Heads up, FYI and Who knew this already??

[Torqsplit]

Mic-ing the lobes on a potential new cam and discovered that the intake and exhaust lobe base circles are different! And even more concerning is that they very by 0.030-0.040" respectively, close to ideal #'s for lifter preload! Discovered its the same on my old cam too! Both are dual-pattern and the exh lobe base circles are smaller than the intakes. This may not be an issue for some who have individually adjustable rockers, but can certainly cause problems for stock or shaft-mount roller rockers. You cant just shim this out. Split the difference maybe, but not ideal. So i was potentially 0.040 preload on the exhaust and 0.080 on the intakes. I had never actually mic'd the base circles and kinda just discovered this while checking the lobe lifts were correct. Wouldnt you assume they were all the same if not told otherwise? I am amazed that this was not pre-warned anywhere on the cam card or spec sheet in BIG, BOLD LETTERS by the cam manufacturer! You wouldn't realize it until the heads already on and your installing the rockers and trying to figure out whats up. I understand that to achieve some profiles the base circles have to be adjusted, but wouldn't you just stick with the smallest one needed and use it for all the lobes vs switching back and forth across the cam? Woulda been nice to know ahead of time you might need new custom length pushrods or adjustable rockers. Seriously, who knew this already??? Not to criminalize anybody but i cant believe i'm just discovering this. And maybe a big factor in previous failures?? Wow.

[IH 392]

I'm still not sure WHY!!!????? everyone in the Jeep stroker world is OBSESSED!! with "lifter preload"????, I'm don't think I've EVER! checked it on the probably 1000 engines I've built over the years!??

[SilverXJ]

Many camshafts have different base circle sizes between the exhaust and intake. I can't remember the exact reason, but I believe they want to keep the lobe nose higher so there is more flank/ramp area.

You may also find that the preload difference may also be different when installed. Especially when the valve tip height is taken into consideration. Oh, and don't forget that the lobe does have a tapered surface.

What do you mean they vary by .030"-.040"? The base circles are that far off from each other? A .010" variance?

[dwg86]

Who is the cam made by?