevy

Thanks for the reply Alan. It is just a lot to give up when we would love to run more camshaft.

Jim

Jim,
Have you tried different length pushrods? What pushrods and springs are you using?

[Stockerchevy]

Originally Posted by Alan Roehrich

Jim,
Have you tried different length pushrods? What pushrods and springs are you using?

Just helping a friend with his 396/L-89 build.

Comp push rods with springs set at 135lbs on the seat and 330 over the nose, "per seat pressure rule" with duel springs with dampner. We just need it to control the valves tight to 6600rpm. We are hoping to run a trap speed of 115mph+ with a 26.9" polyglas tires, 4.10 gears, TH400 , in a 1969 DP 300 deluxe chevelle. Running the infamous manifolds with x pipe for exhaust.

Currently on the road and when I asked him over the phone about his 0 lash lift he said I don't know, I'll call ya back. The lobes on the camshaft should of given him something very close to the max lift if the rocker arms were correct of .530........ Anyway, he called back and he ended up with .511oooops! The conversation continued and after a total of 4 rocker arms on the same lobe he ended up with the .511 with the stamped Nitro's, .514 with a Summit stamped rocker, .527 with a harland 1.70 roller rocker, and a .536 with a 1.72 comp roller........ Anyway, I wasn't there for his pushrod length but he says they are right on with the dry marker on the face of the valve right down the middle with a ware pattern!!!

He is going back to make sure the rocker arm studs are right. Aluminum heads are going to flex more than a cast iron but we are scratching our heads falling so far short on the lift. I'll take any thoughts on trouble shooting this thing..! We're running behind schedule but want to do it right. Thanks again. Sorry for the ramble!

Jim

[Alan Roehrich]

Jim,
Each one of those rockers will likely want a slightly different pushrod length to get the geometry as close to perfect as possible. The right length pushrod for a Crane may not be the same length as the right length pushrod for a Harland Sharp, or a Comp, or a stamped steel. They could require 0.050" or so difference in length. That much difference in length will give a different valve lift. That's how we get the lift correct on a Stock Eliminator engine, by changing pushrod length.

Just checking the pattern across the valve tip will not verify absolute correct geometry. That used to be how people did it, but not any more. The correct pushrod length for a particular combination of parts will almost always yield maximum rocker ratio and lift, and have the least "lost motion". If you change one piece of the combination, the rocker, the lifter, the valve, or the base circle on the cam, the pushrod required to get the best geometry and the most lift will almost certainly change.

There will be slight deflection in a stamped rocker at 330 pounds of pressure, but it will not add up to 0.020" worth of valve lift.

By the way, I will not build a big block any more without 7/16" x 0.125" wall pushrods. I now keep several lengths of each pushrod in order to be able to check my geometry with the valvespring installed, rather than checking springs. I have 8.200" to 8.400" intake pushrods, and 9.250" to 9.450" pushrods, just for that purpose, in 0.050" increments.

Aluminum heads will only be a problem if the rocker studs are wrong, if they are too short (on the 7/16"-14 end that screws in the head), the stud bosses will flex and break. Most commonly on the exhaust.

[Stockerchevy]

Originally Posted by Alan Roehrich

Jim,
Each one of those rockers will likely want a slightly different pushrod length to get the geometry as close to perfect as possible. The right length pushrod for a Crane may not be the same length as the right length pushrod for a Harland Sharp, or a Comp, or a stamped steel. They could require 0.050" or so difference in length. That much difference in length will give a different valve lift. That's how we get the lift correct on a Stock Eliminator engine, by changing pushrod length.

Just checking the pattern across the valve tip will not verify absolute correct geometry. That used to be how people did it, but not any more. The correct pushrod length for a particular combination of parts will almost always yield maximum rocker ratio and lift, and have the least "lost motion". If you change one piece of the combination, the rocker, the lifter, the valve, or the base circle on the cam, the pushrod required to get the best geometry and the most lift will almost certainly change.

There will be slight deflection in a stamped rocker at 330 pounds of pressure, but it will not add up to 0.020" worth of valve lift.

By the way, I will not build a big block any more without 7/16" x 0.125" wall pushrods. I now keep several lengths of each pushrod in order to be able to check my geometry with the valvespring installed, rather than checking springs. I have 8.200" to 8.400" intake pushrods, and 9.250" to 9.450" pushrods, just for that purpose, in 0.050" increments.

Aluminum heads will only be a problem if the rocker studs are wrong, if they are too short (on the 7/16"-14 end that screws in the head), the stud bosses will flex and break. Most commonly on the exhaust.

I'm greatful for the information as usual. I guess even the cup height where the pushrod goes into the rocker arm is all part of these variables. Because of the geometry of the valve train on the BBC I look at this stuff as just critical make these motor RPM and live. I'll keep ya posted on our progress. Thanks again.

Jim

[Alan Roehrich]

Yes, that height is critical. By the way, take a die grinder with a cartridge roll on a mandrel and polish off the burr around the oil hole over the pushrod cups on all of your stamped steel rockers. If you do not do that, it is just a matter of time until you punch a pushrod through the cup in the rocker, because of the stress risers from that burr. I've seen a $20K Stock Eliminator engine destroyed by that exact failure.

[Reed Granrt]

I hope you all don't mind my offering an opinion on this subject. In this case, lets speak of stud mounted roller rockers since they are the easiest to check. All rockers can be check similarly to this but relatively the same way. Rocker ratio is pure physics. Its ratio is built in when it was originally built. Placement of all pivot points will determine its true ratio. When it is installed, its true ratio can only be obtained at one time and that is when all pivot points are lined up and perpendicular to the valve. Anything where these pivot points are above or below this line will result in a ratio that is less than its true ratio and is a result of traveling thru a circular sweep. You must also understand that two (2) places is going thru that circular sweep; the valve end AND the push rod end. So now lets understand that both ends are going thru a sweep that results in loss of true ratio. To obtain the largest area under the curve, proper rocker geometry must be obtained. NOTHING you do with the push rod will make the ratio larger than when the rocker was built. This is why I tell people to make themselves a fixture to check their rocker ratio. Do not start on this journey unless you know where you started from. If you have a rocker that is larger (or shorter) than stock and unless you contort the pushrod to get your specs in, you have a difficult task ahead. If you want correct rocker geometry then you DO NOT use a pushrod to correct lift. If you swap camshaft to see if one is better than the other and you do not set proper geometry on both, how do you know that one is better (or worse) than the other. You should be comparing apples to apples and not apples to oranges. In setting rocker geometry, it does not matter where the rocker rides on the tip of the valve although you better hope it is on the valve tip some place. This will always give you the shortest sweep across the tip and it will give you the shortest amount of motion of the pushrod. Yes the pushrod. As the rocker sweeps across the valve tip, the pushrod will move in and out away from the camshaft. You also want that motion to be the shortest distance possible. If you miss the geometry, you will affect (lessen) duration, rocker acceleration and deceleration and will ultimately change the load on the rocker, pushrod, and cam and lifters, as well as the rocker stud in the head. All loads will be greater. Yes you can use pushrod length to alter acceleration and lift but it will be at the expense of loss of geometry. Your choice. Im just talking forces. In my humble opinion, obtaining proper geometry will result in getting the "best" travel you can get from circle motion and linear travel. There is no way you can speak of proper geometry when you are saying that you are adjusting push rod length to get proper lift. You either do one or the other; proper geometry or correct lift. If you are adjusting puhrods to get lift, you have decided that is more important than geometry and doing that will give you more speed or et. My question is; how do I know which I need; more area under the curve or less. In my opinion, making pushrod length to obtain different lifts should be a tuning mechanism and not a result. Let me git off this horse fore I fall. Mama said shut up and quit.
reed

[Stockerchevy]

Thank you for the post! We are looking for that last bit of horsepower without compromising on durability. We have been running the stock spec lobes and falling short on lift. In theory, the rockers are suppose to be 1.7 and have not been able to get to a .520 zero lash spec. We are finally making that effort and hopefully pay some dividends on the DYNO. We'll let you know what it turns out to be. Thanks again.

Jim