02 magnum 360 - Comp Cam #20-745-9 (212-218 (264/270), .480/.480, on a 114* LSA). Re-used all of OE hardware except the cam and springs. Runs good, but has some light 'chatter' - thinking the OE pushrods are a tad short. Not sure how big a cam has to be to require going lower on a base circle, and my cam isn't really that 'big'...but I've really no idea what I'm talking about. I just 'think' the base circle is lower than the stock OE cam I pulled out, but that doesn't mean there wasn't 'installation' room available for a .480 lobe lift.

Question: Anyone happen to know how Comp makes their cams, specifically how their base circles compare to OE cams? Do they do regrinds, use new cores, or in the end have lower base circles than OE would have?

Thx

I can't give you exact numbers, but I would say most mild performance cams under .500 lift are stock blanks ground on a reduced base circle. At least based on my experience in building several dozen mild small block mopars and having to always get longer pushrods to get proper lifter preload. And it was several suppliers.

I always assume that I'll need new pushrods when I change cams. Base circles are all over the map. Maybe someone has a magic decoder ring but I've never seen it. I just wait until I get the cam installed and then I measure for pushrods.

all good info. As easy as they are to swap...at least on a magnum with 'o-ring-style' valve cover gaskets, and as inexpensive as they are (relatively), I'm just opting to try a set of these .036-longer rods and see how they are. I'll measure the pre-load as best I can when I assemble everything, as well as with the existing rods - see where things are and end up, and I'll report back.

Why not order an adjustable checking pushrod to get the length dead on
I have several for both BB and SB with solid and another one for hydraulic lifters
Manton sells them

Good video on cam grinding and how the base circle diameter is determined.

https://m.youtube.com/watch?v=rz7LAaMtxME

In a nutshell, what it boils down to is the peak height of the lobe can’t be higher than the smallest diameter cam bearing that the cam has to fit through.
On a stock BBM and SBM block, that’s the #4 bearing.

Also, for many applications there might only be one or two different semi-finished cores to choose from. And depending on how far the core is from the desired grind, It will impact how much has to be removed during the grinding process.

Generally, more lift ends up having a smaller base circle.

...so as much as I'd like to I just don't have the super awesome measuring devices or techniques, but I have a pretty good eyeball and a nice new sharpie, and it seems the preload between my pushrods and the Comp cam I stuck in my otherwise stock but freshly restored magnum 360 measures just at 1/8" (of preload), or .125" - that is the apparent distance/difference between 'no slack' at the rockers and tightened to 21 lbs, which I think it 'in range'. No I do not know if I measured things correctly - the rockers were 'loose' but seemingly 'taught', but without a shaft-mounted setup like an LA it's really difficult to get it all 'square' for precise measurements...but this is what I can report so there it is.

The rods I pulled out are just at 6-7/8", and I think OE is 6.92" or something, so they also appear to be 'in range' so I think the OE rods I have are correct, but the valvetrain was just slightly 'chattery', and I'm installing .036-longer rods, which should help quiet things down.

Like I said, I don't have the expertise nor tools nor time to do this as awesomely as I would otherwise like, and I'm taking the advice of several folks that the slightly longer rods should nicely make up for the (likely) reduced base circle of the Comp cam. Yah I know it isn't anywhere near scientific nor how NASCAR would do it...but I just don't have the resources to fully blueprint everything and neither do most everyone else, so for those that have a similar engine with similar parameters and have similar very light chattery sounds coming from somewhere under the valve covers...well maybe a slightly longer pushrod can help you quiet things down too.

I'll be the guinea pig and see how it goes, and I'll report back in a day or so...

- Sam

sometimes, just "doin' it" gets the results one wants.
is it right ? of course not.
does it work ? yes. why ? i don't know............
sometimes this is the only way one learns things.
in my lifetime, i have done things that were said couldn't be done, only to find out that wasn't so.
other times, i have failed spectacularly, [or not that bad] and the "i told ya so's" were richly deserved.
regardless of what happens, i will be [hopefully] learning through you, Max. [now retaining that knowledge without the spiders in my old noggin covering it with cobwebs is another matter.... ]

OK, so I'm no Magnum guy, but I thought that these used the stud mounted rocker arms, and your comment above seems to confirm this.

Further on, based on my understanding of the stud mounted stuff, I think you adjust the lifter preload by tightening down the rocker arm some "more", is it not?

Yes, I would expect that this only gets you a range of adjustment, after all if the pushrod length is all wrong no amount of rocker adjustment will fix that.

Re: current preload of 0.125", that seems like a awful lot, doesn't it? The only thing I can do is to compare it to my retro-fit CompCams rollers and they call for a full turn of an adjuster to preload (per the instructions sheet included with my roller lifters), which ends up being 0.050" (based on rocker adjuster being 7/16-20 TPI, so 1" / 20 = 0.050"). Heck, because CompCams have been all over with their lifter preload recommendations I actually set mine up with an initial 1/4 turn, so that's just 0.0125", will adjust as needed if necessary.

I say this because it seems like the hydraulic roller setups can be pretty noisy and that comes about due to their sensitivity to the lifter preload.

A stock cam usually has the tip of the lobe very close to the max diameter that will fit thru a cam bearing, the only way to significantly increase lift is to decrease the base circle.

Heads been milled? Block been milled? How thick are your head gaskets? Do you know if you want max rocker ratio lift at 1/2 valve lift or some other lift? Some guys like max rocker ratio near the bottom some like it middle and some like it near the max and with stud mount rockers it changes that based on how far up the stud the rocker is... a roller rocker effectively lengthens the valve by 1/2 or the roller tip diameter for geometric consideration and all those things affect the ideal pushrod length.

A lot of the newer cams and lifters are just noisy and not much you can do about it.

Magnum stud mount rockers are desinged to be bolted down...no adjustment.

.125" of preload is about all the travel there is in those lifters so you are about bottomed out. .060 - .080 should be all you need. I don't understand what you mean by "chatter" though.

...the heads were 'shaved' to get 'em flat but not milled to increase compression. Head gaskets are Fel-Pro 519HD with an advertised 0.0499-0.0599" thickness - I've no idea how thick the OE ones I took out were - didn't think to measure them.

Everything is OE except for the cam and springs (Comp), pushrods (now .036 longer), 'shaved' heads, and Fel-Pro head gaskets - everything else is OE, including the lifters.

Installed the p-side rods this morning and as I cranked down the rockers I noticed the lifters compressed a bit then the valves all did - all of them across the head, base circle or not, and I think that is because the lifters are full of oil and haven't bled down, which I think is a good thing (lifter internals are still good, etc). Torqued to spec and cranked it by hand and nothing is contacting so yayee. Spoke with ScramSpeed and they reassure me that what I described is normal because the engine had been running and the lifters are full of oil, so I'm gonna trust the tech.

Don't know how to really describe the 'chatter' but it had a light but noticeable sound when running like a well oiled sewing machine - not a bad sound, but just...well it just kinda sounded like 'loose' - not bad loose but 'could be tighter in there', and that is what all of the R&D is indicating - needed slightly longer pushrods.

It's probably fine - can't imagine how it couldn't be. I'll fire it back up in a day or so, and if it runs nice and smooth and makes less noise then everything I've been told will be validated. If however it makes no compression and won't run then the new (and the OE) rods are/were too long, and it'll be time to pull the intake and buy some measuring tools, and rods. Can't imagine how that could possibly be...but I'm not a magnum engine engineer.

Will know in a day or so

Update: The drivers half is still as-was and I just went back out and ran a little experiment. I loosened a pair of rockers on the base circle (I could spin the pushrods with my fingers) - loosened them all the way out, waited a few, then bolted them right back in. Like the other side, the valves depressed about 1/8"...but then after about 5 minutes both were back in their upright and seated positions. As I understand, that means when I loosened the rockers the lifters extended all the way out to the stops, and when I bolted them back down the lifters couldn't bleed out their internal air/oil pressure as fast as I bolted them back down...which meant the valve had to go 'somewhere'...but then after a few moments the valves were re-seated, meaning the lifters had bled out their internal pressure...which I think means the lifters are good...and everything else is too.

Also, now that they've also bled out, I can now spin the p-side (longer) pushrods with my finger tips, when I couldn't before - it was all due to the internal lifter pressure not being able to bleed out as I was cranking down the rockers...which I think indicates the lifters (and everything else) is good to go...and now I can begin to think straight again.

Interesting experiment.

I would say the noise you described is normal. As for the rest of it I think you have too much preload.

Do you mean valve lift or lobe lift?
Is this cam a standard flat tappet hydraulic lifter or a hydraulic roller lifter cam and lifters?

OE-spec standard hydraulic roller cam for an '02 mag 5.9

...well it doesn't seem to be any worse...not sure if it's any better...maybe a little(?) - really hard to say - may not have been 'pushrods' at all. No idea what the effects of 'too much preload' are - at least too much but not enough to hold the valves open...but if the sounds coming from the engine are normal then I suppose I'd have to think the stock rods that were in there were 'good', and if if that's the case I'd of course now be inclined to think there's much-too-much preload...but hell I dunno. I really don't wanna pull the intake and go through all the motions of measuring pushrod checker and all that if I don't have to...and that's really the question isn't it.

Maybe magnums just sound like a sewing machine...but if it wasn't wrong before it probably is just 'more wrong' now...ugghhh...

https://photos.app.goo.gl/cXWif1A3KsdDnjaL8

Sounds about the same as mine with aluminum roller rockers, the SS ones on it now are only slightly quieter.

...not knowing if there is an actual problem drives me freaking nutzo, so I'm just gonna run the engine as-is - it isn't a terrible amount of noise...more of just a general 'sound'...and there are nearly identical 'sounds' coming from both my wife's 94 Grand Cherokee with a 5.2 magnum and Ben's 01 Dakota with a mag 5.9 and I wouldn't give either of those machines a second thought...so I'm just gonna make a general assumption that it's 'okay enough' to run and focus on the umpteen-kazillion other things I need to get done....and if it blows up I'll finally have a good reason to do a full engine rebuild and go from there.

...so in a bout of frustration...over the weekend I had the thought about just draining out the spec 10W30 and just dumping in something thicker - I figured what do I have to lose? - it'll either like it or blow up...and at this point I'm really fine with either because one version means 'better' and the other means 'will be better'...and since we're (hoping to) keep this truck we'll put whatever investment into it that's required...within reason of course. So...I went to O'riellys and just looked over the oils sitting on the shelfs there...and after a few moments of surfing one in particular just looked 'right' - Valvoline 20-50 VR1 - something in me brain just said "do it - it'll like it", and after pouring I have to say it seems to be just what the engine needed, because after putting it in the engine is now noticeably quieter and has double the hot-idle oil pressure - it now idles at about 40-45 psi hot (it was just under 20psi with 10W30)...and maybe 15 psi more cold but not as much of a cold difference as I thought it would make (maxes out at about 75 psi).

So - it's quieter, and has better oil pressure - good. Yah I know it's kind of a 'shortcut'...but not really - I'll run it as-is now for a while, and if it continues to run good then fabulous, and if the engine ends up needing a rebuild then so be it, but until another reason happens this'll be the oil this 360 uses from now on 👍

I'm figuring the low oil pressure was simply allowing too much 'slack' in the lifters(?), and the higher viscosity enables the system oil pressure to remain high(er)...which by the literal sound of things seems to be just what it wanted. It still makes some 'noise' but it's now more of a normal general 'engine running' sound...like a sewing machine, but it sure seems to have much less 'chatter', and hey if it's this easy and will get me by until I do a full rebuild (if ever) then horray . I know being a roller engine I didn't 'need' the extra ZDDP, but I figure it can't hurt, and VR1 sure has a very good reputation, so there it is.



- Sam