YEEEEEEARRRHG! Coil-bind in valve springs

[cstreit]

I set up to finish the 2L race motor today. Jerry, Dan, and Jerrys BIL came over to observe the cam-timing procedure.

I timed the #1 relatively easy. However every time I set up cylinder #4 it seemed as if the valve was hitting the pistons. The motor just stopped turning.

My first thought was upside down piston. Second was valves too tight or cam timing was out of phase. Just couldn't find any evidence of this. Then I was staring at the assembly and realized the valve spring was going into coil-bind. DAMMIT!

I had to disassemble the right half of the motor, pull out all the heads and check them. Turns out the machine shop set the #4 intake with 1 too many shims. Thank GOD they didn't do it on another where I wouldn't have found it untill I was doing the valve adjust. THe SOB's cost me a good 8 hours of work. Getting there, diagnosis, disassembly, re-assembly... Instead of finishing the motor at 5:00 today, I am "nearly finished" at almost midnight.

I want my money back!

[asphaltgambler]

Man, I f--kin HATE when someone else's poor workmanship interferes with a good build. Just plain carelessness

[cstreit]

Tell me about it. Cost me 8 hours of labor as well... all for a $400 machine shop Bill... One stinkin shim.

[TimT]

Im having trouble believing one shim will casue the valve spring to coil bind.

I typically add a shim or two to increase spring pressure..and have not seen a coil bind issue yet.

interesting problem

[asphaltgambler]

Depends on thickness of the shim

[camgrinder]

I hope they didnt add the extra shim to make up for a weak valve spring. Now the spring may be low on pressure. How much valve lift do you have?

[cstreit]

John,

I don't have the exact numbers, but I measure it somewhere around .475"!

Most of the intakes measured a seat to keeper height (and again, not exact numbers) of like 33.6-33.7mm. The ones that were binding measured at 33.2 or so. The shims were 0.5mm.

I spoke with the machinist and the initial measurements were 33.6-33.7mm so I think the assembly guy must have added one shim to many by mistake. Suprising because I've used these guys a number of times before.

[camgrinder]

I think you are still too close to coil bind. Since it bound up with a .5mm (.019") shim installed, with the shim removed you are now only .019" from coil bind. I like to see at least .050" from coil bind.
Can the machinist give you a seat pressure number for your springs set up at 33.6 mm ?

[cstreit]

John,

I understand. However this is how the motor was built and run for 2 seasons this way. Apparently the close tolerance is how the engine kept from detonating during the overrev. The coils bind up and prevent much valve float. If they had been set at anything bigger than this, say 1.2mm or 0.047" the valves would have hit. There isn't a lot of valve to piston clearance in this motor.

It's designed for about 70 hours and in this case will only have 35 when it's retired again, so I think it's going to stand this way...

EDIT: Negelected to mention that the valve lash was set too tight as well. Not that it's on the looser side of tolerance that gives me a but more room.

Thanks for the advice!

[Tim Walsh]

so the real question is... what was the stroke of that crank?

[camgrinder]

Thats a popular trick, stacking the springs to dampen harmonics. It can also cause the spring to fracture. It is usually done when you run a rule mandated spring thats not up to the job. 70 hours seems like a long time, but 35 hours sounds ok.
When the engine is at operating temperature the valve lash grows and you will have a little more room.
The detonating idea I have a hard time visualizing...

[911pcars]

" The coils bind up and prevent much valve float"

Chris,
Hmm. True. That would prevent the valve from opening farther than it should, but tolerances would have to be mightly close so when the cam says max. "lift", there's enough coil clearance to prevent the coil from stacking. At the same time, if the cam opens the valve to it's maximum and the coil is stacked, somethings going to give, i.e. break.

I assume you've got that covered.

Sherwood

[cstreit]

John,

Doesn't the lash reduce when the engine heats up and the valves get minutely longer?

Sherman,

That's the essence of the design. The spring still has about about 1mm clearance past max lift of the cam. The piston to valve clearance is 1.4mm. Were the valve to try and float, the valve slams to a stop and breaks the cast rocker arms rather than punching into the piston.

[camgrinder]

The thermal expansion of aluminum is roughly twice that of steel. So the rocker arm is moving away from the camshaft.

[cstreit]

gotcha!