BBC Roker Arms

[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.