Stock 3400 Lifter preload

[3400-95-Modified]

So does anyone know this magical number??

I also sent Ben an e-mail with my concerns and why I'm asking but I was just wondering if anyone knew the stock amount of preload that is on a stock valvetrain.

[gectek]

are you testing pushrods or what, the torque spec is of course 124 in lb plus 30*, but i do not think that really equates to much preload at all. the 124 in lb should take up all the lash to zero, and the preload is set by the 30*, but i cant think of an accurate way to measure the actual preload on the lifter itself...it is all hydraulically adjusted so even if you went a little past lash, the lifter would still give an adjustment, it would just stop when either the hydraulic pressure wasnt enough to overcome the force, the orifice is closed off, or the check ball is seated

[3400-95-Modified]

I'm trying to measure new length pushrods for my stage 1 cam. I'm using stock rockers so you dont adjust preload with how tight they are... they are torqued down and tight period... preload is determined by the length of the pushrod.

But my problem is, with the stage one cam and my head on the block, NOT even torqued I get a length of 5.693 at zero lash with my adjustable pushrod... Now I was told to add .030 to that length to get the total length I would need... Well that puts me at 5.727 Which is .033 shy of the stock pushrod length... and that well doesnt make sense because my cam's base circle is .035 smaller in radius than the stock cam......

Get where I'm going with this?

I came close to a number of .093 preload on a stock valve train... and they are telling me to only do .030.... I'm a little leary of that.

[gectek]

your dia of the new cam for the base circle is .070 in smaller?

[3400-95-Modified]

Yes A stage one has to pull from the base circle to get a larger lobe... or else it wouldnt fit into the block... there is no room to add material to a lobe.

Its like that for any cam you buy, especailly a regrind.

[gectek]

no i wasnt questioning the physics of it, i just wanted to know that the dia is .070, because you said the radius reduction was .035, and i wanted to make sure you had not used the wrong term, like meant dia instead of radius

[3400-95-Modified]

yeah dia difference is .070 so thats why originally i figured well radius diff would be .035 so I could get away with just adding .035 to the pushrod length and I would retain stock configuration preload wise, but then I was told to do it the zero lash way and only add .030 to the length. Which doesnt seem to be working out for me.

[gectek]

well not only does the base circle change effect the pushrod, but it also effects the way the rocker arm touches the valve and how the rod will line up with the rocker at various points of lift and how it sits in the lifter cup. so i think they are telling you .030 to keep the valvetrain geometry right for the extra lift to keep the rocker in the right place, and to make sure it stays in place pretty much in the cup and arm without making contact anywhere else, and still keeping the lift of the cam. i think that .030 would sound about right, and that if you wanted to go higher because this didnt seem right then you would have to take in account that the lifter adjusts preload on its own to keep lash the same. but if you want to go more, then ill leave the info and guessing up to someone else, a little more knowledgeable than me on that...a call to comp or manley would be helpful i think

[3400-95-Modified]

well my point is the .030 preload gets me a shorter than stock pushrod... How does that make up for the lost material on my cam?

That just cant be right.

but it also effects the way the rocker arm touches the valve and how the rod will line up with the rocker at various points of lift

This statement doesn't apply to no adjustable rockers. If I could adjust the rocker to fit any length pushrod and get a zero lash than yes that would change your rocker angle and tip placement to the valve. But I cant do that.

[SappySE107]

What is the thread pitch of the nut on the rocker arm? I haven't bought another caliper to measure with but you need I think it was 3/4 turn from the pushrod from not going side to side but sill able to spin it, to "preload". .020 to .040 is the range so perhaps thats what it comes out to.

[3400-95-Modified]

I'll double check the thread pitch tomorrow as well as reinstall the stock cam and check the actuall preload that it puts on the lifter.

I think its more than .030 stock.

[gectek]

well that was assuming that you used a longer rod than what was necessary, because that would add the loak to the lifter, then when it pump up and the engine is at temp, then the rocker arm would prob hit the valve at a different angle and the swept area would be different. and that would also maybe keep the valve open and thats bad for 2 things, compression, and heat dissipation. i would rather have a little too short than too long

[sprucegagt]

I'm just thinking out loud here. So if I'm full of shit just say so. But has anyone realized that since the 3400 uses splayed valves, you can't measure the lifter preload the same way as you would with a traditional valvetrain layout. Just adding say .050 to the pushrod length does not mean your getting .050 of increased lifter preload. Instead your only getting some fraction of it. Now if the splayed layout was designed with a 90* triangle in mind, then you could use geometry to figure out what the actual preload is based off of your thread pitch, number of turns, current pushrod length and lateral distance between the lifter and corresponding valve stem tip. Does anyone know how they measure lifter preload in BBC engines? If I'm right they use splayed valves as well.

[gectek]

well the prob is on the BBC they use adjustable rockers, so you turn to zero lash and then 3/4 turn, im not sure on actual preload though. you cannot really do that on the pedestal mount, but the idea is good though. yes you cant just say the base circle is off so i should add this much, even though the rocker is non adjustable, with the lash being hydraulically adjusted you cannot set it by the diff of the base cirlce and call it good, that would seem to make the geometry way off. about the best way to set the preload that i can think of is to keep the rocker right on the valve tip, and make sure there is no clearance with the lifter in the bottom, then adjust the rod down until it starts pressing in the cup ever so slightly, then i would add .030 to that, but that is just pulling this out of my butt...if im wrong, well im wrong

[Superdave]

Just do what i did.. measure the new cam VS the old one, take the difference in radius and add it to the new pushrods. Simple, effective and way faster.

Logic says that the lifter is .035 (or whatever) further down in the lifter bore.. you need to make up that difference to keep the stock lifter preload.