Mercury flathead 8CM
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Logistics Pro
Joined: Jul 2004
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From: Utah
Mercury flathead 8CM
Ok, I know I have asked a lot of dumb questions lately, so here go's another one.
I think I may have found a running 1951 mercury 8CM I have heard that the stroke is longer 4" ? anyway is pretty much everythuing else the same, If I was to rebuild one would I have just as many options for aftermarket parts, and replacements etc as the 8RT and EAB's ? Thanks in advance
Josh
I think I may have found a running 1951 mercury 8CM I have heard that the stroke is longer 4" ? anyway is pretty much everythuing else the same, If I was to rebuild one would I have just as many options for aftermarket parts, and replacements etc as the 8RT and EAB's ? Thanks in advance
Josh
Laughing Gas
Joined: Oct 2003
Posts: 890
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From: New York
The block is the same, so pretty much everything changes over. The stock 8CM heads are lower compression (to keep the same compression ratio on a larger displacement). Swapping the heads to Ford 8BAs or EABs will give you a compression boost.
Fleet Owner
Joined: Jul 2004
Posts: 27,294
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From: NM
Make sure it's REALLY a Merc; some people think anything with Merc on it is automatically better, so they put Merc heads on Ford blocks, which as Koop says results in lower compression, not higher. I've seen this situation more than once! In at least one case, the guy paid big bucks thinking he was getting a Merc. Unfortunately, there's no quick way to check for the stroke without dropping the pan or pulling a head. Good luck!
Senior User
Joined: Jul 2001
Posts: 432
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From: West London, England
The other guys have pretty well covered everything
One small thing to add
The pistons are different (pin position)
It was a happy day when I took my flatty apart and discovered it had a 4" crank
Steve
One small thing to add
The pistons are different (pin position)
It was a happy day when I took my flatty apart and discovered it had a 4" crank
Steve
Junior User
Joined: Jan 2004
Posts: 66
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Mercury 8CM
The Mercury engines left the factgory with a 4 ich crank istead of the 3.75 inches that the Fords had. That extra .25 of stroke gives the 8Cm 255 CID (when the bore is unchanged) instead of the 239 Ford CID.
I did a Ford 8BA engine over about 10 years ago for a 49 Ford Tudor I had. I had it bored 3 5/16 and had the Merc 4 inch crank for a 276. That motor had a lot more torque and would pull up hill much better than the 3 5/16 X 3 3/4 I have in my 50 convertible now.
It is also possible to have a 4 1/8 crank, by offset grinding the journals on a a 4 inch Merc crank. As you inrease the stoke, you have to change the position of pistons pins.
As others have said, just becasue the head say 8CM don't mean it has a 4 inch crank. I suppose (somehow) you could measure the throw through the spark plug hole or by pulling the oil pan.
Dick
I did a Ford 8BA engine over about 10 years ago for a 49 Ford Tudor I had. I had it bored 3 5/16 and had the Merc 4 inch crank for a 276. That motor had a lot more torque and would pull up hill much better than the 3 5/16 X 3 3/4 I have in my 50 convertible now.
It is also possible to have a 4 1/8 crank, by offset grinding the journals on a a 4 inch Merc crank. As you inrease the stoke, you have to change the position of pistons pins.
As others have said, just becasue the head say 8CM don't mean it has a 4 inch crank. I suppose (somehow) you could measure the throw through the spark plug hole or by pulling the oil pan.
Dick
Lead Driver
Joined: Jan 2006
Posts: 8,423
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From: Colonsay Sk. Canada
EAB is the best stock head.
Too tell if the motor has a 4 inch crank I'm not sure how to word it but the Merc 4 inch crank has indents in the front lobe of the crank itself.
But since crank/pistons are interchangable it is very possible your Ford truck could already have a 4inch crank or your Mercury car could have a crank/pistons from a ordinary 239. I was surprised to find out my 49 halfton Mercury was retrofitted with a 4 inch crank.
Also what Ford/Mercury advertized compared to what they did are 2 different stories not every Mercury truck larger then a 1 ton actually got a 4 inch crank.
Too tell if the motor has a 4 inch crank I'm not sure how to word it but the Merc 4 inch crank has indents in the front lobe of the crank itself.
But since crank/pistons are interchangable it is very possible your Ford truck could already have a 4inch crank or your Mercury car could have a crank/pistons from a ordinary 239. I was surprised to find out my 49 halfton Mercury was retrofitted with a 4 inch crank.
Also what Ford/Mercury advertized compared to what they did are 2 different stories not every Mercury truck larger then a 1 ton actually got a 4 inch crank.
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Senior User
Joined: Jul 2006
Posts: 391
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From: Peterborough, Ontario
You will have to pull a head or the oil pan off and measure the stroke to be sure, the access through the spark plug hole is above the valve. Probably easier to pull a head as the oil pan if the engine is still in the vehicle, if not the pan is the way to go.Good luck
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Cross-Country
Joined: Jan 2005
Posts: 76
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From: Donnellson, IA
The Ford-Merc thing in the 40's and 50's was much the same as it is today. Selling into a higher end market that wants to stay in the Ford "family". As far as the real differences go about any Flathead aftermarket part will fit on the Merc engine. As said, the external dimensions are identical. Internally the crank stroke, piston pin location, and cylinder head combustion chamber volume are different. Merc used a unique carb with a four bolt intake flange but any common Ford intake with a 94 will work just fine. I've located two Merc engines that were in Ford cars dresssed in Ford attire. The only way to know what you got is to measure it up. Have fun.
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