Friend of mine has a Flattie in his Model A. He just realized that the engine has two different heads: a lower compression Merc Head 8CM on the left and a higher compression Merc Head EAC on the right. Is their any logic behind this or was it just ignorance by a previous owner/engine builder? Any ideas. Thanks.
Maybe one block side is relieved and the other isn't? Rumbleseat (flathead guru who wrote many valuable tips and tricks) used to tell about his stock car racing days and how he'd use many different spark plugs in the same engine at the same time. Whatever he could find in the trash would work fine. He modified them so it didn't make much difference for the function, but the made-up reasons he'd tell his competitors sure were funny.
"Rumbleseat" was waaay ahead of the crowd when it came to flatheads. Chances are very good that whoever "built" that engine wasn't in his class. I lean toward the "ignorance" theory.
I would still check it out. No amount of guessing will tell you anything. Pull 'em off and see what you really have. Even if they match, I would find another matching head, if only for anesthetic's sake. You can always rework them for optimum quench.
Could and most likely be that who ever put them on subscribed to the "they are all the same" theory or had a damaged head and replaced it with a spare head. My 51 Merc had 8 BA heads on it with headers when I bought it in 1963 and I never had any Idea that it didn't have Mercury heads on it until maybe ten years ago when I started playing with flatheads again and found that Mercury engines had different heads with slightly lower compression. As for the engine in the Model A he might want to do a compression test and see if there is a distinct difference between right and left bank compression. If they were the same or real close to the same no worries, if they are even down the line on each bank but differ noticeably between banks I'd be looking for a matching head for one side or the other.
Could be that the builder only has those two heads. Could be the way the block is surfaced. Could be a few other reasons. I ran 8CM heads on my 8BA until I found a good pair of 8BA heads. No big deal.
Thanks for the good answers. He has to pull and tear apart the engine anyway. He bought the car recently and has no idea about flatheads. As the car had stood a while he changed oil and the old oil had the dreaded grey milkshake consistence, what is definitely a sign for a bigger problem like a crack. As I have a flathead in my mercury he asked me for advise but I have never heard of different heads before.
How can you rework a head for quench? Is it a matter of blending the quench area into the combustion chamber?
Wonder how/where it was stored? The pics make funny colors Maybe hope for the best that water just made it in there from outside. If access is available pull and clean the pan, change oil just fill up with water and run it around and then check it. Nothing to lose really. Might get lucky. Or might be like me
It's oil gunk mixed with antifreeze. It was stored in a dry garage, I guess. The rest of the car is nice and well preserved.
Back before most here were born, "Milled and filled" was sometimes heard at bench racing sessions. Referred to welding in combustion chamber to alter the shape of things. Mostly done to Chevy six cylinder heads.
Compression and improved "quench" are both obtained by reworking the heads rather than changing pistons as is done with OHV engines. One should try to obtain around .050" over the piston in the combustion chamber. This is usually done by measuring the existing clearance over the piston using clay or foil balls, milling the heads to get a nominal .050", and then using a die grinder to clean up any spots with less than minimum clearance. I've done it on a couple of engines and it really does seem to help. A little research on the work of Sir Harry Ricardo will be informative.
I must be missing something, and been a long time since I had a FH V8 apart, but on an OHV engine the quench portion of the combustion chamber is at the same surface as the head where it bolts to the block. Only way to change quench (other than Rich Fox info) would be to change pistons/piston design. I can see using foil balls, clay etc to make sure no valve to piston issue on PHV or valve to CC issue on a FH. I guess what I'm thinking is if I take a head off and lay a hunk of flat iron over it, the quench are would be the same?
The objective in a flathead is to obtain a uniform minimum clearance over the crown of the piston. The idea is to minimize the volume above the piston top so that the volume of air/fuel mixture forced back into the transfer area as the piston approaches TDC is at a maximum. This creates turbulence in the combustion chamber, which increases combustion efficiency. In the process, the compression ratio is increased; both of these are beneficial. Bolting a chunk of flat iron over the cylinder would not accomplish any of this, while ignoring the obvious clearance problems it would cause.
This thread, well, what this thread turned into- is one of the reasons I joined this site. Great info on a variety of subjects. Little tricks to get a bit more horsepower. Thanks all
The Jalopy Journal
