Topic: cam tolerances

[brandEn]

So I borrowed the tools needed to get my cam degreed in, and thanks to this forum I got things figured out. However, I am confused with the numbers that I got and was hoping to get some clarification of expected tolerances with a new Hi-Po cam. I documented everything in my K4 build thread (starts at post #417) but will give some cliff notes here. I am hoping you guys with lots of aftermarket cam experience will chime in to help me out.


I have read that aftermarket cam specs will vary a bit from the advertised specs. Even Megacycle states in their catalog to split any differences to come within 2-3 degrees of specifications. I will type out the printed specifications of my cam and show you what I came up with.


Is this considered to be within acceptable tolerances? 



Megacycle 125-60 cam


advertised specifications:                                                  My specifications:
INTAKE:
lift = .375"                                                                         lift= .380"
open= 25 btc                                                                     open= 22 btc                                                                   
close= 52 abc                                                                    close= 55 abc
lobe center 103.5 deg                                                        lobe center= 106.5 deg


EXHAUST:
lift = .354"                                                                         lift = .347"
open = 50 bbc                                                                    open = 51 bbc
close = 23 atc                                                                    close = 19 atc
lobe center = 103.5 deg                                                     lobe center = 106 deg

[MRieck]

Branden....that is fine. Always check piston to valve clearance but that looks very good.

[Jerry Rxman Griffin aka MuthaF'er]

Extra intake duration and lift

[johno]

Branden,
+1   all good.

Quicky waye     look at overlap ie in open, exh close numbers subtract one from other and divide by two equals advance or retard.          if inlet is biggest number is advanced ......if exhaust largest number retarded

your case
meg specs 25 io,    ex close23........... 25minus 23 =2   then divide by two = 1 advanced   ( in biggest no)

your spec  23 io,   ex close 19...........23 minus 19 =4 divide by two = 2 degrees advanced   

Yours is set up nice, camchain will settle in and retard slightly and is just where I would like to set it up.
cheers Johno

[brandEn]

Wow thanks for the responses.

I look at these cam numbers and they are just Greek to me. I figured out how to get it degreed in but interpreting the results and understanding them is a whole other lesson in itself.  What I have a hard time understanding with today's technology why is that grinding a cam to advertised spec seems impossible? Seems like industry standard to expect this.  I really appreciate the help on this. Thanks for the replies.

[johno]

Branden, pretty rare to find the exact specs match up
WHY    tolerances.... every part ever produced on the planet is to a tolerance.  In the case of the cam grinding getting down to a degree is pretty serious stuff, it then compounds by the tolerance associated with radius of the rocket arms, then the tolerance of the rocker arm ratio, then its compounded by the  tolerance of the adjuster tip radius, valve length tolerance and the angle of the guide tolerance.........see simple ...doesnt take long before the lift and the io and ec points dont line up with the cam card.   Especially after a couple of remachining and rebuilds.

Thats why If its a symmetriacally ground cam I just use the overlap advance or retard thing, if its a polydyne or assymetrical design cam it pays to use the system you just used.

Just like maths and geometry at school....drives you nuts.    I reckon they should teach geometry at school by analysing valve train geometry
ciao johno

[brandEn]

johno, thanks for your posts. You too Mike. As long as my cam is good to go and does not need to be sent back to Megacycle I'm happy. Math was never my strong point and well geometry? What's that?... haha

[MRieck]

Johno's last post explains it all perfectly. Watch a cam being ground on youtube.....it all makes sense then. 

[brandEn]

I never thought of doing that Mike. Will do. It's amazing what you can find on You Tube. How did we ever survive without the Internet?

[FunJimmy]

Great thread!

[turboguzzi]

at what tappet clearance are you measuring? if your too close too the ramps it gets difficult to be precise...

[brandEn]

I measured everything at zero lash.