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Unread 01-30-2014, 03:54 PM   #1
elks96
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Flatness specs on a 258 head?

Ok so in my other recent post I had a kid break out the stud holes for the exhaust on the 258 he was working on. Well I found another head in the shop and when we checked it for flat using the machinist edge we found the very center of the head was running over .004 and under .0055. I have a spec on 350 heads of anything more than .004 need machined. What is an acceptable spec on a 258? Is it any different.

The head looks good everywhere else.

I also have access to a machinist block/flat surface (one of those big rock flat surfaces that is true to .0001 inch).

Since we are on a total budget can we use that surface and lay down medium grit sand paper and slide the head in a figure eight to bring it back into tollerance? Again money is tight for this kid and I think the and hour of sliding the head around would do him a little good.

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Unread 01-30-2014, 09:44 PM   #2
80cj
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First thing that comes to mind is where are you going to get a piece of emery cloth longer than the cylinder head and can you slide that heavy head against the emery cloth without wrinkling it? Just wondering.
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Unread 01-30-2014, 11:05 PM   #3
chet44
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I'm a tool and die maker by profession, and what you are describing we call "Lapping". Instead of emery cloth, we use what is called Lapping Compound. It is basically abrasive grit, suspended in a light lubricant. You spread the compound on a steel lapping plate (Not a good idea to use it on a granite plate). Then you spread some compound on the surface you want to lap/hone. Then, like you say, move the head around in slow figure eights on the granite plate.

However, If the head is steel, you'll be old and gray before you can take .004" off of it, not if you want it flat. A typical rookie mistake is to go to fast, and as a result you end up slightly rocking it, causing a bow in the surface you want flat. Or you press too hard on one side, making it uneven, or bowed off to one edge. You have to keep turning it end for end every few figure eights, and occasionally check it with a indicator.

Since you have .0055", and .004" is the minimum machining spec, then it is doable, but still a lot of work. Might want to pop a few Aleve before you start.

Emery on a granite plate is a bad idea, especially for something that big. It will wrinkle, tear, move around, and at some point grit will get under it, so you won't be on a flat plane anymore.

I lapped an aluminum head (4-banger) for a guy at work when I was an apprentice. It had .010-.012 low spots, which I had to take out with a surface grinder first. It lapped pretty easy being aluminum though.

Shame I don't live closer to you, I'd do it for you for a few beers.
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Unread 01-31-2014, 08:18 AM   #4
elks96
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Quote:
Originally Posted by chet44 View Post
I'm a tool and die maker by profession, and what you are describing we call "Lapping". Instead of emery cloth, we use what is called Lapping Compound. It is basically abrasive grit, suspended in a light lubricant. You spread the compound on a steel lapping plate (Not a good idea to use it on a granite plate). Then you spread some compound on the surface you want to lap/hone. Then, like you say, move the head around in slow figure eights on the granite plate.

However, If the head is steel, you'll be old and gray before you can take .004" off of it, not if you want it flat. A typical rookie mistake is to go to fast, and as a result you end up slightly rocking it, causing a bow in the surface you want flat. Or you press too hard on one side, making it uneven, or bowed off to one edge. You have to keep turning it end for end every few figure eights, and occasionally check it with a indicator.

Since you have .0055", and .004" is the minimum machining spec, then it is doable, but still a lot of work. Might want to pop a few Aleve before you start.

Emery on a granite plate is a bad idea, especially for something that big. It will wrinkle, tear, move around, and at some point grit will get under it, so you won't be on a flat plane anymore.

I lapped an aluminum head (4-banger) for a guy at work when I was an apprentice. It had .010-.012 low spots, which I had to take out with a surface grinder first. It lapped pretty easy being aluminum though.

Shame I don't live closer to you, I'd do it for you for a few beers.

Thanks for the reply Chet. I am not sure what to do. I will talk with a machinist today and see what he says. I am working with a kid who is just trying to get back on the road. I thought that maybe we could figure something out.

Maybe the machinist will help us out on the head. The machinist in my home town just ended up in the hospital and the next closest one is an hour away. I will give the guy a call.
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1985 CJ 7 258 bored/Cammed, MotorCraft Carb, Mallory Ignition, NP435/Dana 300, 4.5 RE Ex, 1.25 JKS Body Lift, Detriot AMC 20 Rear(detroit locker and Warn Hub Conversion), Herculined Tub, Chevy Heater Blower, 34" Swampers, Custom Rear Bumper, more to come....

1972 K5 8" lift, 36" swampers, lockers, custom paint.
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Unread 01-31-2014, 10:36 AM   #5
SLO_Ken
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258 Head Specs from the 84-86 FSM attached.
258-head-specs.jpg

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Unread 01-31-2014, 11:33 AM   #6
LumpyGrits
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I check'em after torquing to a known flat plate. Similar to a stress plate that is used when a eng. block gets its final cyl honing.
Cast iron will not stay 'flat' after many hot/cold cycles. But will 'pull-down' under headbolt torque.
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