Head and Block, gasket surface finish

L

Larry Brumfield

BANNED
Gasket surface finish

I'll bet many of you never give a thought (or even know to give it a thought) as to the surface finish applied to your blocks and heads when they are resurfaced. Many automotive machine shops as well as conventional machine shops may not know which surface texture or finish is best without you telling them. Many of the automotive shops today do not even know what a flathead is, and the "youngster" in charge will often argue that the surfaces must be smooth as possible, almost like glass, or it won't seal. Wrong! ..... especially when dealing with cast iron high compression heads for Model A's or even the standard original head.

The truth is head and block surfaces cause sealing problems if they are too smooth OR too rough.

Too smooth a surface allows lateral motion of the head gasket which can deform the combustion seal resulting in an eventual blowout. This can happen even easier with a Model A because there are no precision dowel pins to help hold the gasket if it tries to move. There are only the studs which have 1/32" clearance between the stud and the stud holes in the head. There must be enough surface texture or "tooth" in the finish for the head and block to grip the gasket.

On the other hand, too rough a surface will grip like hell but it creates small leakage paths like little channels.

All resurface work should be checked with a surface comparator by the engine rebuilder or machine shop employee. For best results with cast iron blocks and flatheads, the surface should be in the 90 to 110 RMS. range.

That is just rough enough to feel with your fingernail.

Larry Brumfield
 
Join Date: May 2010
Location: Fresno, Ca.
Posts: 87

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Re: Head gasket failure/ Brumfield Head
Larry Brumfield
Please answer this, what do you recommend, if any for the lube on the studs and nuts? I have an opinion on this but have been buried on more than one occasion. Thanks, Dudley
 
Dudley,

Unless otherwise noted, torque values on the studs and nuts are "dry" torque values. Any lube will change the torque. However, I do recommend using a drop of motor oil on the contact surfaces bewteen the nuts and the stud bosses of the head. This allows the base of the nut to slide. The surface should also be free of paint, i.e., smooth clean metal.

Also, the nuts have a tendency to dig into the metal of the boss on cast iron heads and roughen it up or make a groove. This is especially a problem on some aluminum heads and washers have to be used. When people remove a head they typically ignore this and "put her back together." As a result, the torque can come out uneven or insufficient from nut to nut. The gasket later blows out and the head gets the blame (call Ol' Brumfield and complain or get on the Fordbarn and blow your mouth! Competitor minions will come out of the woodwork and come to your aid).

If the stud bosses are rough after use they should be milled smooth before reinstalling the head.

Larry B.
 
Larry - Very useful information. I really appreciate your knowledge.

Call me dumb, but what does "RMS" stand for.

Thanks,
 
I believe RMS is Roughness Measuring System (some also call it root mean square).

Examples:

The grooving on an LP record is about 125 (you can feel it with your fingernail)

Cover of the Time magazine is about 16

A typical business card is about 16 to 8 depending on quality of the cardstock

The window on your A is about 4.

A burn out torch cut is about 250



What friend Larry is saying I believe is correct.

You must have the block and head surfaces rough enough to "bite" into the head gasket; or the gasket on the A, because it does not have precision dowel pins, is held by the head studs.

A glass smooth head or block surface will let the gasket "move".

Just my humble opinion.


Dennis L Oberer
Green Bay WI
 
Hello Ron

Anybody who is inquisitive is not dumb!

RMS stands for Root Mean Square; actually it is Root Mean Square (RMS) Average. It is calculated by measuring the distance of peaks and valleys on a surface, adding the square of these measurements and then calculating the square root of the total.

Larry B.
 
In the oil refining industry flange surfaces where gaskets are used RMS is a specification that has to be adhered to. Flanges are circular in shape. When the flange temperature increases due to fluid flow the expansion is circular and the same for both halves. When we built refining units we checked the surface finish with a gauge and a jewelers loupe. I believe that there is a gauge available that we can purchase. I will check on this with some of my former colleagues and report when I find out. Larry - you have more machining experience than most of us. What do you use to check the RMS? What grade of cutter or stone do you recommend for the surfacing? If we follow your RMS recommendation I suppose that we can minimnize the blown head gasket problem.

Thanks for the information.

Vic in E-TN
 
I use an electronic surface roughness measuring gage. For the average old car guy, such an instrument would be an outrageous expenditure. All you guys really need to perform a decent check is a surface comparator. They are not expensive. I've seen some made out of plastic for as little as 10 bucks. Fel-Pro makes one. The better ones cost more but they're still fairly inexpensive. I bet if you asked around someone would have one that you could borrow.

The final finish surface on Brumfield Heads is done with a multi-blade cutter head on my milling machine.
 
Tho I disagree with Mr. Brumfied on the lack of lubricant, almost 100%, he is 100% correct on the surface finish! If a machine shop does not have one(surface roughness gauge), walk out, and if they use a " belt " run! Dudley
 
Dudley,

Some people believe that dimensional variations of the threads can cause two identical fasterners to not provide the same clamping force when equal torque is applied. So, they'll use a lubricant which is fine. BUT, if they don't reduce the torque values accordingly, they'll over-torque!

It is my belief that NEW studs and NEW nuts torque and clamp just fine with dry threads.
 
Larry's right. You must stick with the condition of the threads ("WET" - lubricated - or "dry") under which the manufacturer's critical torque values were established and specified. The clamp-up force can be damagingly-higher when a "dry torque" value is used on a lubricated thread.

It's bad enough that we work on "greasy old cars" with "greasy old hands" in a "greasy old shop" environment. Thus, it's almost impossible to have absolutely "DRY" threads when doing a job. Don't intentionally make it worse! I'm paranoid and use a rag with some laquer thinner and lightly wipe down the studs and the nut threads before I apply the torque. I don't do this everywhere, but as Larry points out, head torqueing is critical for leakage, gasket life, stud breakage and engine performance.

Since there are so many other variables that affect the torque such as manufactured thread roughness, roughness of the head surface that the nut turns against, level of any corrosion present, accuracy of the torque wrench - why introduce another major factor like intentional lubrication? Because of the BIG difference it makes, if a manufacturer intended a torque value to apply when lubricated, they would specify it as such.

Earle
 
I opened the door, so here we go. When you tighten, say a head nut, you can feel the HEAT from the friction, on the nut! The deck on an A is about .300, not .500,BIG downfall and to few head studs, another! With GOOD studs (stud with rolled threads )and nuts, with a good quality lube( not engine oil ), I think a torque around 45 pounds should work and the deck stays flat. The deck is a problem, plus I DON'T like galled threads.
By torquing dry, you are using the friction to set the torque. Larry, I'm sure you make a fine product ( your here standing up for your work ), I just don't like to screw dry. Dudley
 
Dudley,

The higher torque necessary for dry thread puts no more pressure on the block than a lesser amount of torque on a lubed thread. The ultimate clamping power or downward push on the head as well as pull on the block will be the same if all determinations are done correctly.

I have nothing against lubed threads. I like lubed threads! It's just that there are too many variables which can affect the torque (including all the variables of each different type of lube) such as the type of alloy, cadmium plating on the nuts, zinc plating on the nuts, no plating on the nuts, etc. for me to trust the outcome.

Larry B.
 
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