The Ford Barn

Merc Cruzer · 07-09-2018, 03:06 PM

What is the difference between the Mercury EAC and 8CM heads? Both sets have the raised Mercury on them.

51 MERC-CT · 07-09-2018, 05:25 PM

Not much, they both have compression ratios in the 6.5 ± range.
The only major difference is if it came off a '49 which has no dedicated mount boss's for the oil filter.

tubman · 07-09-2018, 05:48 PM

Really? I thought 8CM's were 6.8:1 and EAC's were 7.2. Are they that far off or do I have some bad information?

rotorwrench · 07-09-2018, 06:03 PM

They changed to a simpler mouting for the filter housing that didn't include any head bolts. 1CM changed the choke stove set up for the new Merc-O-Matic carb. EAC was an identity change for the 1952/53 Mercury line up since they decreased clearance volume and increased compression to get more horse power. Ford Listed 6.8 CR for the Mercury 255 with 8CM heads. They increase the clearance volume for the 255 so that the compression ratio would still be about the same as the 239 Ford engine with 8BA heads. If the 255 heads are installed on a 239, the compression ratio would be down around 6.3. The EAC Mercury heads were an improvement but not by as much as the EAB heads for the Ford line. The EAB heads have the highest compression of standard stock heads. The Denver AS heads have the highest CR but they were for special order only.

Merc Cruzer · 07-09-2018, 06:11 PM

Thank you all for the clarification.

51 MERC-CT · 07-09-2018, 06:44 PM

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Quote:

Originally Posted by tubman

Really? I thought 8CM's were 6.8:1 and EAC's were 7.2. Are they that far off or do I have some bad information?

Ford EAB's are the ones above 7.0 CR.

tubman · 07-09-2018, 06:58 PM

Boy, every specification I have seem for '52-'53 Mercury's lists them at 7.2:1 compression ratio, the same as Fords. I realize that since they were for the 4" stroke, they weren't as good as the EAB's, but it seems they were better than the 8CM's. I'm having a hard time believing the '52-'53 Mercury's had a lower compression than than contemporary Fords. The original poster was asking only about Mercury heads, not Fords.

51 MERC-CT · 07-09-2018, 07:27 PM

Quote:

Originally Posted by tubman

Boy, every specification I have seem for '52-'53 Mercury's lists them at 7.2:1 compression ratio, the same as Fords. I realize that since they were for the 4" stroke, they weren't as good as the EAB's, but it seems they were better than the 8CM's. I'm having a hard time believing the '52-'53 Mercury's had a lower compression than than contemporary Fords. The original poster was asking only about Mercury heads, not Fords.

I stand corrected (looked at the wrong reference)
8CM's listed at 6.67 CR
EAC's listed at 7.14 CR

r_reed · 07-10-2018, 06:24 AM

no attempt to hijack here, but i have a merc crank 4" and i put the EAB heads on it. my understanding was that the EAB has a smaller combustion chamber than the EAC and the EAB would give me a little more compression. does that make sense? any idea of est compression with the 4"/EAB combo? thanks.

51 MERC-CT · 07-10-2018, 06:55 AM

Quote:

Originally Posted by r_reed

no attempt to hijack here, but i have a merc crank 4" and i put the EAB heads on it. my understanding was that the EAB has a smaller combustion chamber than the EAC and the EAB would give me a little more compression. does that make sense? any idea of est compression with the 4"/EAB combo? thanks.

With 4" crank listed as 8.05 CR.

rotorwrench · 07-10-2018, 11:03 AM

There is a compression ratio cylinder head chart that has been posted on the HAMB for quite a while so that folks can download it and use it for reference. I covers most of the heads that are out there both stock & aftermarket. It also lists CRs with different strokes and there are more than just two. This is a very comprehensive chart and it took a long time to put it all together.

40 Deluxe · 07-11-2018, 02:20 AM

Quote:

Originally Posted by rotorwrench

There is a compression ratio cylinder head chart that has been posted on the HAMB for quite a while so that folks can download it and use it for reference. I covers most of the heads that are out there both stock & aftermarket. It also lists CRs with different strokes and there are more than just two. This is a very comprehensive chart and it took a long time to put it all together.

If this is the same comprehensive chart I have (don't remember where I found it), it's not totally accurate. I have found one glaring error so far. The chart says "41T" heads have 62 cc chambers. BUT, they actually measure about 84 cc's!

So the safe thing would be to physically measure the cc's of any head you use!

34PKUP · 07-11-2018, 03:23 AM

Quote:

Originally Posted by 40 Deluxe

If this is the same comprehensive chart I have (don't remember where I found it), it's not totally accurate. I have found one glaring error so far. The chart says "41T" heads have 62 cc chambers. BUT, they actually measure about 84 cc's!

So the safe thing would be to physically measure the cc's of any head you use!

Unless the heads being measured were actually '81T' heads and not "41T" heads.

rotorwrench · 07-11-2018, 09:44 AM

This one shows the 41T heads at a higher CR. Here is a Link to Ryan's post of the 18 page file. Post # 2: https://www.jalopyjournal.com/forum/...hoices.482189/

40 Deluxe · 07-11-2018, 10:22 AM

Quote:

Originally Posted by 34PKUP

Unless the heads being measured were actually '81T' heads and not "41T" heads.

Nope, 41T heads, measured with a burette, using a Plexiglass plate over the chamber. 84 cc's. This subject came up a week or two ago on another thread. Someone said 41T heads were used by old time stock car racers for the higher compression. I thought that strange because the three I have look like bath tubs. Very deep chambers. I dug out my chart and sure enough, it lists 41T heads at 62 cc's. Chart is wrong! Another barner on that thread, Fordestes, measured one of his 41T heads at 80 cc's. Again, chart is wrong.

40 Deluxe · 07-11-2018, 10:28 AM

Quote:

Originally Posted by rotorwrench

This one shows the 41T heads at a higher CR. Here is a Link to Ryan's post of the 18 page file. Post # 2: https://www.jalopyjournal.com/forum/...hoices.482189/

This is the exact same chart I have. It says a 41T head is 62 cc's. They are not! They are around 84 cc's, determined by actual measurement with a burette and Plexiglass plate! So they are among the largest chamber heads, not the smallest.

Lesson learned: Measure your heads to know what you really have.
Shop where I worked has a plaque on the wall with two Cummins rod inserts and the logo "Trust but verify". Tech did an in frame on a big cam Cummins. It knocked. After much head scratching, turned out he was given small cam rod inserts which have same crank diameter but rod bore is larger on a big cam engine. Took awhile for him to catch on.

tubman · 07-11-2018, 10:51 AM

My advice would be to measure them all, with the possible exception of heads you positively know are new and unused. It's been over 50 years since these heads have been made and I would bet that a lot, if not the majority, have been resurfaced at one time in their life.

JSeery · 07-11-2018, 12:05 PM

Me too!

Always best to measure, only way to know what you have.

rotorwrench · 07-11-2018, 01:20 PM

Maybe the one they checked was one that had already been milled for racing. Truck heads usually all have large volume chambers. Even the 85 HP 221 engines. They might have been OK on the prewar 239 engines and are listed that way in the chart pages 228 & 229 in the green bible. There are a lot of old racer tales about Ford flatheads. It's likely that not all of them are fact. The 41T heads were replacement heads to work with the 221 engines or the 41A replacement 221 blocks made starting in late 1944. The old farm trucks had to keep going for the war effort.

drolston · 07-12-2018, 10:50 AM

My '34 pickup has a '53 Merc engine with an 8BA head on the driver side and EAC head on the passenger side. Bothered me a bit when I bought it, but it is the smoothest running flathead I have ever had; and very strong.

Anybody know how the cc volume compares between 8BA and EAC?

07-09-2018, 05:25 PM	#2
51 MERC-CT Senior Member Join Date: May 2010 Location: East Hartford, Ct Posts: 5,898	*Re: Difference between Mercury CM and EAC heads** Not much, they both have compression ratios in the 6.5 ± range. The only major difference is if it came off a '49 which has no dedicated mount boss's for the oil filter. __________________ DON'T RECALL DOING SOMETHING FOR MYSELF BASED ON SOMEONE ELSE'S LIKES OR DISLIKES Last edited by 51 MERC-CT; 07-09-2018 at 05:31 PM.

07-09-2018, 05:48 PM	#3
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	*Re: Difference between Mercury CM and EAC heads** Really? I thought 8CM's were 6.8:1 and EAC's were 7.2. Are they that far off or do I have some bad information?

07-09-2018, 06:03 PM	#4
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	*Re: Difference between Mercury CM and EAC heads** They changed to a simpler mouting for the filter housing that didn't include any head bolts. 1CM changed the choke stove set up for the new Merc-O-Matic carb. EAC was an identity change for the 1952/53 Mercury line up since they decreased clearance volume and increased compression to get more horse power. Ford Listed 6.8 CR for the Mercury 255 with 8CM heads. They increase the clearance volume for the 255 so that the compression ratio would still be about the same as the 239 Ford engine with 8BA heads. If the 255 heads are installed on a 239, the compression ratio would be down around 6.3. The EAC Mercury heads were an improvement but not by as much as the EAB heads for the Ford line. The EAB heads have the highest compression of standard stock heads. The Denver AS heads have the highest CR but they were for special order only.

07-09-2018, 06:11 PM	#5
Merc Cruzer Senior Member Join Date: Jul 2010 Location: Conifer, Colorado Posts: 2,429	*Re: Difference between Mercury CM and EAC heads** Thank you all for the clarification.

07-09-2018, 06:58 PM	#7
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	*Re: Difference between Mercury CM and EAC heads** Boy, every specification I have seem for '52-'53 Mercury's lists them at 7.2:1 compression ratio, the same as Fords. I realize that since they were for the 4" stroke, they weren't as good as the EAB's, but it seems they were better than the 8CM's. I'm having a hard time believing the '52-'53 Mercury's had a lower compression than than contemporary Fords. The original poster was asking only about Mercury heads, not Fords.

07-10-2018, 06:24 AM	#9
r_reed Senior Member Join Date: May 2010 Location: east glenville NY Posts: 158	*Re: Difference between Mercury CM and EAC heads** no attempt to hijack here, but i have a merc crank 4" and i put the EAB heads on it. my understanding was that the EAB has a smaller combustion chamber than the EAC and the EAB would give me a little more compression. does that make sense? any idea of est compression with the 4"/EAB combo? thanks. __________________ 1949 F-1 Flatrod II (flat paint, flat motor) Keeping one more flathead alive It smoked, but gave it up 7/26/2013 East Glenville NY

07-10-2018, 11:03 AM	#11
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	*Re: Difference between Mercury CM and EAC heads** There is a compression ratio cylinder head chart that has been posted on the HAMB for quite a while so that folks can download it and use it for reference. I covers most of the heads that are out there both stock & aftermarket. It also lists CRs with different strokes and there are more than just two. This is a very comprehensive chart and it took a long time to put it all together.

07-11-2018, 09:44 AM	#14
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	*Re: Difference between Mercury CM and EAC heads** This one shows the 41T heads at a higher CR. Here is a Link to Ryan's post of the 18 page file. Post # 2: https://www.jalopyjournal.com/forum/...hoices.482189/

07-11-2018, 10:51 AM	#17
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,319	*Re: Difference between Mercury CM and EAC heads** My advice would be to measure them all, with the possible exception of heads you positively know are new and unused. It's been over 50 years since these heads have been made and I would bet that a lot, if not the majority, have been resurfaced at one time in their life.

07-11-2018, 12:05 PM	#18
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	*Re: Difference between Mercury CM and EAC heads** Me too! Always best to measure, only way to know what you have.

07-11-2018, 01:20 PM	#19
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	*Re: Difference between Mercury CM and EAC heads** Maybe the one they checked was one that had already been milled for racing. Truck heads usually all have large volume chambers. Even the 85 HP 221 engines. They might have been OK on the prewar 239 engines and are listed that way in the chart pages 228 & 229 in the green bible. There are a lot of old racer tales about Ford flatheads. It's likely that not all of them are fact. The 41T heads were replacement heads to work with the 221 engines or the 41A replacement 221 blocks made starting in late 1944. The old farm trucks had to keep going for the war effort.

07-12-2018, 10:50 AM	#20
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	*Re: Difference between Mercury CM and EAC heads** My '34 pickup has a '53 Merc engine with an 8BA head on the driver side and EAC head on the passenger side. Bothered me a bit when I bought it, but it is the smoothest running flathead I have ever had; and very strong. Anybody know how the cc volume compares between 8BA and EAC?