Valve lash / Base Circle problem
Valve lash / Base Circle problem I have a intresting problem, my head is a 98 head and it has aftermarket valves in it that are probably not stock length.
When I first got my 408 with TREX there was lots of slop in the valvetrain, I found a little shim that was under my rocker guide and removed it and everything was fine.
Fast forward to current, I have a LS1 comp cams grind that is pretty generic, 235/235 550/550 114ish and now when I put that cam in without any shims my valves are not closing all the way so I get no compression, if I put a little shim under my rocker guide it does get compression but there is some slop on some of the rockers.
I'm not sure what the problem is, it seems like both cams had the same base circle size.
BTW push rods are thunder racing 7400
When I first got my 408 with TREX there was lots of slop in the valvetrain, I found a little shim that was under my rocker guide and removed it and everything was fine.
Fast forward to current, I have a LS1 comp cams grind that is pretty generic, 235/235 550/550 114ish and now when I put that cam in without any shims my valves are not closing all the way so I get no compression, if I put a little shim under my rocker guide it does get compression but there is some slop on some of the rockers.
I'm not sure what the problem is, it seems like both cams had the same base circle size.
BTW push rods are thunder racing 7400
I know all about pushrods length, what im curious about is the possibility of the cam thats in my car now not being the same base circle as trex. I really don't want to pull the bitch out and mesure it.
Originally Posted by Boostaholic
I know all about pushrods length, what im curious about is the possibility of the cam thats in my car now not being the same base circle as trex. I really don't want to pull the bitch out and mesure it.
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However, if that's the case, -.080" valve lift comes out to ~-.048" at the pushrod. All things being equal which they are probably not, finish grind, touch-up if there was one, would give ~+.024" higher lifter point on the cam heel or +.024" of additional preload. Not much for a hydraulic unless you were close to the edge.
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I would just buy a pr length checker, the correct length pr's, and set it up right.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
Your case circle must be way larger than Trex XE-R base.
Your lobes aren't even XE, more like old 216/220 lobes.
You better watch out, lifter collapse is imminent with your size p-rods.
No only you want to close your valves, but you also want proper preload not to exceed .100 cold on stock lifters.
BTW shimming you rocker pedestals only adjusts you rocker geometry on valves and does little in preload department. That method is used to center wipe contact on valve stems.
Once this is done, only then you should use a checker.
Take 0 lash measurement and add preload to that. That is what your p-rod length should be.
Assuming stock rockers.
But what rockers are you using?
Your lobes aren't even XE, more like old 216/220 lobes.
You better watch out, lifter collapse is imminent with your size p-rods.
No only you want to close your valves, but you also want proper preload not to exceed .100 cold on stock lifters.
BTW shimming you rocker pedestals only adjusts you rocker geometry on valves and does little in preload department. That method is used to center wipe contact on valve stems.
Once this is done, only then you should use a checker.
Take 0 lash measurement and add preload to that. That is what your p-rod length should be.
Assuming stock rockers.
But what rockers are you using?
