Thread: Cylinder head flatness spec??

In the process of checking my l67 heads i got for free. What is the max allowable clearance? I can almost fit a .003in feeler gauge. But a .002 will slide under the straght edge at a few points.

EDIT--One of the other heads i can fit a .003in feeler. It will slide under the straight edge with hardly any resistance. This is between the cylinders.

I'm showing .004 inches as the max allowable warpage.

Nice

Being an engineer and trained as a machinist, I would imagine anything less than .005". But that is just my "gut feeling". Sabre probably posted the correct answer

But I would also consider checking them in a different manner as the surface you have them sitting on probably isn't flat (unless you are using a surface plate). I would suggest getting a known straight edge, laying it on the head surface, and checking with feeler gauges. Again, don't just assume a ruler is straight either. Make sure you are using a straight edge that is straight within .001".

When I was working at the engine machine shop, and even in college and the shop we used there for machining, if .002" - .003" feeler fit between the precision straight edge and the head then it was getting surfaced. The service manual shows you all the ways to lay a straight edge across the head to check this and basically you want to go long ways, diaganol, short side to side, and anything in between and especially checking between combustion chambers. A precision straight edge is important here as just anything flat may have a lot more variation than just the cylinder head and the precision straight edge is usually rated to .0005"-.001" tolerance. .004" may be the max spec but is best to have them surfaced at that point. All in all Im sure it will work ok but way take the chance? It is cheaper to have them surfaced and know it wont be a problem than it is to put it all together and a month down the road have coolant going where it isnt supposed to!

Originally Posted by Trannyman95

When I was working at the engine machine shop, and even in college and the shop we used there for machining, if .002" - .003" feeler fit between the precision straight edge and the head then it was getting surfaced. The service manual shows you all the ways to lay a straight edge across the head to check this and basically you want to go long ways, diaganol, short side to side, and anything in between and especially checking between combustion chambers. A precision straight edge is important here as just anything flat may have a lot more variation than just the cylinder head and the precision straight edge is usually rated to .0005"-.001" tolerance. .004" may be the max spec but is best to have them surfaced at that point. All in all Im sure it will work ok but way take the chance? It is cheaper to have them surfaced and know it wont be a problem than it is to put it all together and a month down the road have coolant going where it isnt supposed to!

I never surfaced heads before, but good to know I wasn't too far off on my thinking.

Specs are just half of the madness! Then you get into what type of surfacing equipment is being used at the shop and what RA (Rough Average) is being put on the surface. Belt sanders are about the most crude version of surfacing and does work but can leave high or low spots and a few local guys with GNs used one of the local shops that does this and had numorous head gasket failures. The shop I worked at had a nice grinding table that had a large spinning stone in the center and you would drag the head around the table in a circular motion and left a very nice flat and smooth finish on the head but was not good for wanting to remove a lot of material in a timely manor. Most larger machine shops use a larger cutter head on a mill and this is typically what you see and also what the shop I use now does with heads and comes out very accurate and true..... But as we found at the shop I worked at for a while they used a Bridgeport mill with a large cutting head and the table of the mill was worn and would mill out the center more than the end about .001-.002" so we quit using it even though the owner persisted it was fine.

I was using a designated snap on (surface plate?) Is that what you call it. Borrowed it from a fellow technician.

That plate is perfect, that is what I as well as most shops use

thats rather large tolerance! I am used to seeing .00005-.0001 at work.

sorry for barging in.

Good to know!!

How much does one of those surface plates run? Any particular type of straight edge that should be used if the surface plate is not available.

IIRC the Snap On straight edge is (was) in the $75-$100 range though I have heard of a few of these being in the $200 range. I havent looked in a number of years though so things may have changed. Other than the SnapOn straight edge Im not sure what else is out there that is easily available to most guys but a quick internet search will likely turn up something. There is a placed called Goodson products that sells a lot of engine rebuilding specialty equipment that we bought from out the machine shop I worked at and they have just about anything and everything you need for machining so they may be worth checking in to.