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Discussion Starter #1
How much variance is acceptable between installed heights? Just pulled springs off of a set of heads to take the inner springs out for cam break in. While doing this I ck'd installed heights and have over .020 between valves. Aluminum heads and they used a spring seat that locates on the boss, shims under the seat and you can't remove the seat without removing the seals. Is this the best way for this setup? Springs are 978-16 Comp for a solid FT cam. Springs recommend 1.850 height, cam is under .600 lift.
 

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No expert here but .020" is huge! I would be fixing that for sure. That will have your seated and open pressure all over the place.
 

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I am no expert either, but I feel that 1.850 plus or minus 0.010 is fine for a flat tappet. I don't feel that the pressure would vary enough to warrant getting excited about.
 

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Installed height and spring pressure work together. I try to get + or - .007" using a last .015" shim under the cup. But I also measure the seat pressure of all the springs and put the weakest ones in the positions of the shortest heights to try and equalize. You can also check open pressure and tune around that.
 

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Discussion Starter #5
The only problem I have with changing the heights is I have to pull the seals to get the spring seats off as the shims are under the seats. After the cam is broken in when I put the inner springs back I'll get them right. Just wanted to know what the tolerances should be.
 

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As the spring (978-16 Comp) has a rate of 403 lbs/inch a variance of 0.020" is 8 lbs.

An 8 lb difference - now is that worth fussing around? How close does one have to get?
I suspect if you measured all 16 springs at exactly 1.850" you would see +/- 4 lbs difference.
Run the springs for a week or two, take them out and re-check. Again the pressure would likely change for each spring.
 

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Discussion Starter #7
Ok good point on the pressure variance which is around 2%. So what would the allowable variance be in general no matter what the spring rate?
 
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