Ford Flathead Centre Main Cap Rplacement

Biggles

Biggles

Member
A friend has asked a question which I can't answer:

"My engine's centre main cap bolt has sheared and the cap fractured. I am going to risk changing this for another centre main cap with the engine in situ, hoping that these were not line-bored by Ford. Do you know if they were made closely enough to spec? Also, this cap is different to the others I have which have a raised ridge each side and a smooth finish, while this one does not have the ridges and is a rough cast finish."

What do you think?


LMBCentreMainSmall.jpg
 
Yes, they were align bored by ford. I have seen this happen several times, and you MIGHT get lucky with another cap, but most I have seen are usually a couple of thou out. the smooth finish cap is probably LB 36, the ridged type were 37 and later. Shouldn't be an issue.
 
I have seen a few of them also. By rights the caps are bored with each engine. BUT, a couple of week ago a friend came in my shop with a center cap just like yours. I told him the story then gave him a couple of center caps from junk engines. Told him to plastic gauge them and if he gets somewhere near .002 clearance and the engine turns over give it a try. The next day he came back with the unsued cap and said one of them checked out fine and the car is on the road. Walt
 
If you've ever bolted up mains without checking match marks, you'll know that yes, they are line bored. Caps from the same engine must be replaced in the original position. As fordscript says, though, you might get lucky. Sure worth a try!
 
Me too! Those Ford engines were machined VERY similarly. I have had several examples of cap swapping that worked out just fine.
 
Re: Ford Flathead Centre Main Cap Replacement

My questions would be 1. Why did it break like this? 2. When did it break like this? 3. How did you discover it? Was there ovetorqueing or uneven torque that caused one side to snap? Was the engine running and started to make "funny noises" which could indicate all kinds of other problems. I'm certainly not a flathead expert like many on this chatline but when I've seen this kind of damage in other engines it's usually been catestrophic. I would at least check the crank for run out as it seems it would certainly start flexing with no center support and under power. I've come to believe that Ford had some extremely high quality castings and metal in it's engines but this one would give me the heeby-geebies. Me? I'ld want it linebored, put a dial indicator on it somewhere (like the center journal) and check it for run out to make sure a bent crank didn't stress the main cap. If someone broke it during installation that might be another story.

Like I said, I'm new to flatheads and would like to hear from old hands who've seen this kind of damage before. Casting flaws?
 
If you have several caps to fit you can get lucky but in my honest opinion I like the idea of taking it to the machine shop for a line bore check. Its cheap insurance. At the same time I would also have the crank shaft checked out.
 
I made a fixture to cut the cap prior to align boring, some shps mill the block this requires a special timing gear.
 
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