81A Heads
 

Suffering from acute CRS this morning.
Are the combustion chambers on 81A heads smaller than on 59AB heads? It runs in my mind that they are and that the 81A heads require some clearance modifications around the valves to work on a 59AB engine. Hopefully some of our accomplished engine builder can answer this without too much trouble. Thanks in advance.

Re: 81A Heads
 

You may need to cut for clearance around the back of the valves, as the valves on a 59A and later are farther from the bore.
As for volume of the combustion chamber, I haven't yet measured a 59A head, I believe they are around 76-80ish cc. I have measured 81A aluminium and iron heads, they measured 80cc. Also measured some 8BA heads and got the same 80cc.
you gotta remember though there all castings and as such they will not all be identical.
I'm sure Ron has measured 81A heads and came to a smaller cc than me. He often recommends them to use on 59A engines for a slightly higher compression and tighter chamber.
The only precise measurement of your heads, is if you measure them, takes the guess work out.
Martin.

Re: 81A Heads
 

Henry, without question you will need to modify the early heads to work on the 59AB engine. The center line distance between the valves and pistons in your 59AB is .090" wider than the early engines.
There is a KRW fixture for doing just that modification. However, without the fixture, any decent machine shop can do it fairly reasonably.

Re: 81A Heads
 

What about going the other way, 59AB heads on an early say 39/40 motor?

Re: 81A Heads
 

That'll work but the combustion chamber will be slightly larger than stock. I am certain not larger enough to make any noticeable difference in performance.
I have modified early heads to go on the later blocks... very little material is actually removed.

Re: 81A Heads
 

I said maybe need clearance cut because I have fitted 81A heads on a 59A, as I have done this swap without cutting the head around the back of the valves. So not all heads need the cut.
Martin.

Re: 81A Heads
 

If they are stock 81A heads, they need to be cut. If not, they already were.

Re: 81A Heads
 

The chamber on the heads appeared stock and uncut.
Martin.

Re: 81A Heads
 

Good information as I have a set of the 81a heads on my bench waiting for my avatar 59ab.

Re: 81A Heads
 

Martin, I suppose anything is possible but the specifications for 81A heads simply will put them in an interference situation with the later blocks (valves) if they have not been cut.
Understand that cutting these heads was OFTEN done when these engines were rebuilt years ago.

Re: 81A Heads
 

OK lets do it the old fashion way. Place a 59 or 8ba gasket on the head with a few bolts holding it in place. Notice the distance between the gasket and the valve pocket. You don't have to take all this out, just about .060"using a die grinder or even a Dremel. Check clearance with some alum foil. Yes I've measured the 81A heads about 5cc smaller. However as you say they are all cast and can vary. If you do the arithmetic, you'll find stock engines are about a point low from the advertized CR. Now the icing on the cake here is to have the heads milled to give .040/.050" piston to head clearance.

Re: 81A Heads
 

What about the 81T truck heads with that large combustion chamber. Was the idea behind it the same as relieving the block but in the head.?I have a pair but have never ran them.

Re: 81A Heads
 

Hi Ken,
The depth of the T heads is deeper than the A heads thus, lowering the compression ratio. The combustion chamber "shape" is the same. The T heads, like the A heads would also require modification to be installed upon a later (59ab) engine.
The T heads were authentically used on trucks as well as 39 - 41 Mercury's.

Re: 81A Heads
 

My 39 3/4 ton truck had the 81a heads on the engine. That's where I borrowed them from. Wonder why the engine didn't have the t heads?

Re: 81A Heads
 

Thanks Mike, The heads I have are on an early mercury engine . I didn't realize they were standard issue for the early Mercury's, I figured they probably got changed sometime over the last 75yrs. Learn something new every day!