81A Big A Heads
 

On a 81A big A head does anyone have some experience with how much material can be die grind out for valve clearance . This motor (59ab) has a lift of .365 and to achieve the piston to head clearance I have to mill the head .80 + thou.

Another question can I angle mill the head leaving some material above the valve area but taking off material to achieve the piston to head clearance .

I CC the heads ( crude liquid volume ) and found the combustion chamber smaller on the Big A head . I’m guessing from a 221 engine . The AB head were larger .

The seal power pistons aren’t helping as I feel the deck height at TDC are a
Smidge low in regards to deck height .

I check with clay with no gasket and it pretty consistent 95-.100 piston to head

Re: 81A Big A Heads
 

This may be an "apples to oranges" comparison, but I have done several 8BA's and had to mill the heads on all of them (stock and aftermarket) to get them where I wanted (.050" nominal) so I could start grinding. On none of these did I run into any valve to head clearance problems after the milling. One engine had an 8CM cam (.338" lift) and another had a "MAX-1" (.364" lift).

I think "Ol' Ron" is the resident expert on "angle milling" here; maybe he will chime in.

Re: 81A Big A Heads
 

Take a picture of your pistons and deck at TDC - would like to see what you have going on. I've never had that amount of clearance - on any head, especially without a head gasket in place. I'm wondering if your pistons have the correct pin location (compression height) - as they should not be that LOW in the manner that you're describing. (At least that is my first impression).

Give us the details on bore, stroke, etc.. Also, would like to see if there is any nomenclature on the tops of the pistons? Don't start whacking the heck out of the heads until we can take a bit closer look at your piston situation. Best of luck!

Re: 81A Big A Heads
 

Excellent point.

Re: 81A Big A Heads
 

3-3/4”

Re: 81A Big A Heads
 

I think B&S has a good point, When it comes to clearance for the cam lift I do this cut at a 4 deg angle which is the approximant angle to the valve head to cyl head. This way you remove very little material from the combustion chamber. Like adding an eye brow. Now angle milling the head is another story. On an 8ba head taking .070" off the chamberside of the head is about all you can do, THis brings both the piston and valves closer to the head. but gives good turbulence to the chamber and raises the Cr about a point, The shop that does this has a broach and all you have to dis put a shim on one side of e head and clamp i in place.

Re: 81A Big A Heads
 

Photo # 5

Re: 81A Big A Heads
 

there is a kr wilson tool that is used to open up the valve chamber to allow A heads to be used on a 59ab engine. I have the tool and have done this several times

Re: 81A Big A Heads
 

I wish I had more history on the motor . Motor was given to me and someone spent A lot of money . I did knock her down and all seemed good . I bought new rings with light hone .
I match the deck height to pin on piston against a Mercury piston
The piston has marking to show offset .

Thanks for everyone comment

Thanks again

Re: 81A Big A Heads
 

Hard to tell from the photo, but it looks to me like the piston is "down in the hole" at least 1/16. That might be the 'smidge' you are referring too.

Can you take a closer picture of the piston edge to the bore. I'm wondering if these pistons were designed for an 1/8" or even a 1/4" stroker crank - putting the pin higher and therefore putting the piston down in the hole on a 3 3/4" stroke crank.

I looked at the one picture - can't quite tell, but it looks like I see at least 1/16" of cylinder sticking ABOVE the piston edge.

If this thing has the wrong pistons in it, don't try to "fix it" with the heads - that is the wrong way to go. :rolleyes: Get a correctly configured set of pistons with the right compression height (if my suspicions are correct).

PS: Since it appears you have mics, put just a rod and a piston pin in a cylinder and figure out what your compression height actually is (remember to subtract 1/2 the pin diameter).

Best of luck!
D

Re: 81A Big A Heads
 

What your looking for is a very turbulant small combustion chamber. By angle millimg the head you take morematerial off the chamber side of the head and the cut for valve clearance at a 4degree angle also only removes material fron thepoint where the valve might hit the head.. Now i do have some numbers you might find interesting, I did this a few years back to an8BA head and ended up with a 64cc chamber and enough clearance for a Max#1 cam. Not sure what size engine it went on but probablu a 258. this made inexpensive street engines with quite abit of low eng power and rev to 5K no problem.

Re: 81A Big A Heads
 

I see a lot of the top ring and a large end gap in that picture.
Bill

Re: 81A Big A Heads
 

I outlined the angle milling of heads and the other necessary operations, which must follow the face machining, in my book. About the maximum I have ever removed from stock iron heads is .060 and I cannot say I have ever put the sonic micrometer on them. I can say the nominal blueprint thickness is .260 and some which I have converted into hone plates have shown at least the print specified thickness.

Re: 81A Big A Heads
 

What’s the difference between A and T embossed heads?

Re: 81A Big A Heads
 

Thanks for all the interest .
This motors been a mystery as I said earlier I inherited the motor. I got distracted when I had motor apart with other issues .
By process of elimination I narrowed down the piston to be Sealed Power 1003NP-30 comp 1.529/crown.210 7.0:1cr .375

I do agree TDC is low
See close up .

Next step is to pull pistons and rod and buy egg piston .030

Where do we buy egge piston from ?

Thanks again
Matt
PS- those piston have an receded rim edge around the edge of the piston which I don’t think us happening .

Re: 81A Big A Heads
 

That edge recession is one reason I do not like the Sealed Power pistons. I don't know what's the reasoning is that design.

Re: 81A Big A Heads
 

The SP pistons in oversize diameters are "decompressed"(SP WORD) so as to maintain original standard size compression ratio.

Re: 81A Big A Heads
 

Good luck with your new pistons . . . glad you didn't try to make up the difference with milling the crap out of the heads. :D

Re: 81A Big A Heads
 

A=Passenger car T= Truck.
I don't know what the difference is, but often the truck version of a head has lower compression, via a bigger chamber.

If you want a definitive answer, sorry I can't give it.

Mart.

Re: 81A Big A Heads
 

There is also a very rare 81AS head - which a friend of mine from Finland has a set of. They are a special high-compression design - and look just about the same as most of our aftermarket chambers (like Edelbrock, Navarro, etc).

I'd love to find a set of these . . . well, just because . . .

Attachment 470705

What is interesting is that the depth of the piston dome area is about .267 deep and 3.110 or so wide (for 221 engines). The dome height is such that pistons I've seen from Ford or anybody else are not going to work well (too much clearance over the piston dome). These heads really need a piston with a higher dome - less radius, more height.

The 81AS chamber is roughly .085" higher than it should be to run a Ross type piston with a .187 raised dome (and have about .045 squish)

If you ran these with stock style pistons or even Ross, there would be a completely wrong quench/squish area above the piston (height and radius). So, there must have been some different pistons . . . anybody have a clue?