Cylinder head thickness?

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I’ve run into yet another issue with my project. I accidentally buggered the valve guide boss on the intake ports trying to remove the raised lip. I grabbed another head in great condition on the ebay after pulling two cracked heads at a salvage yard. However the refurbished head I have had been milled 15 thousandths. eBay head measures 1.070 while the ZZP head measures 1.055. Both measurements were taken from the lip (narrowest part of the head). I’m going to recheck the flatness as I used the .005 gauge but I read on engine builder mag pushrod cast iron V6 heads shouldn’t be over .003.

Is it okay to run the heads as is? If you’ve read my old posts the machine shop screwed up gasket matching my lower intake which led to a lengthy CC dispute so milling is out of the question. The only thing I can do is use emery cloth but .015 is going to be murder in this heat.
 
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Not an expert, but wouldn't that difference cause a difference in compression between the sides? Probably why the manufacturer says max difference .003.

I'd get the taller head milled to match the shorter.
 
.003 is flatness for proper gasket sealing. My concern is indeed whether running different compressions from side to side will cause issues. I’m already running 93 to counter knock after installing mods. From what I can figure the 15 thousandths will raise compression from 9.5 to 9.87
 
the correct approach is to measure the combustion chamber volume of each head and equalize for that. The thinner head could have a bigger chamber because the valves are seated deeper or a casting difference.
You can use a sheet of plexiglass to seal the chamber, and a turkey baster labeled in CC to apply the fill fluid.

Rod
 
I have seen a small block Chevy with a closed chamber head on one bank and and an open chamber on the other. One side is 10:1 and the other side was 8:1..seemed to have run ok. I'm sure worse has happened
 
3.8 bore, 3.4 stroke, 6 cylinders with 64cc heads
 
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I'd say you have two possible options.

One, find a machine shop you trust to mill the one head to match.

Two, just bolt them on and don't worry about it.

My personal preference would be option one, do not try and sand the head down yourself with any sort of abrasive, not only will it take a silly amount of time, you likely won't keep it flat.
 
Back in the day, we built a cheater Super Modified engine with a 0.040 bigger bore on one side (Ford FE w/ Medium Riser heads). 355 inches was the rule. The right bank was all normal and the one we knew they'd pick for challenge tear down... The left bank was the oversized, but steering and brakes made it a hassle to make us pull that head
laugh.gif


Ran three seasons with that motor on dirt tracks. Loped something awful, but ran like a freight train and never missed a beat. We have run mismatched heads on a number of claimer motors. And we spin the living poo out of them. No easy street life. They all live.

If you CC the heads you will be amazed at the variation. I've seen two open chamber heads off a Detroit built OEM motor with chambers 3cc off from tightest to biggest. Not normal, but core shifts and casting errors do happen. They still racked up 100,000 miles. Most are within 1 CC or less. You are likely to be in there somewhere...

The biggest deal to stopping ping is to kill the hot spots. So sand away any rough or sharp edges. The chambers don't have to be polished, but smooth rules for ping control. Sommender grooves 0.030" deep and 0.250" wide on the quench flat aimed at the exhaust valve can help. Controlling the total quench height and limiting it to no more than 0.040" will be a huge help. 0.030" would be even better.

Have you done any porting work? Your machine shop can mill the heads to match. But if you measure all combustion chambers carefully, I'll bet you'll find some valves shrouded a bit tighter than others. That controls, or limits, air flow into that cylinder. So even if the chambers are the same volume, they won't fill the same ... So your effective compression ratio will be off cyl to cyl. If you really want anything resembling actual even compression, you'll have to have them make un-shrouding cuts identical on each chamber.

There is a lot going on there. I'd look at a lot of factors before I'd get concerned about 0.015" difference in machined height ...
 
Check the valve clearances to piston (you can do it with no gasket) and if OK, you can run the milled head with a thin head gasket.

If not, depending on the clearance needed, you might be able to use a thicker head gasket to obtain the needed clearance.

You can also obtain valve clearance from the other side, using shims, buckets, etc to increase valve to piston clearance, although broadly speaking you do want to maximize valve lift, generally speaking the small changes are not terribly detrimental to performance. It can always be restored by cam machining, although now we're talking "real money" and might be too much effort for too little reward. A third option is to machine piston tops for larger valve pockets.

Make sure the head is truly flat or your head gasket won't properly seal. If not, you might need to mill it further, which makes the above even more important to do.

15 thou is nothing, really as far as head milling goes. The OEM almost certainly has engineered in greater clearance than that to allow for correcting heads that are not truly flat. I don't think you will have much problems.

You don't say (in this post) what motor. Some OHV engines are interference fit and some are non-interference. Certain valve-to-piston adjustments can change a non-intereference motor to an interference one. Something to think about. ("interference" refers to what the valves hit if you lose a cam timing belt / chain. A non-interference design will result in no damage; an interference design will bang valves to piston).
 
Originally Posted By: Chris142
I have seen a small block Chevy with a closed chamber head on one bank and and an open chamber on the other. One side is 10:1 and the other side was 8:1..seemed to have run ok. I'm sure worse has happened


Years and years ago I wondered about intentionally doing things different on the two banks of cylinders, so as to broaden the torque curve. Didn't think of compression as much as exhaust differences. Probably not as good as a cam that actually used different lobe patterns for the two banks though.
 
Only available in .010-.020, not .015. I’m just gonna do it the PITA way and work it with the grade 50 emery paper and finish it off with some wet sand 2000 grit to give it a smooth finish.
 
Ditching the emery for wet/dry sand paper. A guy over on the corral fixed a blown head on his race vehicle using oil and sanding. Start with 280, then 400 and 800 for the final pass. The key is attaching the sandpaper to a surface that’s as flat as you need the head to be. He managed to get it down to 30Ra and the .0015 feeler gauge couldn’t slip under the straight edge. From the pics it looked to be aluminum too.
 
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