"New" 8CM

M

MikeJ1702

Junior Member
Hello Barners,

I recently purchased an 8CM from a fellow Early Ford V8 Club member. He had owned this engine for number of years and kept it as backup should he ever need a replacement for his running 59AB. I have no immediate plans for the engine but did disassemble it to inspect its condition and thought I would share what I found. Nothing startling but still interesting.

p.s. I've been a member of Ford Barn for a number of years but haven't made many posts. Typically I've read and tried to absorb the knowledge that is shared here. I'm also trying to reach 9 posts so please forgive the numerous posts I will make.

Mike
 
As I understand from my reference materials the 8CM was used in 1949 & 1950 Mercury's. The heads on this engine have 8CM and the word Mercury in script. The pulleys are 3/8" wide so I believe this engine is from a 1950 Mercury as opposed to 5/8" wide pulleys used in 1949.
 
The head bolts came out well except for two. I worked them, soaked them with penatrant and heated them for a few days but two still twisted off. One was in the center of the head entering the exhaust passage for heating the carburetor so not really a surprise.

What I did find surprising was that the cylinders had never been bored. There was a very slight ridge at the top of each bore but very minimal. Measurement of the bores found all to be about 0.003" oversize at the bottom with taper to the top of approximately 0.010" - 0.015" on all bores. One cylinder have slight grooving apparently from a broken top compression ring. It appears that the bores would easily clean up with a 0.030" overbore.
 
The top compression ring was cracked on all pistons when they were removed. More interesting was two of the pistons had burned / melted through the top land down into the top ring groove.

One of these was cylinder #4 (per flathead cylinder numbering convention) and the other was #7. In the attached photos piston #4 is marked 8 and piston #7 is marked 5. The pistons are marked this way as that is the position of the piston from the front of the engine to the rear.
 

Attachments

  • 20241018_103150.jpg
    20241018_103150.jpg
    30.9 KB · Views: 67
  • 20241018_105230.jpg
    20241018_105230.jpg
    37.1 KB · Views: 71
Not the worst news in the world. Now, you can bore it .125" over and have a 276 ci engine, which is a nice size.:cool: The pistons are virtually the same cost.:)
 
Phil Gillespie said:
Possible cause could be detonation!! Had similar happen was linked to detonation.
Phil NZ
Click to expand...
Phil, I agree with your thoughts on possible detonation. It might have been running lean or the timing was way advanced ( trailing or "break" point set not closing) or maybe just running very hot. When these got to be old, used-up cars they often got very little maintenance.
 
tubman said:
Not the worst news in the world. Now, you can bore it .125" over and have a 276 ci engine, which is a nice size.:cool: The pistons are virtually the same cost.:)
Click to expand...

Yes Tubman, lots of possibilities. It's a clean looking engine.
 
Last edited:
The block decks look good, only a couple of small cracks from coolant holes to head bolt holes. No visible to the eye cracks between valves or from valves to cylinders.

The pan and rear valley were full of sludge, like one would expect in an older engine like this.

The crank revealed that this engine had been rebuilt at some point in time. The main and rod journals had both been ground 0.010" undersized. Both have nice looking
surfaces but are worn slightly. The mains measure better while some of the rod journals are somewhat out-of-round. Not sure if they wore that way or if the crank grinder was having a bad day. Either way they all look like they will clean-up at 0.020" under. All the bearing inserts have the Ford oval stamped in their backside so it's likely been awhile since the last rebuild.
 
MikeJ1702 said:
Phil, I agree with your thoughts on possible detonation. It might have been running lean or the timing was way advanced ( trailing or "break" point set not closing) or maybe just running very hot. When these got to be old, used-up cars they often got very little maintenance.
Click to expand...

They often detonate when well worn and using oil but I think you will find the ring grooves are worn out causing ring failure. Look at the broken ring ends ito tell if they were already broken prior to removal. They often break on removal of pistons when the catch the lip.
 
Mike: CONGRATS on getting a merc engine.
As I recall 1950 Merc engines had wide belts and pressed steel crank pulley. 1951 thru 53 used narrow belts and a cast iron pulley.
Also, check and see if there are squirt holes in the connecting rods and make a note of how they are positioned in the block. Take plenty of photos. Number the main bearing caps so as not to mix them up. Don't loose them. Is there a date code stamped on the block intake surface on the pass. right rear of engine block? Here's a thread about flathead engine date codes.
https://www.fordbarn.com/forum/showthread.php?t=308121&highlight=ENGINE+BLOCK+DATE+CODE
and
https://www.bing.com/search?q=flath...EDSAQg5NjMzajBqNKgCCLACAQ&FORM=ANAB01&PC=HCTS
 
Last edited:
You must log in or register to reply here.
Back
Top